Bulletins of American Paleontology

Transcription

Paleobiology, Paleoecology,
and Systematics of Solemyidae
(Mollusca: Bivalvia:
Protobranchia) from the
Mazon Creek Lagerstätte,
Pennsylvanian of Illinois
Jack Bowman Bailey
Number 382, September 2011
BULLETINS OF AMERICAN PALEONTOLOGY
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Paleobiology, Paleoecology,
and Systematics of Solemyidae
(Mollusca: Bivalvia:
Protobranchia) from the
Mazon Creek Lagerstätte,
Pennsylvanian of Illinois
Jack Bowman Bailey
Number 382, September 2011
ISSN 0007-5779
ISBN 978-0-87710-496-4
Library of Congress Catalog Card Number 2011936760
© 2011, Paleontological Research Institution, 1259 Trumansburg Road, Ithaca, New York 14850, U. S. A.
PALEOBIOLOGY, PALEOECOLOGY, AND SYSTEMATICS OF SOLEMYIDAE
(MOLLUSCA: BIVALVIA: PROTOBRANCHIA) FROM THE MAZON CREEK LAGERSTÄTTE,
PENNSYLVANIAN OF ILLINOIS
Jack Bowman Bailey
Department of Geology, Western Illinois University, Macomb, Illinois 61455, USA, email [email protected]
abstract
The most abundant bivalve of the Essex biofacies (Mazon Creek fauna, Pennsylvanian of Illinois), misidentiﬁed by past authors as the
marine pholadomyoid Edmondia de Koninck, 1841, is herein named Mazonomya mazonensis n. gen., n. sp., and assigned to the family
Solemyidae, based on: (1) anterior elongation of the shell as deduced from brevidorsal placement of the hinge-axis, preserved traces of
the external ligament, and supporting structures; (2) preserved traces of a longidorsal extension of the ligamental outer layer and periostracum; and (3) sedimentary backﬁll marks left by the large foot near the longiterminus of the shell. The second most abundant Essex
solemyid (Solemya radiata of past authors), showing traces of the periostracal frill and external ligament, is emended as Acharax radiata
(Meek & Worthen, 1860) n. comb. Other Essex solemyids previously unreported include two probable solemyids left in open nomenclature, and Acharax (Nacrosolemya) trapezoides (Meek, 1874), for which Meek's original, non-Essex specimen is designated as lectotype.
Systematic revisions herein challenge open-marine and open-estuary depositional models of the Essex biofacies. Unlike coeval euhaline oxic communities in which solemyids are rare, the Essex bivalve community is dominated by solemyids, a recurrent phenomenon in
carbonaceous roof-strata immediately overlying Pennsylvanian coal seams. Extant solemyids are common in shallow euryhaline waters,
forming dense chemoautotrophic populations in organic-rich dysoxic/anoxic muds. Within the Essex, the prevalence of solemyids along
with an admixture of thin-shelled euryhaline bivalves and growth-inhibited stenohaline bivalves is suggestive of a transitional paleoenvironment, such as a drowned coal-swamp or restricted estuary, in which superabundance of organics and nutrient pollution had induced
eutrophication.
Arguably, a persistent suite of traits (amphidetic ligament, edentulous hinge, periostracal frill, mantle fusion, reduced gut, and
enlarged gills hosting bacterial chemosymbionts) has characterized the Solemyidae since the Early Ordovician. Whereas the diagnostic
internal ligament of Solemya Lamarck, 1818, is apparently a post-Paleozic trait, the prevalence of external ligaments among Paleozoic
solemyids requires that species previously placed in Solemya be transferred to Acharax Dall, 1908, or other genera. Emended examples
herein are: S. [Janeia] primaeva Phillips, 1836, sensu Hind (1900) (Carboniferous, United Kingdom) is emended as Acharax primaeva
n. comb., a probable senior synonym of S. parallela Beede & Rogers, 1899 (Pennsylvanian, Kansas) (non S. parallela Ryckholt, 1853
[1854]); Carydium elongatum Clarke, 1907 (Lower Devonian, New Brunswick) is emended as Dystactella elongata n. comb. Additionally,
several European Carboniferous species of "Solemya" (e.g., S. puzosiana de Koninck, 1842, S. saginata Ryckholt, 1853 [1854], S. costellata
M’Coy, 1844, and S. excisa de Koninck, 1885) should be reassigned to Acharax.
INTRODUCTION
The Mazon Creek fauna occurs in sideritic concretions
within the lowermost part of the Francis Creek Shale,
Liverpool Cyclothem, Carbondale Formation, Pennsylvanian
(Westphalian D; see Peppers, 1996), at numerous localities
distributed in northeastern and north-central Illinois (Smith,
1970; Baird, 1979: appendix 1; Baird et al., 1985a; Baird &
Sroka, 1990; Baird, 1997a), western Illinois (Foster, 1979)
and west-central Missouri (Baird et al., 1985b). Comprising
the "roof shales" (i.e., shaly strata immediately overlying a
coal seam) just above the Colchester (No. 2) Coal Member,
the lower Francis Creek is analogous to roof shales above coal
seams of other Pennsylvanian cyclothems (see below).
Often compared to the Burgess Shale, the Mazon Creek
Konservat-Lagerstätte is acknowledged to be among the
most significant Paleozoic faunas known (Sepkoski, 1981).
Preservation of the bivalves is remarkable; specimens occasionally show replaced periostracum and ligament, traces of
internal softparts, and preserved burrows with the bivalves
still inside. Despite the deserved attention that other taxonomic groups have received, the bivalves, which are unique
in many respects, have attracted relatively little study. Prior
to the reports of Bailey (1992) and Bailey & Sroka (1997),
much of the existing data on the bivalves have been limited
to faunal lists (Johnson, 1962; Johnson & Richardson, 1966,
1970; Schram, 1979; Thompson, 1979), occasional mention
in general articles or paleoecological studies (Richardson &
Johnson, 1971; Baird et al., 1985a, b, 1986; Baird, 1990,
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Bulletins of American Paleontology, No. 382
1997c; Baird & Sroka, 1990), and two unpublished Masters
theses (Dickson, 1974; Sroka,1984).
The Mazon Creek fauna was divided by Johnson &
Richardson (1966) into two concretionary biofacies, the
Braidwood (freshwater/terrestrial) and the Essex (marinedominated). Whereas Braidwood bivalves include mostly
freshwater myalinids such as Anthraconaia Trueman & Weir,
1946, and Anthraconauta Pruvost, 1930, the more diverse
Essex bivalves, beginning with Johnson & Richardson (1970:
table 1), have been generically described as "marine" (see also
Schram, 1979; Baird, 1997b) or "saltwater" (Baird, 1997a).
The most abundant Mazon Creek bivalve is Mazonomya mazonensis n. gen., n. sp., commonly misidentified as "Edmondia"
in earlier reports, and popularly called the "clam-clam" by
collectors because the valves of specimens are generally "butterflied," that is, conjoined in the natural in situ death pose
with the valves splayed open widely due to post-mortem relaxation of the adductor muscles. Mazonomya mazonensis n.
gen., n. sp. normally accounts for a high percentage of the
total fauna. At many Essex localities, it is second in abundance only to the ubiquitous scyphozoan Essexella asherae
Foster, 1979 (commonly called "blobs") (Baird, 1997a; Baird
& Anderson, 1997). At an Illinois coal mine at Diamond
in Grundy County, Baird (1979: 61) noted that it accounts
for as much as 70% of the total invertebrate assemblage. At
Locality 22 of Baird (1979), the remarkable abundance of M.
mazonensis is mirrored in the nickname of the site, humorously called "Chowder Flats" by collectors.
In early works, Mazonomya mazonensis n. gen., n. sp. was
referred to Solemya radiata Meek & Worthen, 1860. However,
Eagar (1970: 683) indicated this identification to be in error.
Beginning with Dickson (1974: 15), subsequent researchers
(including Baird, 1979, 1997c; Schram, 1979; Sroka, 1984;
Baird et al., 1985a, b, 1986; Baird & Sroka, 1990) incorrectly identified it as the pholadomyoid Edmondia de Koninck,
1841, owing to the superficially similar profile and edentate
hinge. Although not S. radiata, my analysis suggests that early
placement of the bivalve species in Solemya Lamarck, 1818,
was more plausible (Bailey, 1992; Bailey & Sroka, 1997).
Using the criteria of Pojeta (1988) and Carter (1990), preserved morphology permits M. mazonensis n. gen., n. sp. to be
confidently placed within the family Solemyidae Gray, 1840,
and subfamily Solemyinae.
MATERIALS AND TERMINOLOGY
Fossil Repositories
Repositories cited herein are: ISM, Illinois State Museum,
Springfield, Illinois (stored by Illinois State Geological
Survey, Urbana); FMNH, Field Museum of Natural History,
Text-fig. 1. Directionally neutral terms as proposed by Bailey (2009:
fig. 1) for the description of bivalve shells; these can be especially
useful in problematic or poorly preserved bivalve taxa where posterior and anterior directions are either undetermined or equivocal. The bivalve example shown is the Mississippian protobranch,
Spathelopsis Hoare Heaney, & Mapes (1989). Spathelopsis shows an
unexpected mix of solemyoid and nuculanoid characters (see text).
Chicago, Illinois; NEIU, Mazon Creek Project, Department
of Earth Sciences, Northeastern Illinois University, Chicago;
NMMNH, New Mexico Museum of Natural History,
Albuquerque; NYSM, New York State Museum, Albany;
UM, University of Missouri, Columbia; UNC, University
of North Carolina, Chapel Hill; UNM, University of New
Mexico, Albuquerque; USNM, National Museum of Natural
History [United States National Museum], Washington, DC;
and WIU, Museum of Geology, Department of Geology,
Western Illinois University, Macomb.
Shell Orientation Terms
To circumvent the ambiguities inherent in conventional terminology applied to the various regions of the bivalve shell
exterior, an alternative scheme (Text-fig. 1) was proposed by
Bailey (2009). Besides being more precise and etymologically
more consistent, terms in the new scheme are directionally
neutral – a distinct advantage in the description of problematic or poorly preserved bivalve taxa. Several of these terms are
Bailey: Mazon Creek Solemyidae
used herein, including:
breviaxis/breviaxial – referring to the shorter of two anteroposterior axes of shell expansion as measured from the
original growth point (beak/apex);
longiaxis/longiaxial – referring to the longer of two anteroposterior axes of shell expansion as measured from the
original growth point (beak/apex);
inequiaxial – applied where the breviaxis and longiaxis are
significantly different in length; replaces inequilateral
(antonym, equiaxial – replaces equilateral);
breviterminus/breviterminal – referring to the breviaxial extremity of the shell;
longiterminus/longiterminal – referring to the longiaxial extremity of the shell;
brevidorsum/brevidorsal – referring to the breviaxial portion of the dorsal margin;
longidorsum/longidorsal – referring to the longiaxial portion of the dorsal margin;
adapical – proximal placement, toward the beak/apex;
abapical – distal placement, away from the beak/apex.
PALEOENVIRONMENTAL SETTING
The Francis Creek Shale is a depositional system of distributary channel, interdistributary bay, and proximal prodelta facies associated with the Mazonian Delta Complex (Shabica,
1979; Baird et al., 1985a), part of the largest known peat mire
complex in Earth history (Wanless & Wright, 1978; Greb et
al., 2003). Directly overlying the Colchester (No. 2) Coal
Member, the Francis Creek roof-shales represent a deltaic progradation event preceding and partly concurrent with transgressive inundation of the coastal Colchester peat swamp (Baird
et al., 1985a, b, 1986; Baird & Sroka, 1990). As such, it essentially represents a drowned delta (Shabica, 1979). Whereas
there has been widespread agreement that the Braidwood
biota is largely freshwater/terrestrial in character, the Essex
biota has been more problematic, being characterized by various descriptors: marine-dominated (Johnson & Richardson
1966); marine storm-surge (Johnson & Richardson, 1966,
1970; Richardson & Johnson, 1971); interdistributary bay
(Shabica, 1970); near-shore marine (Schram, 1979); crevassesplay (Shabica, 1979); semi-restricted estuarine embayment
(Baird et al., 1985a, 1986); river-influenced (positive) estuary
(Baird & Sroka, 1990); marine, open-water, prodelta-estuary (Baird, 1997c); saltwater (Baird, 1997a); brackish water
(Baird, 1997c); salt marsh/marshy estuary (Bailey & Sroka,
1997); marginal marine (Martin, 1999); or marginal marine/
open marine (Schellenberg, 2002).
Uncertainty regarding the depositional character of the
Essex paleoenvironment is attributable in part to the confusion surrounding a common bivalve (Mazonomya, a new
3
solemyid genus described herein), with the pholadomyoid
Edmondia, a genus considered to be marine (Johnson &
Richardson, 1966: table 1; Waterhouse, 1969). An Edmondiarich Essex community would favor a depositional model in
which open marine species were periodically swept into the
delta and deposited by sediment-laden storm surges as posited by Johnson & Richardson (1966; 1970), Richardson
& Johnson (1971), and others. It would likewise be in harmony with Kansas Pennsylvanian cyclothems, in which
Edmondia frequently occurs in near-to-offshore open-marine
biofacies, such as the Beil-type (Pulchratia) and Drum-type
(Euconospira) assemblages described by Moore (1964), or the
Biofacies 3 association of the Ames marine zone described by
Lebold & Kammer (2006). Although broadly adapted, many
extant pholadomyoids are stenohaline heterotrophs associated
with aerobic euhaline, often shallow-to-deep water substrata
(Waterhouse, 1969; Prezant, 1998a; Coan et al., 2000).
Systematic analyses of the Essex bivalve assemblage here
and elsewhere (Bailey, 1992; Bailey & Sroka, 1997) show it
to be overwhelmingly dominated by solemyoids, not pholadomyoids. Although present, pholadomyoids are consistently
small (probably stunted) and few in number. Their reduced size
suggests they might have been autochthonous rather than allochthonous marine species swept in to the coastal shallows by
marine incursions. Today, solemyids are rare or absent among
stenohaline open-marine faunas. Due to their nutritional
dependence on sulfur-oxidizing endosymbiotic chemoautotrophic bacteria (see below), they occur in dense Essex-like
concentrations in restricted, rather euryhaline conditions,
inhabiting organic-rich, dysaerobic muds. Thus, because the
majority of the bivalves were probably autochthonous, they
do not support the storm-surge hypothesis. Coupled with the
generally diminuitive body sizes of the invertebrates, as well
as the lack of euhaline shelf taxa such as crinoids, articulate
brachiopods, bryozoans, and trilobites, the in situ occurrence
of solemyids and their dominance of the Essex macrobenthos
is in harmony with restricted estuary/interdistributary bay
models, but is also highly suggestive of drowned delta/saltmarshes. As discussed herein, a solemyid-dominated benthos
suggests brackish waters somewhat reduced in oxygen with
dysaerobic to anaerobic muds below.
BIVALVE TAPHONOMY
Like most Mazon Creek fossils, the bivalves commonly occur in syngenetic concretions composed dominantly of siderite (FeCO3), possibly formed from chemical reduction of
Fe2O3 by sulfate-reducing bacteria and subsequent reaction
with bicarbonate (HCO3–) derived from anaerobic decomposition of underlying peat or other organic substrates (see
Sellwood, 1971; Allison, 1988; Coleman et al., 1993; Ehrlich
4
& Newman, 2008). Within the Essex, the concretions occur
in thin zones at several levels, suggesting periodicity. Episodic
genesis of sideritic concretions has been elsewhere been studied in the shale sequences of the Lower Lias (Pliensbachian)
of Robin Hood's Bay, Yorkshire, England, and linked, in part
to stablization of the sediment/water interface during intervals of reduced sedimentation (Sellwood, 1971). However,
the remarkable preservation of biota within Essex concretions
has been attributed to rapid deposition followed by early
diagenesis of both the concretions and the enclosing muds
(Johnson & Richardson, 1970; Richardson & Johnson, 1971;
Shabica, 1971; Woodland & Stenstrom, 1979; Baird et al.,
1986). Schopf (1979) suggested that onset and cessation of
siderite precipitation correlates with fluctuations in both Eh
and ion supply. The diagenetic mineral reactions shown in the
classic Eh-pH fence diagram of Krumbein & Garrels (1952)
suggests sideritic concretions formed in moderately alkaline
(pH between 7.0 and 8.0) and reducing (eH between 0.0 and
-0.3) conditions. Interestingly, optimal carbon-fixation in the
nutrition of solemyids is reported to occur within the same
range (pH 7.5-8.4) favorable for siderite diagenesis (Scott &
Cavanaugh, 2007).
Sideritic iron could have been derived from the underlying
peat; genesis of the Essex nodules has been postulated to result
from sudden burial (obrution) in fresh-to-brackish water conditions (Woodland & Stenstrom, 1979; Baird et al., 1986).
Siderite concretionary diagenesis is elsewhere associated with
estuarine and tidal marsh sediments depleted in sulfates and
calcium-carbonate, although it has been suggested that they
could also form in marine waters where rapid burial in impermeable muds has blocked downward diffusion of SO4 and
CO3 from the seawaters above (Berner, 1971; Woodland &
Stenstrom, 1979; Baird et al., 1986). In tidal marsh sediments
of East Anglia, Pye (1984) reported that most siderite concretions form at shallow sediment depths (< 1 m), just beneath
the oxidized zone, within the upper part of the reduced zone
where intense metabolic activities of sulfate-reducing bacteria have resulted in near-total depletion of sulfates (Berner,
1981; Pye, 1984). More recent studies confirm that reduction
of iron as well as depletion of sulfates in the diagenesis of siderite concretions are microbially mediated by sulfate-reducing
bacteria that are capable of reducing iron directly (Coleman
et al., 1993; White, 1993; Lovley, 1995; Duan et al., 1996).
Interestingly, Lesquereux (1870) presciently suggested microbial mediation as a mechanism for sideritic precipitation of
the Essex concretions well over a century ago (Schopf, 1979).
It is notable that sideritic pseudomorphs after pyrite are abundant within Essex concretions. This suggests early anaerobic
reduction of sulfates to sulfides, resulting in strongly reducing
(eH < -0.3) conditions that were subsequently moderated (eH
> -0.3 < 0.0) either by microbial activity or utilization of iron-
bound sulfides by the large population of solemyid bivalves
(see Reid, 1998: 243).
Whereas skeletal parts composed of chitin and bone are
well preserved among other Mazon Creek metazoans, calcium-carbonate shells of bivalves, in most cases, are preserved
only as casts and molds (external, internal, and composite).
Following burial, original shells were ordinarily dissolved, but
facsimiles sometimes remain as thin calcite-sphalerite replacements. Within the concretions, hardparts such as valves are
usually a dark red-brown color, whereas organic remains of
the uncalcified ligament, periostracum, and softparts, including adductor, pallial, and visceral musculature, are lighter tans
or other contrasting stains (see also Richardson & Johnson,
1971: 1226).
Within the concretions, most solemyid bivalves lie in
situ, butterflied in a natural death pose with no indication
of post-mortem disturbance or transport. This is at variance
with previous reports that mischaracterized them as either
allochthonous marine taxa swept shoreward by storm surges (Johnson & Richardson, 1970; Richardson & Johnson,
1971) or "reoriented from original burrowing postures by decay processes in water-rich mud or by effects of subsequent
burrowing"(Baird et al., 1986: 276). Richardson & Johnson
(1971) and several subsequent authors have alleged that many
bivalves were buried in living position (in contrast to almost
all other Mazon Creek fossils which typically lie in death poses
parallel to the bedding) and used these assertions as evidence
for obrution events – rapid or catastrophic burial (Shabica,
1970). Apparently referring to so-called "Edmondia" (i.e.,
Mazonomya n. gen. herein), Richardson & Johnson (1971:
1226) noted that the "bivalves are occasionally found at the
end of a trail, where they seem to have been overwhelmed
and buried alive." As a result, subsequent authors have interpreted these traces as fugichnia (escape structures) suggestive
of catastrophic burial (Baird et al., 1986; Baird, 1997c; Baird
& Speyer, 1997; Schellenberg, 2002).
However, my study reveals several significant disparities in
the obrution model:
1. Early diagenesis of siderite is elsewhere associated
with reduced depositional rates and stable mud bottoms (Sellwood, 1971).
2. Most bivalves are found parallel to bedding, not in
living position as alleged; specimens of Mazonomya
n. gen. in particular lie on bedding planes in a natural, in situ death poses outside of their burrows with
the adductors relaxed and valves butterflied, gaping
widely. Such poses require that Mazonomya individuals at death were not confined within the sediment.
Moreover, mortality must have been followed by an
undisturbed period of decay (see also Allmon, 1985;
Savrda & Bottjer, 1991) with no scavenging, biotur-
bation, or transport.
3. Bivalves found in living positions are relatively uncommon (epifaunal myalinids are an exception –
these are occasionally found byssally attached to wood
fragments).
4. There is no firm evidence that the "trails," which are
less common than the literature suggests, are fugichnia. The closed valves and sedimentary backfill within
the structures are also compatible with dominichnia
(see below).
5. Bivalves found in burrows are neither appropriate
evidences for mass mortality nor for escape behavior,
whereas horizons marked by large numbers of normally infaunal bivalves consistently lying parallel to
bedding in natural death poses outside their burrows
indeed suggest mass mortality events, but not necessarily due to catastrophic burial.
Mass-mortality of Mazonomya populations might have
been related to temporal perturbations in salinity. Although
extant solemyids are euryhaline and are broadly tolerant to
direct transfer from salinities as high as 34 ppt to as low as
20 ppt or less, mortality rates are nevertheless high without
a period of acclimation at intermediate salinites (Castagna &
Chanley, 1973). However, salinity fluctuations alone do not
account for the apparent lack of post-mortem scavenging or
bioturbation. Alternatively, it has been proposed that periodic
mass-mortality events could result from poikiloaerobic-aerobic
episodes (Oschmann, 1991; Wignall & Hallam, 1991; Brett
et al., 1997). It could be that Essex mass-mortality events,
especially among the butterflied Mazonomya, was due to extremes in the temporal oscillations of biogeochemical (H2S,
O2, CO2) regimes associated with eutrophic cycles (see below). Because extant solemyids require both O2 from the water above and H2S from the muds below, their burrows must
span the dysaerobic-anaerobic interface (i.e., exaerobic tier of
Savrda & Bottjer, 1987). Thus, mass mortality cycles could
result from fluctuating insufficiencies in either O2 or H2S. If
the rate of microbial reduction of iron exceeded the rate of
sulfate reduction, the resulting deficiency in dissolved sulfides
might be linked to mass death of solemyids whose chemoautotrophic endosymbionts are inherently sulfide-dependent.
More likely, preservation of softparts, lack of post-mortem
disturbance, and mass mortality are all related to episodes of
total anoxia, with the solemyids emerging from their burrows
in an unsuccessful search for oxygen.
MAZON CREEK BIVALVE ASSEMBLAGES
Mazon Creek bivalves were comprehensively reviewed by
Bailey & Sroka (1997). The Essex bivalve assemblage is distinctive in several ways (Table 1):
5
1. Shells are very thin and poorly calcified.
2. Many are either edentate or lacking in well-calcified
hinge teeth, instead relying mostly on ligament or periostracum to maintain valve articulation. Weak calcification suggests that shell secretion might have been
inhibited by environmental factors such as hyperenrichment of the water by organic molecules, calcium
insufficiency, low pH, and especially hypoxia (Rhoads
& Morse, 1971; Clark, 1976; Wilbur, 1983; Økland
& Økland, 1986; Byrne & Dietz, 1997). Only bivalves with strong ligaments and thick periostracum,
such as solemyids, were able to prosper.
3. The assemblage is dominated by solemyids, eurhyaline chemoautotrophic bivalves that are rare in normal-marine assemblages.
4. Most marine heterotrophic taxa are small; many appear stunted when compared with similar taxa occurring in other coeval faunas. Stunting suggests marine
taxa were indigenous, but due to the transitional nature of the Essex environment, were not able to reach
average size.
Bivalves are found in both Braidwood and Essex biofacies.
Small in size, and low in diversity, Braidwood bivalves consist
mostly of edentate, thin-shelled freshwater myalinids, such
as members of Anthraconaia, Anthraconauta, and occasionally the euryhaline genus Myalinella Newell, 1942, which has
been elsewhere reported in marine, brackish, and freshwater
strata (Newell, 1942; Kues, 1992a; Bailey & Sroka, 1997). As
such, the bivalves generally support the traditional view of the
Braidwood as essentially freshwater. In contrast, Essex bivalves
were formerly regarded as a nearshore marine assemblage (e.g.,
Johnson & Richardson, 1966). Prior to my study (Bailey,
1992; Bailey & Sroka, 1997), this judgment appeared to find
support from the bivalves, or rather, their misidentification.
Aside from a few euryhaline solemyids, Essex bivalves were
viewed as essentially marine in character based on the predominate taxa which, or so it was believed, were edmondiids,
followed by myalinids, various pterioids, a few heteroconchs,
and an occasional nuculoid. However, several misconceptions
regarding Essex bivalves need to be rectified:
1. Most of the marine "edmondiids" are really solemyids, i.e., Mazonomya n. gen., Acharax Dall, 1908a,
and Acharax (Nacrosolemya) Carter, 1990. Although
true edmondiids do occur in the Essex, they are relatively rare, and all are stunted.
2. Nuculoids and nuculanoids are conspicuously absent
in Essex collections; alleged occurrences (Johnson &
Richardson, 1970; Baird et al., 1985a: 259) are unconfirmed. Extant species are common in open and
protected nearshore marine muds in waters of mostly euhaline salinities (Castagna & Chanley, 1973;
6
Table 1. Census of Bivalvia within the Mazon Creek Fauna (after Bailey, 1992; Bailey & Sroka, 1997). [Note: Aviculopecten janewiedlini Bailey
& Sroka, 1997: 107, is a probable junior synonym of Heteropecten exemplarius (Newell, 1937)]. Biofacies distribution: B, Braidwood; E, Essex.
Approximate bivalve abundances (based on > 12,000 bivalves from several Mazon Creek collections examined by Sroka, 1984: appendices
III-IV): AA, very abundant, 26-50% or greater; A, abundant, 10-25%; C, common, 1-9%; U, uncommon, 0.05-1.0%; R, rare, < 0.05%.
Approximate average sizes (maximum dimension): S, small, < 2.5 cm; M, medium, 2.5-4.0 cm; L, large, > 4.0 cm.
Nuculiformii
Protobranchia
Bivalve Taxa
Solemyidae
Mazonomya mazonensis n. gen., n. sp.
Acharax radiata (Meek & Worthen, 1860)
A. (Nacrosolemya) trapezoides (Meek, 1874a)
Acharax? sp. A
Acharax? sp. B
Myalinidae
Anthraconaia ohioense (Morningstar, 1922)
Anthraconauta sp.
Myalinella meeki (Dunbar, 1924)
Myalinella sp.
Pteriomorphia
Aviculopectinidae
Aviculopecten mazonensis Worthen, 1890
Heteropecten exemplarius (Newell, 1937)
Euchondria pellucida (Meek & Worthen, 1860)
Acanthopecten sp.
Pterinopectinidae
Dunbarella striata (Stevens, 1858)
Dunbarella sp.
Posidoniidae
Autobranchia
Posidonia fracta (Meek, 1875)
Limidae
Palaeolima retifera (Shumard, 1858)
Pterineidae
Anomalodesmata
Heteroconchia
Leptodesma ohioense (Herrick, 1887)
Permophoridae
Permophorus spinulosa (Morningstar, 1922)
Permophorus sp.
Schizodidae
Schizodus cf. wheeleri (Swallow, 1863)
S. cf. affinis (Herrick, 1887)
Edmondiidae (= grammysiidae sensu Johnston, 1993: 93)
Edmondia ovata Meek & Worthen, 1873
E. aspinwallensis (Meek, 1871)
E. oblonga M'Coy, 1855
Sedgwickia? sp.
Grammysioidea hayi Bailey & Sroka, 1997
Distribution
Size
Thickness
(calcification)
Dentition
E(AA)
E(C)
E(R)
E(R)
E(R)
S-M
S
M
VS
M
medium-thin
thin
thin
thin
thin
edentate
edentate
edentate
edentate
edentate
B(C)
B(U)
E(A) B(C)
E(R)
S
S
S
M
thin
thin
thin
thin
edentate
edentate
weakly dentate
weakly dentate
E(A)
E(R)
E(U)
E(U-R)
S-M
S
S
S
thin
thin
thin
thin
edentate
edentate
weakly dentate
edentate
E(C)
E
S-M
S
thin
thin
edentate
edentate
E(R)
S
thin
edentate
E(U)
S
thin
edentate
E(U)
S
thin
weakly dentate
E(R)
E(R)
S
S
thin
thin
dentate
dentate
E(U)
E(U)
S
S-M
medium-thin
thin
dentate
dentate
E(U)
E(R)
E(R)
E(U)
E(R)
S
S
S
S
M
thin
thin
thin
thin
thin
edentate
edentate
edentate
edentate
edentate
7
Table 2. Relative abundance and diversity of bivalve superfamilies within six Midwestern Carboniferous faunas. The Essex and Springfield are
brackish faunas; the others are nearshore marine (euhaline). Aside from the Imo Formation, all are Pennsylvanian in age. The Essex is dominated
by euryhaline taxa: ambonychioids, pectinoids, and especially solemyoids. Materials from Illinois except as shown. Essex – Francis Creek Shale,
Carbondale Formation, Will-Kankakee-Grundy counties; FMNH, WIU, and NEIU collections. Springfield – concretionary shale (Member 98
of Wanless, 1958) above the Springfield (No. 5) Coal, Carbondale Formation, Fulton and Stark counties; WIU collections. Oak Grove – Oak
Grove Limestone (Carbondale Formation), Wolf Covered Bridge, Knox County; Knox College collections. Shumway – Shumway Limestone
(Mattoon Formation), Effingham County; WIU collections. Wewoka – Wewoka Formation, Wewoka and Coalgate quadrangles, Oklahoma;
data from Girty (1915) and WIU collections. Imo – Imo Formation, Late Mississippian (Chesterian), Searcy and Van Buren counties, Arkansas;
data from Hoare et al. (1989) and WIU collections. Abundances are subjective estimates. Approximate average sizes (maximum dimension): S,
small, < 2.5 cm; M, medium, 2.5-4.0 cm; L, large, 4.0-5.0 cm; VL, very large, > 5.0 cm.
Superfamily
Essex
Springfield
Oak Grove
Shumway
Wewoka
Imo
Nuculoidea
0
0
?
C•D1•S
A•D4•M
A+•D4•M/L
Nuculanoidea
0
0
A•S
C•D1•S
A+•D4•L
A+•D4•M
Solemyoidea
A+•D3•M/L
A•D2•M/VL
0
0
0
0
Ambonychioidea
A•D3•S/M
R•S
0
A•D2•VL
C•L
0
Mytiloidea
0
0
0
C/R
C•M
R•S/M
Pectinoidea
A•D4•S/M
C•D2•S/M
C•S
0
R•VL
R•S
Pterioidea
R•D1•S
0
0
0
0
0
Limoidea
R•S
0
0
C•D1•S
0
0
R•D4•S/M
C•D2•S/M
C•S/M
C•D1•M/L
C•D3•L/VL
A•D4•L/VL
Pholadomyoidea
?
R•S
C/R•D1•S
0
C/R•D1•M
R•L/VL
Carditoidea
Crassatelloidea
R•D2•S
0
?
0
R•M
R/C•S
Trigonioidea
R•D2•S/L
R•D2•S/M
C•S/M
C•D1•S/M
R•D1•S
A•D2•M/L
Driscoll & Brandon, 1973; Howes & Goehringer,
1996).
3. Heteroconchs are few; most are thin-shelled or
stunted. Although Astartella Hall, 1858, has been reported from the Essex, the few purported examples
that I have seen in the Mazon Creek Project and Field
Museum collections are misidentified. Most are probably small Edmondia ovata Meek & Worthen, 1873
(Bailey & Sroka, 1997).
4. Whereas typical marine myalinids of the late Paleozoic
are relatively large with robust shells and thick hinge
plates (see Newell, 1942), Essex myalinids such as
Myalinella are all small and poorly calcified with
shells that are little more than periostracum. They
were so thin and flexible that, owing to sedimentary
compaction, the umbonal region (the thickest part of
the shell) is marked by numerous rounded wrinkles,
resembling a rumpled napkin (Bailey & Sroka, 1997:
fig. 8A.3E). Possibly, Myalinella is a stressed-environment ecophenotype of one of the larger, thick-shelled
marine myalinids such as Septimyalina Newell, 1942.
5. In marine environments, pectinoids are larger, fewer
and thicker-shelled than in the Essex. However, shells
of Essex pectinoids like Dunbarella Newell 1937,
Euchondria Meek, 1874b, and Aviculopecten M’Coy
1851, are typically small and thin. Their distribution
elsewhere (especially Aviculopecten) among a wide variety of lithologies suggests that they were euryhaline
with broad environmental tolerances (Newell, 1937;
Kues, 1992b; Kammer & Lake, 2001). They are also
common in nearshore biofacies occurring in pyritic
carbonaceous shales where they are associated with
wood fragments as well as eurytopic brachiopods
like Orbiculoidea d'Orbigny, 1847, and Marginifera
Waagen, 1884 (see Johnson, 1962).
The weight of evidence suggests that the Essex bivalve assemblage represents brackish-dysoxic rather than euhaline-oxic conditions. When Essex bivalves are compared with bivalve
assemblages from Carboniferous nearshore marine communites of the Midwest, there are few similarities (Table 2). The
Essex is populated mostly by solemyids and, to a lesser extent,
by small myalinids and pectinoids, all euryhaline taxa with
thin, poorly calcified shells; although the Essex does contain
marine genera, they are relatively rare, and most are stunted.
By contrast, bivalves from coeval marine nearshore communities are typically larger with thicker shells, and they are domi-
8
nated by different taxa – mostly nuculoids, nuculanoids, pholadomyoids, and heteroconchs.
SOLEMYID AUTECOLOGY AND
FUNCTIONAL MORPHOLOGY
Autecology
Data provided by various authors on the ecological distribution
of extant solemyids is well summarized elsewhere (Shepard,
1986; Pojeta, 1988; Reid, 1998; Zardus, 2002; Stewart &
Cavanaugh, 2006). Living solemyids are euryhaline freeburrowers and, rarely, facultative short-distance swimmers.
Although most protobranchs are not well adapated for salinities less than 20 ppt, Solemya (Petrasma) velum Say, 1822, is
tolerant of brackish salinities as low as 15 ppt, with a few individuals surviving at 12.5 ppt after acclimation (Castagna &
Chanley, 1973). It charactistically digs a Y-shaped ventilated
burrow (Stanley, 1970: pl. 3, fig. 6) spanning the dysaerobicanaerobic sedimentary interface necessary for accessing the
essential metabolic substrates H2S, O2, and CO2 (Stewart &
Cavanaugh, 2006). In S. reidi Bernard, 1980, bridging this
interface permits alternate uptake of oxygen and sulfide, both
of which can be reversibly bound and partitioned by haemoglobin in the large (hypertrophic) gills (Reid, 1990). Although
solemyids have successfully colonized deep-sea reducing environments near hydrothermal vents, cold seeps, and beneath
dead whale carcasses (see Fujiwara et al., 2007), they also form
dense populations in dysoxic muds rich in sulfides and organic detritus in shallow coastal environments such as estuaries, tidal salt marshes and mud flats, intertidal eelgrass meadows, and log storage depots ("booming grounds") near pulp
mills, as well as ecosystems that are anthropogenically damaged through eutrophication and sewage contamination. In
the fjords of Vancouver Island, Shepard (1986) reported that
solemyids form dense populations in substrata characterized
by an abundance of large-particle forest debris with at least
some measure of oxygen in the waters above; however, they
are absent where wood fibers are finely disseminated and the
overlying water is totally anoxic (Shepard, 1986). Reid (1998)
found analogous occurrences in deposits of human hair associated with sewage outfall from Los Angeles, California.
Globally, solemyids have been reported from depths from as
little as 0.5 m (intertidal) to more than 4,000 m, but they are
most abundant at relatively shallow depths – less than 40 m
for booming grounds (Reid, 1980) and 2-4 m for salt marshes
(Stanley, 1970) (see also Zardus, 2002: fig. 7). Extant Acharax
exhibits the greater bathymetric range, with Solemya most
commonly limited to shallow depths (Zardus, 2002).
Although minor suspension feeding on small cyanobacteria is known in Solemya (Petrasma) velum (see Krueger et al.,
1992), and deposit-feeding occurs in both S. (P.) velum and
S. (Solemyarina) velesiana Iredale, 1931 (Reid, 1990), all living solemyids are nutritionally dependent on symbiosis with
intracellular chemoautotrophic Proteobacteria. These occur
within the gills where they both oxidize sulfides and fix carbon
(Cavanaugh, 1983; Felbeck, 1983; Felbeck et al., 1984; Reid,
1989, 1990; Neulinger et al., 2006; Stewart & Cavanaugh,
2006). Sulfide-oxidizing mitochondria occur in peripheral
soft tissues (Powell & Somero, 1986; Anderson et al., 1987).
Consequently, the gills, largest among the Protobranchia, are
hypertrophied, with the bacteria harbored there accounting for
as much as 98% of the total carbon requirements of their solemyid hosts (Waller, 1998; Conway et al., 1989). Accordingly,
the labial palps and gut are either reduced or absent among
the families of living Solemyoida, including the Solemyidae
(Pelseneer, 1891; Owen, 1961; Reid, 1980; Reid & Bernard,
1980) and extant Manzanellidae (= Nucinellidae) (Kuznetsov
& Shileiko, 1984; Reid, 1990; Waller, 1998; Amano et al.,
2007; Kiel et al., 2008). In these respects, solemyoids differ
from other burrowing protobranchs, such as the nuculoids, in
which chemoautotrophic endosymbionts are lacking, the gut
is entire, and the gills are smaller (Reid, 1990). Utilization of
chemoautotrophs permits some solemyids to survive under
conditions of total anoxia for extended periods, sometimes
several days (McMahon & Reid, 1984; Reid, 1990: 198).
Concentrations of sulfide minerals (e.g., sphalerite, pyrite, and
pyrite pseudomorphs) within the valves of Essex Mazonomya
mazonensis n. gen., n. sp., and Acharax radiata (Meek &
Worthen, 1860) suggest a feeding mode in which endosymbiotic chemoautotrophic bacteria served a similar role.
A fairly predictable association of solemyid populations
with organic-rich, dysoxic substrata persists through much of
the Phanerozoic. In the Silurian of Möllbos, Gotland, solemyids occur with other infaunal protobranchs in the Halla beds
(calcilutites with interbedded marlstones) where tiered feeding
strategies are evident: deposit-feeding taxodont protobranchs
burrowing in shallow aerobic tiers and solemyids (presumably
chemoautotrophic) burrowing in the deepest, dysaerobic tier
(Liljedahl, 1991). In the Llangynog Inlier (Early Ordovician
of Wales), the autecology of solemyids is unclear, because they
occur in bioturbated mudstones in mixed association with
euhaline invertebrates including trilobites, articulate brachiopods, red algae, and bryozoans (Cope, 1996a; Riding et al.,
1998); possibly they were limited to dysaerobic conditions associated with very deep burrows. In the Pennsylvanian, large
solemyid populations are mostly limited to shallow, restrictedmarine, carbonaceous shales and limestones, especially those
lying immediately above coal seams that represent drowning
of peat swamps during initial phases of marine transgressions
(Kues, 1992a; Johnson, 1962; Hoare et al., 1979; Wanless,
1958). In late Paleozoic, Mesozoic, and Cenozoic deposits,
solemyoids are significant benthic components in chemosymbiotic invertebrate communities associated with extreme environments, including hydrocarbon seeps, deep hydrothermal
vents (Campbell, 1992; Callender & Powell, 1999; Aitken et
al., 2002; Peckmann et al., 2001; Kiel et al., 2008; Goedert et
al., 2003; Kanie & Nishida, 2000), and "stagnation deposit
Lagerstätten" such as the bituminous marls of the Holzmaden
Posidonienschiefer (Lower Jurassic, Toarcian) of Germany
(Wild, 1990; Seilacher et al., 1985) and black shales of the
Oxford Clay (Middle Jurassic, Callovian) of England (Duff,
1975).
Solemyid fossils and burrows are regarded as useful indicators of dysaerobic depositional environments (Seilacher,
1990). Solemyid burrows (ichnogenus Solemyatuba Seilacher,
1990) have been described from Upper Devonian (Goldring,
1962) and Lower Ordovician strata (Seilacher, 1990). Burrows
of Paleozoic solemyids are characteristically U-shaped, distinguishable from worm burrows by their elliptical cross sections
and smoothly curving forms. In post-Paleozoic strata, the
burrow assumes the form of the characteristic Y-shape by the
addition of a descending shaft that functions as a sulfide conduit (Seilacher, 1990). The large piston-like foot can be used
to actively pump sulfides and other reduced chemicals needed for endosymbiont metabolism up the shaft from below
(Seilacher, 1990; Campbell, 1992), a mechanism analogous
to that used by chemosymbiont-bearing thyasirids (Dufour
& Felbeck, 2003). Where sulfides seep directly into U-shaped
burrows from the surrounding sediment, a descending shaft is
unnecessary (Reid, 1998).
Functional Morphology
Essential parts of the functional morphology of living
Solemyidae include:
1. a strong ligament with the hinge axis located posterodorsally for maximum deployment of the large foot;
2. tube-like anterior elongation of the mantle cavity;
3. ventral fusion of the mantle lobes;
4. large anterior and small posterior mantle openings;
5. hypotrophy (reduction or absence) of the gut and labial palps;
6. hypertrophy (enlargement) of the gills and hypobranchial gland;
7. large foot with papillose margins; and
8. highly flexible shell with overhanging periostracal
frill.
Aside from a large anterior opening for inhalant currents and
passage of the foot and a small posterodorsally placed exhalant
aperture, the lobes of the mantle are largely fused in extant
Solemyidae (Allen & Hannah, 1986; Reid, 1980).
Although there is no direct evidence that comparable
mantle fusion was present in fossil solemyids, a case can be
9
made for fusion among Pennsylvanian or earlier solemyids
with the following characters:
1. gaping shells;
2. cylindrical shape (strong anterior but not anteroventral expansion);
3. high placement of adductor muscles for accommodation of the enlarged gills, hypobranchial gland and
foot (Reid, 1980, 1998: fig. 5.8);
4. radial pleats/plicae/radii (implying thin shell with
flexible periostracum);
5. pallial band consisting of radial muscle tracks;
6. overhanging periostracal frill; and
7. pyriform composite scar of the muscle complex associated with the viscera and foot (suggesting functional
and dynamic similarities to extant solemyids).
Several studies (Beedham & Owen 1965; Reid, 1980;
Reid & Bernard, 1980) suggest that these features, which are
similar in all extant solemyids, are related to reorganization
of the mantle cavity and fusion of the mantle lobes to form a
closed flexible tube with sphincter-like openings at each end.
Taken together, they comprise an adaptive suite of characters
related to life within a deep, ventilated burrow spanning the
dysaerobic-anaerobic sedimentary interface (i.e., exaerobic
tier of Savrda & Bottjer, 1987), and nutritional reliance on
chemosymbiotic sulfide-oxidizing bacteria within the enlarged gills and attendant reduction of the gut (Reid, 1980,
1998; Reid & Bernard, 1980). Because access to both O2 and
H2S is needed, the burrow is usually kept open with a mucous
lining secreted from the large pedal gland (Reid, 1998). Aside
from the valve-like openings, fusion of the anteriorly expanded mantle lobes effectively seals the mantle cavity, enabling
it to function like a pump cylinder, with the foot serving as
a piston to supply gill chemosymbionts with sulfides drawn
upward from the lower shaft of the Y-shaped burrow, which
serves as both a sulfide conduit and sump (Seilacher, 1990;
Campbell, 1992). Mucous secretions from the hypobranchial
gland cleanse the mantle cavity by consolidation and expulsion of waste through the exhalant aperture (Morton, 1977).
Because the large gills are directly attached behind the pedal
retractor muscles, kinetic energy produced by pumping motion of the foot is transferred to them, thereby enhancing
circulation across the gill filaments (Reid, 1980). The small
size and high placement of the posterior adductor muscles
correlates with enlargement of the gills and attached hypobranchial gland lying directly below. Radial muscle tracks that
mark the pallial band are suggestive of ventral fusion of the
mantle lobes and the ability to vigorously infold their margins (Liljedahl, 1984a, b). Radial pleats of the periostracum
establish the pattern of radial plicae in the underlying calcified portion of the valves. Working together, the elastic pleats
and radial muscles of the mantle provide flexibility to adjust
10
the cross-sectional shape of the mantle cavity during pumping operations with the periostracal frill serving to anchor the
anterior and ventral margins of the shell during thrusts of the
foot (Seilacher, 1990). The frill can likewise function as a kind
of pump gasket, to seal the walls of the burrow during pumping operations. The papillae of the foot can serve to enhance
the seal and stabilize the pumping action; they are observed to
assist the bivalve in gripping the wall of the burrow (Stanley,
1970).
The presence of "fully evolved" solemyids in the early
Paleozoic (Liljedahl, 1984a, b, 1991; Cope, 1996, 2000)
strongly suggests that mantle fusion, endosymbiosis with
chemoautotrophic bacteria, hypertrophy of the gills, hypotrophy of the gut, and sulfide pumping are basal traits of
the Solemyidae already present in Ordovician and Silurian
species (Liljedahl, 1991; Cope, 1996a, 2000; Waller, 1998;
Callender & Powell, 1999). Ironically, this runs contrary to
Cox's (1960: 72) view that reduction of the gut was too recent a specialization to have been present in Paleozoic solemyids. Liljedahl (1991) posited both chemoautrophy and gut
reduction to have been present among Silurian (Wenlockian)
solemyids from Gotland. Going further, Cope (1996a, 1997,
2000) argued that they were fully established in Ovatoconcha
fragilis Cope, 1996a, by the Early Ordovician (Arenigian of
Wales). The subdorsal location of the small posterior adductors in O. fragilis (see Cope, 1996a: pl. 4, fig. 2, text-fig. 4)
is comparable to that of extant solemyids, suggesting that the
requisite volume in the posterior part of the mantle cavity to
accommodate hypertrophied gills was already present by the
Early Ordovician. Likewise, the shape and elevated placement
of the composite scar associated with the anterior adductor
and adjoining visceral/pedal muscle complex suggest biomechanical similarities of the foot. Similarly, the periostracal frill,
because it is related to sulfide pumping, is predictably an ancient trait. Cope (2000: fig. 1d) reported a possible frill in O.
fragilis. Unambiguous periostracal frills closely comparable to
those of extant solemyids are herein documented for the first
time among Pennsylvanian solemyids. The radiating plicae
commonly seen in shells of living solemyids are derived from
overprinting by radial pleats in the periostracal frill extending
beyond the calcified shell. Thus, the occurrence of radial plicae in the shells of fossil solemyids likely implies the presence
of an extended frill, even if the frill itself is not preserved.
SYSTEMATIC PALEONTOLOGY
Class Bivalvia Linnaeus, 1758
Subclass Protobranchia Pelseneer, 1889
(= Subclass Palaeotaxodonta Korobkov, 1954)
The biology, ecology, and zoogeography of protobranch bi-
valves were well summarized by Zardus (2002). Lack of
a clear consensus on the cladistic status of members of the
Protobranchia, coupled with a long history of taxonomic instability at the suprageneric level, has made formulation of
a concise diagnosis of the subclass problematic (Reid, 1998;
Coan et al., 2000). Because the discussion of Carter (1990:
143) briefly summarizes both anatomical and conchological
characters of the subclass, it is especially noteworthy:
"Diagnosis by Allen and Hannah (1986, p. 226): "Bivalvia
with foot sagitally and longitudinally grooved, sole margins
papillate, fringed; gill filaments usually plate-like, unreflected
with abfrontal cilia; byssus gland absent. (A so-called pedal or
'byssal' gland in heel of foot is also present. This does not produce a byssus and has a structure totally different from that
of the lamellibranch byssus gland.)" Pojeta (1988, p. 210) described the conchological features of this subclass as follows:
"Shell usually equivalved, inequilateral, and aragonitic; ultrastructure variable, often with nacreous inner layer; most lack
shell gapes; beaks prosogyrate or opisthogyrate, subcentral in
a few, in most shell anteriorly or posteriorly elongated. Most
with taxodont teeth, a few edentulous, some with actinodont
teeth or other dentition types. In some ligament external,
opisthodetic, short parivincular, convex-upward, attached in
external grooves, and supported by nymphs; in some ligament
amphidetic, both external and internal, supported by chondrophore; in many ligament internal, amphidetic, and largely
confined to resilifer. Most isomyarian; some anisomyarian,
having posterior adductor muscle reduced; some monomyarian, having posterior adductor muscle absent. Often having visceral muscles forming prominent scars in shell; pallial
line integripalliate in most; sinupalliate in some." This subclass description is presently modified to indicate that some
Devonian taxa had largely unmineralized ligaments, but
Carboniferous to Recent taxa have (1) only a simple ligament
with a fibrous sublayer, or (2) a mineralized or unmineralized
simple ligament plus a mineralized resilium. The resilium may
be entirely lamellar, laterally mineralized, or entirely fibrous.
Mineralization in the resilium may consist of granules, irregular crystallites or oriented fibers. The shell is entirely aragonitic and ancestrally nacreous internally. Later representatives
have a variably prismatic or homogeneous outer shell layer,
and nacreous, diffuse CL [crossed lamellar structure], crossed
acicular, homogeneous or CCL [complex crossed lamellar
structure] middle and inner shell layers."
Living protobranchs (solemyoids, nuculoids, nuculanoids,
and manzanelloids) share protobranchiate gills that recall the
aspidobranch or bipectinate ctenidia of archaeogastropods.
Using gill structure as a taxobasis, Pelseneer (1911) considered the Solemyidae Gray, 1840, and Nuculidae Gray, 1824,
to share a common ancestry and placed the two, together
with the Ledidae Gray, 1854 (= Nuculanidae H. & A. Adams,
1858) in the Protobranchia, a system later supported by several important studies (Yonge, 1939, 1959; Owen; 1959;
Purchon, 1959) and in agreement with the earlier studies of
Stempell (1898a, b, 1899). In modified form, this scheme
was subsequently followed by Franc (1960), who included the
Nuculidae, Nuculanidae, Malletiidae H. & A. Adams, 1858,
and Solemyidae within the Protobranchia.
Cox's (1960) polyphyletic understanding of the Protobranchia included the orders Palaeotaxodontida Korobkov,
1954 (= Nuculoida Dall, 1889), Lipodontida Iredale, 1939
(= Solemyoida Dall, 1889), and the Cryptodontida Neumayr,
1884 (problematic Paleozoic bivalves with edentate or unknown hinges). Although solemyids are edentate, a condition that Cox incorrectly believed to be primitive, he placed
them in a separate order owing to the hypotrophy of the gut,
which, he posited, was probably a post-Paleozoic specialization. Because of their edentate hinge and early Paleozoic occurrence, it seemed probable, Cox argued, that cryptodonts
also had protobranchiate gills.
Newell (1965), who treated protobranchiate gill structure as an evolutionary grade, not a symplesiomorphic trait,
did not recognize the Protobranchia as a natural group, and
thereby argued against its recognition as a valid subclass.
Consequently, the Treatise classification (Newell, 1969a; see
also Vokes, 1980) placed taxodont protobranchs (Nuculoida)
in the subclass Palaeotaxodonta, edentate protobranchs (order
Solemyoida) in the polyphyletic subclass Cryptodonta, and
the heterodont protobranchs [Manzanellidae Chronic, 1952
(= Nucinellidae Vokes, 1956)] in the subclass Pteriomorphia
Beurlen, 1944. Thus, the Palaeotaxodonta sensu Newell
(1965, 1969a) is a paraphyletic taxon. Although the Treatise
classification is still in wide use, in several later systems nuculoids and solemyoids are combined in the Protobranchia
and both the Palaeotaxodonta and Cryptodonta were abandoned. Johnston & Collom (1998), and others (Nagel-Myers
et al., 2008) revived the Cryptodonta to contain the extinct
order Praecardioida Newell, 1965, whereas Amler (1999)
left it in open nomenlclature. Kříž (2007) eliminated the
Cryptodonta, combining the Praecardioida together with
the order Antipleuroida Kříž, 2007, to form the superorder
Nepiomorphia Kříž, 2007, subclass Autolamellibranchiata
Grobben, 1894.
Studies since the Treatise rekindled support for recognition
of the Protobranchia as a natural group distinct from other bivalves (e.g., Sanders & Allen, 1973; Allen, 1978; Scarlato &
Starobogatov, 1979; Maxwell, 1988; Morton, 1996; Zardus,
2002). Shared similarities extend beyond the gills. Aside from
differences of hinge and palp structure (Allen, 1978, 1985;
Sanders & Allen, 1973), solemyoids and nuculoids show
numerous anatomical similarities suggestive of kinship, including the Type I stomach and digestive diverticula, palp
11
proboscides, circulatory system, smooth periostracum, and
longitudinally grooved foot with papillose fringe, respiratory
pigments, and larval development, and certain aspects of shell
microstructure (Quenstedt, 1930; Yonge, 1939; Purchon,
1956; Owen, 1959; Stasek, 1961; Allen & Sanders, 1969;
Boss, 1982; Carter, 1990; Carter et al., 1990; Zardus, 2002).
The early Paleozoic fossil record linking nuculoids and
solemyoids is also in harmony with this judgment (Pojeta,
1978, 1988; Liljedahl, 1984a, 1994b; Pojeta & Runnegar,
1985):
1. The posteriorly restricted lateral profile with concomitantly diminished gill space, and attachment pattern of the pedal musculature of the Silurian bivalve
"Janeia" silurica Lilljedahl, 1984a, are all intermediate between solemyoids and ctenodontid nuculoids.
In view of their relatively late occurrence, these traits
could be interpreted as either iterative apomorphic or
persistent plesiomorphic.
2. Nucinella Wood, 1851, and Manzanella Girty, 1909,
are heterodont solemyoids with taxodont subumbonal hinge teeth suggestive of an ancient phyletic
link to the Nuculoida (Allen, 1978), although Pojeta
(1988: 210) remarked that taxodonty in these genera
might be a later, iterative character.
3. Paleozoic Solemyidae share similar primitive ligament
(opisthodetic, parivincular, externally set in nymphae)
with taxodont Ctenodontidae Wöhrman, 1893, from
the Early Ordovician, the edentate hinge adopted by
the Solemyidae being an early apomorphic trait.
Broad kinship among protobranch groups was accepted in
most studies (Owen, 1959; Sanders & Allen, 1973; Scarlato &
Starobogatov, 1979; Bernard, 1980; Allen & Hannah, 1986;
Maxwell, 1988; Pojeta, 1978, 1988; Carter, 1990; Waller,
1990, 1998; Starobogatov, 1992; Morton, 1996; SalviniPlawen & Steiner, 1996; Purchon, 1987; Campbell et al.,
1998; Prezant, 1998b; Reid, 1998; Coan et al., 2000, Carter et
al., 2000, Steiner & Hammer, 2000; Schneider, 2001; Zardus,
2002; Bieler & Mikkelsen, 2006). However, the conclusions
of Cope (1995, 1996, 1997) and Cope & Babin (1999) run
contrary to the consensus. Cope (1995: 363) argued that certain Ordovician palaeotaxodonts with gradidentate taxodont
dentition might have had filibranch rather than protobranch
gills. He further argued for placement of solemyoids within a
separate subclass, the Lipodonta Iredale (cf. Cox, 1960) based
upon: (1) few morphological and anatomical similarites to
palaeotaxodonts; (2) observed differences in the protobranch
gills of palaeotaxodonts and solemyoids; and (3) a stratigraphically earlier (Early Ordovician) occurrence of a true solemyid,
Ovatoconcha fragilis, than allegedly transitional genera, such
as Psiloconcha Ulrich, 1894 (early Middle to Late Ordovician)
and late Middle Silurian (Homerian)"Janeia" silurica that are
12
morphologically intermediate between palaeotaxodonts and
solemyoids (Liljedahl, 1984a, b; Pojeta, 1988). However, allowing for preservational differences, one figured paratype of
O. fragilis (see Cope, 1996a: pl. 4, fig. 2) strikingly resembles
several specimens of the probable congener, Psiloconcha figured by Pojeta (1988: pl. 20, figs 1-5, P. subovalis), thus extending the range of Psiloconcha down into the middle Lower
Ordovician. Both O. fragilis and Psiloconcha share characters
seen in extant solemyids with hypertrophied gills: (1) high
placement of the posterior adductors to accommodate the
underlying large gills and attached hypobranchial gland; (2)
elevated placement and pyriform shape of the anterior composite scar (hereafter abbreviated ACS) formed by the anterior adductor, contiguously placed visceral mass containing
a gonad and digestive gland, and visceral/pedal retractor and
protractor muscle complex (cf. P. grandis, Pojeta, 1988: pl. 17,
figs 1-2, 10).
Although subclass Lipodonta continued to be used by
Amler (1999), Cope (2000) subsequently abandoned it, reinstating most palaeotaxodonts, including the Solemyoida,
Nucinelloida, and Nuculoida, in the subclass Protobranchia.
Likewise, Carter et al. (2000) rejected Cope's separate subclass
argument, demonstrating taxodont-hinged Ctenodontidae,
including Ctenodonta Salter, 1852, and Tancrediopsis Beushausen, 1895, to be near-basal plesions within the order
Solemyoida.
Superorder Nuculiformii Carter, D. Campbell,
& M. Campbell, 2000
(nom. transl. Starobogatov, 1992, ex Nuculidae Gray, 1824)
A House Divided?
Within the Nuculiformii, Carter et al. (2000) placed the orders Solemyoida, Nuculoida, and the families Tironuculidae
Babin, 1982, and Praenuculidae McAlester, 1969, into a single unnamed clade that Giribet (2008) subsequently named
the Opponobranchia.
Cladistic analyses of the Bivalvia have resulted in conflicting and even contentious phylogenetic scenarios. The molecular-based (18S rDNA) study of Adamkewicz et al. (1997)
could not establish protobranch monophyly; the same study
also yielded implausible results: two gastropods appeared as
a sister group to a protobranch (Solemya) and an anomalodesmatan (Periploma Schumacher, 1817), with other protobranchs (Nucula Lamarck, 1799, and Yoldia Möller, 1842)
appearing in separate lineages. The 42-character cladistic
analysis of Salvini-Plawen & Steiner (1996), based largely on
soft anatomy, reached a very different conclusion, arguing for
monophyly of the Protobranchia. Similarly, Waller's (1998)
mostly nonpaleontological analysis placed all Protobranchia
in a single subclass, and assigned all other bivalves to a sister
subclass, the Autobranchia (= Autolamellibranchiata Grobben,
1894). Waller's contention of a basal dichotomy between the
superfamily Nuculanoidea H. & A. Adams, 1858, on the
one hand and the superfamilies Nuculoidea Gray, 1824, plus
Solemyoidea Gray, 1840, on the other gained later support
through the hardpart-based cladistic study of Carter et al.
(2000) who proposed the superorder Nuculaniformii to accept the former and Nuculiformii to contain the latter. This
view is consistent with 18S rRNA analysis of Taylor et al.
(2008: fig. 8). However a nonmonophyletic interpretation
of the Protobranchia found support in the combined morphology and DNA sequence studies of Giribet & Wheeler
(2002) who concluded that protobranchiate bivalves are paraphyletic: the Nuculoidea + Solemyoidea comprise a single
clade, the sister group of which, they asserted, is not the
Nuculanoidea as might be expected, but rather, a clade comprising the Nuculanoidea + Autolamellibranchiata. Although
nonmonophyly was also suggested by the molecular analyses
of Girbet & Distel (2004), a clear consensus was not achieved,
inasmuch as the topological outcomes were contingent on
the parameters chosen and datasets selected. The Nuculoidea
+ Nuculanoidea formed a single clade separated from the
Solemyoidea using their 18S rRNA and 28S rRNA dataset,
whereas the Solemyoida and Nuculoidea were grouped closer
together but apart from the Nuculanoidea if either histone
H3 or cytochrome c oxydase (COI) was selected (Giribet
& Distel, 2004: figs 3.2-3.4). However, topological disparities of the protobranch groups appear to be driven by COI
data (Hoeh et al., 1998; Giribet & Wheeler, 2002; Giribet &
Distel, 2004). Nevertheless, Giribet (2008) continued to argue for division of the Protobranchia, even by proposing that
the Bivalvia be divided into three clades: (1) Opponobranchia
(= Solemyoida + Nuculoida); (2) Nuculanoida; and
Autolamellibranchiata. However, Giribet remained undecided
whether the Opponobranchia + Nuculanoida form the clade
Protobranchia or whether the Opponobranchia is the sister
clade to the Nuculanoida + Autolamellibranchiata.
Although molecular-based kinship among the
Protobranchia and phylogenetic relationships to other bivalves remain unresolved, there are significant reasons for yet
suggesting that the protobranchiate bivalves (Nuculoidea +
Nuculanoidea + Solemyoidea) constitute a natural group:
1. Giribet & Wheeler (2002) based nuculoid-solemyoid
monophyly on a single character, the presence of a
shared adoral sense organ. However, an adoral sense
organ has also been reported in the nuculanoids Yoldia
and Nuculana Link, 1807 (Schaefer, 2000).
2. Hemocyanin, the respiratory pigment shared by all
protobranchs, does not otherwise occur among the
Bivalvia (see numerous studies cited by Zardus, 2002:
30). In various forms, it is elsewhere common among
3.
4.
5.
6.
7.
8.
9.
gastropods, cephalopods, chitons, and arthropods
(Decker et al., 2007).
Similar larval types (pericalymma) shared by nuculoids, nuculanoids, and solemyoids differ from veliger
larvae of all other bivalves (Drew, 1897, 1899a, b; Salvini-Plawen, 1973, 1980; Gustafson & Reid, 1986;
Cragg, 1996; Zardus & Morse, 1998; Zardus, 2002).
However, Salvini-Plawen (1973, 1980) observed
that "pericalymma," in the broad sense, are larval
types also shared by other molluscan outgroups (Scaphopoda and Aplacophora) as well as nonmollusks,
including sipunculids and polychaetes. In a narrower
sense "pericalymma" has been used only in reference
to larvae of protobranchs and certain aplacophorans
(Jabolonski & Lutz, 1983; Gustafson & Reid, 1986;
Zardus & Morse, 1998; see also Waller, 1990: 61-63),
whereas larvae of scaphopods and some aplacophorans
have been characterized as "stenocalymma" (Scarlato
& Starobogatov, 1978; Salvini-Plawen, 1980; Haszprunar et al., 1995; Okusu, 2002; Mizzaro-Wimmer
& Salvini-Plawen, 2001). Among protobranch pericalymma, morphological comparisons suggest that
the phyletic link between the nuculoid Acila castrensis
(Hinds, 1843) and the nuculanoid Yoldia limatula
(Say, 1831) is closer than to the solemyid Solemya
(Petrasma) velum (see Zardus, 2002: fig. 5).
The molluscan cross occurs in the developmental
cleavages of the pericalymma of solemyoids Solemya
reidi and S. (Petrasma) velum (see Gustafson & Reid,
1986; Gustafson & Lutz, 1992), whereas the nuculoids (Nucula delphinodonta Mighels & Adams, 1842,
N. proxima Say, 1822, and Acila castrensis) and nuculanoids (Yoldia limatula) resemble each other in the
absence of this feature (Zardus & Morse, 1998).
In juvenile nuculoids and nuculanoids, gill buds arise
at the posterior extremity of the mantle cavity, whereas in solemyids, they arise in the midposterior region
(Gustafson & Reid, 1988).
Extra- and intracellular digestion in the midgut is
shared by both nuculoids and nuculanoids (Giribet
& Wheeler, 2002: 290).
Nuculoids and nuculanoids show important similarities of the digestive diverticula (Owen, 1959) and the
heart-kidney complex (Morse & Meyhöfer, 1990).
Atkins (1938) noted shared similarities in structure
and organization of lateral ciliated cells of the gill filaments among nuculids, nuculanids, and solemyids.
Giribet’s (2008) assertion that the Nuculanoida differ from the Opponobranchia in their non-nacreous
shell is not fully supported by microstructural studies.
Among extant protobranchs, solemyoids, like nucu-
13
lanoids, are also non-nacreous; only nuculoids, both
extant and fossil, are typically nacreous (Taylor et al.,
1969; Carter & Tevesz, 1978; Bailey & Sandberg,
1985; Carter, 1990; Carter et al., 1990). Additionally,
nacre has been reported in fossil shells of both nuculanoids and solemyoids (see Carter, 1990; Carter et
al., 1990): (a) nacreous inner layers occur in the Early
Jurassic nuculanoids Ryderia graphica (Tate, 1870)
(Cox, 1959; Taylor et al., 1969) and Dacromya ovum
(J. Sowerby, 1825); (b) inner nacreous films occur in
several Cretaceous nuculanoids; (c) middle and inner
nacreous layers occur in Pennsylvanian nuculanoids,
including Phestia bellistriata (Stevens, 1858), Phestia
corrugata Hoare, Heaney, & Mapes, 1989, Paleyoldia
glabra (Beede & Rogers, 1899), and Paleyoldia angustia Hoare, Heaney, & Mapes, 1989 (Carter, 1990;
Carter et al., 1990); and (d) middle and inner nacreous layers likewise occur in the Pennsylvanian solemyid Acharax (Nacrosolemya) trapezoides (Meek, 1874a)
(Carter, 1990; Carter et al., 1990). Although there is
considerable variation in shell microstructure among
Paleozoic protobranchs, a general pattern, consisting
of outer prismatic (or other) layers with inner nacreous layers, is shared by Pennsylvanian members of all
three groups. Loss of nacre appears to be a parallel
apomorphic character shared by both solemyoids and
nuculanoids alike.
10. Several Paleozoic nuculoid, nuculanoid, and solemyoid genera share other shell microstructure similarities, consisting aragonitic columnar prisms in at
least part of the outer layer (Runnegar & Bentley,
1983; Carter, 1990), a structure also seen in the Early
Cambrian bivalve Pojetaia runnegari Jell, 1980, a possible protobranch (Runnegar & Bentley, 1983; Cope,
1996b; Geyer & Streng, 1998; but see Carter et al.,
2000).
11. Nuculanoids are said to differ from solemyoids and
nuculoids in their posterior mantle fusion and possession of siphons (Giribet, 2008; Giribet & Wheeler, 2002). Although lacking in nuculoids, posterior
mantle fusion is well developed in both solemyoids
and nuculanoids. Whereas nuculanoids have extensible siphons, solemyoids have a single posterior
mantle aperture, described in early literature as a "siphon," bearing an elaborate system of marginal papillae (i.e., tentacles), putative sensory structures, and
cilia (Owen, 1961; Taylor et al., 2008). Although the
structure is nonextensible and exhalent in function,
in Solemya (Zesolemya) parkinsoni Smith, 1874, the
papillae on the apertural margins interlock, thereby
dividing the aperture into a smaller dorsal opening
14
and a larger ventral one (Owen, 1961). The obvious
correspondence with the exhalent/inhalent siphonal
arrangement of nuculanoids suggests that phyletic relations between nuculanoids and solemyoids might be
closer than supposed. Perhaps the solemyoid aperture
represents a basal structure from which nuculanoid
siphons were later derived.
12. Paleontological studies are divided on the relationship
of the Early Ordovician taxodont genus Ctenodonta
to other protobranchs. Whereas the cladistic study
of Carter et al. (2000) placed Ctenodonta within the
Solemyoida, Pojeta (1988) suggested that Ctenodonta
was a possible nuculanoid basally linked to the Solemyoida (see also Cope, 2002). Further evidence of a
solemyoid-nuculanoid connection is observed in the
Mississppian nuculanoid Spathelopsis Hoare, Heaney,
& Mapes, 1989 (Text-fig. 1), which shows a persis–
ent intermediate mix of solemyoid and nuculanoid
traits (Peck et al., 2009). Whereas the taxodont hinge,
resilifer, and posterior rostrum are nuculanoid in
character, other features are like those of solemyoids:
(a) the shell has a large anterior (pedal/inhalent) gape
and a small posterior (exhalent) gape (in between, the
valves are tightly appressed); (b) the adductor muscles
are anisomyarian and are shifted to a subdorsal position; (c) the shell is anteroventrally expanded as in the
solemyid clinopisthins; and (d) the pallial line forms
a wide muscular band suggestive of ventral fusion of
the mantle lobes (see Discussion below).
Order Solemyoida Dall, 1889
Diagnosis.–Equivalve to subequivalve protobranchs, anteriorly or anteroventrally elongated with anterior and posterior gapes variably developed to absent. Mantle margins
sometimes fused. Foot enlarged; labial palps reduced; palp
proboscides rudimentary or absent, nonridged, triangular,
close to mouth; ctenidia typically large; gut simple, reduced
or absent. Ligament variable, opisthodetic to amphidetic; primary ligamental component opsithodetic, short, thick, either
attached at external nymphae in primitive taxa or attached at
internal nymphae (i.e., chondrophores of most authors) in derived taxa; often with secondary ligamental components consisting of posterior and anterior extensions of outer ligamental
layer; anterior extension sometimes partly internal (subumbonal) and/or external, attaching along anterodorsal margins
of valves. Posterior adductor muscle scar variable, sometimes
well impressed, often reduced or absent. Hinge either edentulous or with a few subumbonal taxodont teeth separated from
anterior lateral teeth by an edentulous space. Chemosymbiotic
sulfur-oxidizing bacteria usually present within the ctenidia.
Remarks.–The preceding diagnosis is a modified amalgam
of those by Allen & Hannah (1986: 227), Pojeta (1988: 213),
Carter, 1990: 169), and Reid (1998). Reid (1998) suggested
that features of probable ordinal value include enlargement of
the foot as well as the near-ubiquitous presence of chemosynthetic gill bacteria, which correlates with the hypertrophy of
the ctenidia, and concomitant reduction of both the gut and
labial palps.
Discussion.–Diagnostic characters of various authors cannot be consistently applied. Whereas Allen & Hannah (1986)
described the shells as equivalved, they are sometimes only
subequivalve and slightly overlapping in certain genera of
the Solemyidae. Although Pojeta (1988) emphasized reduction or elimination of the posterior adductor muscles, they
are occasionally prominent and strongly impressed in certain fossil examples of Acharax including the Cretaceous A.
mikasaensis Kiel, Amano, & Jenkins, 2008 (Albian, Yezo
Group, Hokkaido, Japan) and A. (Nacrosolemya) trapezoides
(herein). Allen & Hannah (1986) emphasized that, if present, the hinge teeth are not chevron-shaped, whereas Pojeta
(1998: pl. 21) showed both peg-like and chevron-shaped
teeth in Manzanella; Reid (1986) called these cardinal teeth
rather than taxodont teeth. Whereas Allen & Hannah (1986)
described the ligament as usually external, it is internal (an
apomorphic character) in extant Solemya. Newell's (1969a:
N241) ordinal diagnosis emphasized shell gape. Although
anterior and posterior shell gapes are common among solemyoids, they are not universal and they are also observed in
other protobranch groups, notably the nuculanoidean families Siliculidae and Lametilidae of Allen & Sanders (1973),
as well as the Mississipian nuculanoid Spathelopsis (see Peck
et al., 2009). Among the ordinal criteria of Allen & Hannah
(1986), emphasis was placed on the equivalve shell. However,
because valve overlap is common among solemyoids, equality
of the valves can be only approximate.
Sanders & Allen (1973) divided the order Solemyoida into
two families, Solemyidae and Nucinellidae. Subsequently,
Scarlato & Starobogatov (1979) redivided the order into
two suborders, the Nucinellina (containing three superfamilies – the Afghanodesmatoidea, Manzanelloidea, and
Huxleyioidea) and the Solemyina (containing two superfamilies – Acharachoidea and Solemyoidea). Whereas the former
system is in wide use, the latter system has received little support.
Allen & Hannah (1986) argued that use of the superfamily Solemyacea [sic] in earlier classifications (e.g., Cox,
1960, 1969) is unnecessary because it contains only one family (Solemyidae) bearing the same definition. Retaining the
superfamily, they contended, would necessitate erection of
a second superfamily Nucinellacea [sic] also containing one
family (Nucinellidae) of the same definition. Nevertheless,
Pojeta (1988) applied the Solemyacea [sic] to a single family,
the Solemyidae, but added the superfamily Nucinellacea [sic]
Vokes to embrace two families, the Nucinellidae Vokes, 1956,
and the Manzanellidae Chronic, 1952. A similar system was
used by Carter (1990) and Cope (1996a). Coan et al. (2000)
recognized two superfamilies within the order Solemyoida,
namely, the Solemyoidea Gray, 1840, and the Manzanelloidea
Chronic, 1952, the latter consisting of a single family, the
Manzanellidae, posited to be the senior synonym of the
Nucinellidae Vokes, 1956.
Superfamily Solemyoidea Gray, 1840
Diagnosis.–"Small to large, anteriorly or anteroventrally
elongated solemyoids lacking teeth; adductor muscles anisomyarian, having posterior adductor muscle smaller than
anterior one; in many a shelly buttress present anterior to
posterior adductor muscle. Usually extending posterodorsally
from ventral side of anterior adductor muscle is an arcuate
to straight muscle scar marking attachment of integument of
the visceral mass; this scar is dorsally continuous with scar of
muscles of foot. Ligament variable, external or both internal
and external, with or without nymphs and chondrophores"
(Pojeta, 1988: 213).
Remarks.–Although the earliest species are anisomyarian, the condition is variable; some extant species are nearly
isomyarian (Owen, 1959: fig. 7; 1961: fig. 3; Reid, 1998:
fig. 5.8). Among solemyoids, chondrophores are also called
internal nymphs (see below). Eleven apomorphies of living
Solemyoidea were listed by Waller (1998: 18):
1. labial palp lamellae absent/vestigial;
2. gills large;
3. gill filaments very narrow, elongated;
4. outer demibranch of the gill dorsally directed;
5. posterior adductor smaller than the anterior;
6. alimentary tract reduced;
7. sorting area in the stomach absent;
8. two openings of the digestive diverticula into the
stomach;
9. inner fold of the mantle relatively deep and well developed;
10. anterior and posterior mantle papillae present on the
middle fold of the mantle; and
11. sulfur-oxidizing bacterial symbiosis present.
Family Solemyidae Gray, 1840
Diagnosis.–"Shell elongate, gaping, posterior and anterior margins rounded, weakly calcified particularly at ventral
15
margin; hinge teeth absent; extensive ventral mantle fusion;
lumen of hind gut extremely narrow or wanting" (Allen &
Hannah, 1986: 227).
Remarks.–An attendant feature of extant solemyoids is
hypertrophy of the gills and hypobranchial gland, which are
commonly much larger than those of the Nuculoida (Scott,
2005). Five apomorphies of living Solemyidae were listed by
Waller (1998: 19):
1. hinge edentate, taxodont dentition lacking;
2. frill of periostracum extending beyond the edge of the
calcified shell and/or presence of secondary prismatic
shell microstructure;
3. pericalymma larva uniformly ciliated;
4. apical tuft absent on the larva; and
5. ventral fusion of the mantle lobes.
Shell microstructure of extant and fossil Solemyidae was described by Taylor et al. (1969), Carter (1990), and Carter et
al. (1990).
Systematic background.–Beginning with its namesake, the
genera and subgenera of the Solemyidae have a perplexing history. The accepted type species of Solemya Lamarck, 1818, is S.
togata (Poli, 1795) (= S. mediterranea Lamarck, 1818, by subsequent designation by Children, 1823; = Tellina togata Poli,
1795). However, the first of two species listed by Lamarck
(1818: 489) were S. australis Lamarck, 1818, with S. mediterranea listed second. Not surprisingly, both Bowdich (1822)
and Blainville (1825: 570) chose S. australis to represent the
genus, and Dall (1908a, b) treated the latter as the type species. However, Dall (1908b) further noted that Lamarck's
S. mediterranea was earlier published twice; first under the
name Mytilus solen Salis-Marschlins, 1793, and later as Tellina
togata Poli, 1795. Dall (1908b: 362-363) thereby regarded
both S. mediterranea and S. togata as junior synonyms of S.
solen (Salis-Marschlins, 1793). Given that Salis-Marschlin's
description (1793: 405) of the shell is adequate and his color
figures (1793: pl. 9, figs 15a, b) are quite good, Dall's opinion
is compelling. Thus, the type species of Solemya is arguably
S. solen.
The most characteristic features of solemyid shells are
the extreme anterior elongation of the shell and far posterior placement of the ligament (Pojeta, 1988: 202). Generic
taxobases of Dall (1908a, b) and later authors emphasized the
morphology of the ligament and its supporting structures.
Using Holocene species, Dall (1908a) first divided Solemya
into three subgenera: S. (Solemya) Lamarck, 1818 (to include S. australis, S. parkinsoni Smith, 1874, and S. solen); S.
(Petrasma) Dall, 1908a (type species: S. borealis Totten, 1834);
and S. (Acharax) Dall, 1908a (type species: S. johnsoni Dall,
1891). Iredale later added two genera, Solemyarina Iredale,
16
Text-fig. 2. Simplified internal views of ligamental areas in Acharax Dall, 1908, and five subgenera of Solemya Lamarck, 1818. Diagrams based
on Taylor et al. (2008: fig. 1), Kamenev (2009: table 1, fig. 108), and J. D. Taylor and E. A. Glover (pers. comm., 2010). Not to relative scale:
(1) Acharax s. s., type species A. johnsoni Dall, 1891; (2) S. (Solemya) [= Solemya s.s.], type species S. (Solemya) togata Poli, 1795; (3) S. (Petrasma
Dall, 1908), based on S. (Petrasma) velum Say, 1822; (4) S. (Solemyarina Iredale, 1931), type species S. (Solemyarina) velesiana Iredale, 1931;
(5) S. (Zesolemya Iredale, 1939), type species S. (Zesolemya) parkinsoni Smith, 1874; (6) S. (Austrosolemya Taylor, Glover, & Williams, 2008),
type species S. (Austrosolemya) australis Lamarck, 1818. Key: black, inner (fibrous) ligamental layer; dark gray, outer (lamellar) ligamental layer;
stippled, nymphae. Abbreviations: ALE (AB), anterior outer-layer ligamental extension, abapical part; ALE (AD), anterior outer-layer ligamental
extension, adapical (subumbonal) part; B, shelly buttress; EN, external ligamental nymph; IN, internal ligamental nymph (= "chondrophore" of
other authors); LL, ligamental lobes (lateral expansions of the adapical part of the ALE); PA, posterior adductor scar; PE, posterior periostracal
extension; PL, primary ligament; PLE, posterior outer-layer ligamental extension.
1931 (type species: S. velesiana Iredale, 1931), and Zesolemya
Iredale, 1939 (type species: S. parkinsoni Smith, 1874). Vokes
(1955), studying Tertiary and Quaternary solemyids, followed Dall's subgeneric system, but minimized the usefulness
of Iredale's genera. Fleming (1957: 136) retained Solemyarina
as a separate genus but regarded Zesolemya as a subgenus of
Solemyarina. Cox (1969: N243) accepted Solemyarina as a
subgenus of Solemya, but placed within it not only S. velesiana
(the type species), but also both S. parkinsoni and S. australis.
Unfortunately, Cox mistakenly figured the ligament of S. australis as the type species of Solemyarina instead of S. velesiana
(see Taylor et al., 2008: 78). Additionally, Cox raised Acharax
to generic status, and added both Adulomya Kuroda, 1931, and
the problematic Paleozoic genus, Janeia King, 1850. Bernard
(1980) essentially followed Cox's system, except placing S.
(Solemyarina) in synonymy with S. (Solemya), as did Pojeta
(1988). However, both Allen & Hannah (1986) and Carter
(1990) continued to treat S. (Solemyarina) as a separate subgenus with Zesolemya treated as a junior synonym. Pojeta (1988)
expanded the Solemyidae to include several Paleozoic genera,
including Clinopistha Meek & Worthen, 1870, Dystactella
Hall & Whitfield, 1872, Paleosolemya Pojeta & Runnegar,
1985 (= Dystactella; Pojeta, 1988: 216), and Psiloconcha
Ulrich, 1894. To Pojeta's list of Paleozoic solemyids can be
17
Table 3. Simplified character key for Holocene Solemyidae based in part on Taylor et al. (2008) and Kamenev (2009: table 1).
Characters
Primary ligament
(opisthodetic,
parivincular)
Acharax s.s.
narrow with
external
nymphae
Proximal
narrow; lateral
(subumbonal) part of lobes absent
anterior extension of
ligamental outer layer
Solemya s.s.
S. (Petrasma)
S. (Solemyarina)
S. (Zesolemya)
S. (Austrosolemya)
wide with
internal
nymphae
wide with
internal
nymphae
narrow with
internal
nymphae
wide with
long internal
nymphae
wide with internal
nymphae
narrow; lateral
lobes absent
expanded;
lateral lobes
absent or
small, irregular,
sometimes
elongate along
dorsal margin
expanded;
lateral lobes
symmetrical,
forming
bifurcating
narrow strips
expanded;
lateral lobes
symmetrical,
forming
elongated
narrow strips
expanded, lateral
lobes symmetrical,
forming
prominent ovoid
pads
Posterior extensions
of ligamental outer
layer
small, not
expanded
elongate
expansion
above each
posterior
adductor
small, not
expanded
small, not
expanded
elongate
expansion
above each
posterior
adductor
small, not
expanded
Internal shelly
buttress (rib) along
anterior margin of
posterior adductor
absent or weak
absent
weak, narrow,
extending to
nymph; also
running along
dorsal margin
of posterior
adductor
weak, low,
narrow
weak, low,
broad; not
extending to
nymph
strong, broad,
elevated, extending
to nymph
Posterior adductor
weakly
impressed,
small,
subdorsal
placement
weakly
impressed
impressed
weakly
impressed
weakly
impressed
weakly impressed
Type species
S. johnsoni
Dall, 1891
S. togata (Poli,
1795)
S. borealis
Totten, 1834
S. velesiana
Iredale, 1931
S. parkinsoni
Smith, 1874
S. australis
Lamarck, 1818
added "Janeia" silurica Liljedahl, 1984a, Ovatoconcha Cope,
1996a, and Acharax (Nacrosolemya) Carter, 1990. Adulomya
Kuroda, 1931 (= Ectenagena Woodring, 1938), is no longer
considered to be a solemyid; Kanno et al. (1998) and Kiel
(2007) placed it in the heteroconch family Vesicomyidae Dall
& Simpson, 1901.
Because the important studies of Taylor et al. (2008) and
Kamenev (2009) have done much to unravel the generic and
subgeneric confusion that has plagued Holocene Solemyidae,
their nomenclature is applied to all extant species of Solemya
treated herein. In their studies, Iredale's genera, Solemyarina
and Zesolemya, are treated as separate subgenera of Solemya
(Text-fig. 2; Table 3). Furthermore, Taylor et al. (2008) added
an additional subgenus, S. (Austrosolemya) Taylor, Glover, &
Williams, 2008 (type species: S. australis Lamarck, 1818).
Ironically, S. australis was, as previously noted, treated by early
authors as emblematic of Solemya s.s.
In addition to the structure of the ligament (see below),
past authors have stressed the presence or absence of:
1. an internal shelly buttress (i.e., rib or clavicle of other
authors; ridge or prop of Dall, 1908a) sometimes joining the primary ligament support structure (nymph/
chondrophore), either limited to the anterior edge of
the posterior adductor, or sometimes extending above
and slightly behind it;
2. bounding of the primary ligament support structure
by the posterior adductor;
3. radial shell ornament (radial ribs, plicae, or radii);
4. anterior and posterior shell gapes;
5. intercalations in the comarginal ornament; and
6. rounding of the anterior and posterior margins of the
shell.
Other possible criteria include: (1) left-right asymmetry or
dorsal overlap of the valves; and (2) numerical ratios, includ-
18
ing aspect ratio (length/height) and relative distance of the
beaks and umbones from the posterior extremity (i.e., longiaxis/breviaxis ratio). Additionally, Taylor et al. (2008) suggested that in extant solemyids, several characters associated
with the posterior aperture of the mantle are potentially of
systematic significance.
Cox (1969: N242) considered the scars marking muscle
attachment of the integument of the visceral mass and foot to
be distinctive features of Solemya and the Solemyidae. In each
valve, these scars form arcuate bands beginning with the pedal
protractor and visceral retractors along the anteroventral margin of the anterior adductor, extending posterodorsally, and
ending with the attachment site of posterior pedal retractor
in front of the umbo. Just below the anterodorsal margin of
the valve, pedal elevator and anterior pedal retractor muscles
occupy a band extending from the posterior pedal retractor to
the posterodorsal edge of the anterior adductor (Cox, 1969:
figs 31, B1). In extant S. (Austrosolemya) australis, and Acharax
agassizii Dall, 1908a, the bands are conspicuous (see Pojeta
1988: pl. 1, fig. 5, pl. 2, fig. 7). But because solemyid shells
are very thin, these features, like other muscle scars, are usually
indistinct in fossil specimens. Pojeta (1988) has nevertheless
documented them in Paleozoic genera of both solemyid subfamilies: the Solemyinae Gray, 1840 (Solemya, Acharax, and
Psiloconcha) and the Clinopisthinae Pojeta, 1988 (Clinopistha
and Dystactella). In many fossil solemyids, including those described herein, the weakly defined scars of the anterior adductor and adjoining pedal and visceral muscles visually merge,
resulting in a single, large anterior composite scar (ACS) that
is pyriform or teardrop-shaped in outline, anteriorly broad
but narrowing posterodorsally to a vertex in front of the umbones. The form and placement of the ACS closely matches
the characteristic position occupied by anterodorsal softpart
complex seen in living solemyids (Owen, 1959: fig. 7; 1961:
fig. 3; Kamenev, 2008: fig. 76). Bordered by associated visceral and pedal muscle attachment sites, the complex consists of
the anterior adductor muscle and contiguously placed visceral
mass (consisting of a large digestive gland and gonad). Thus,
the ACS proves to be an invaluable feature in diagnosing fossil
solemyids.
One common but variably developed shell character not
discussed by past authors is herein dubbed an auricule (L.,
auricula – "little ear"), in reference to the slightly protruding brim or lip of the posterodorsal margin of the shell and
typically associated, in part, with the relatively high placement of the posterior adductor muscle (Reid, 1980: fig. 4)
and primary ligament. The auricule can rise as high or higher than the umbones, which are typically very low in most
solemyids. The contrast of the elevated auricule juxtaposed
against the depressed umbo results in a distinctive posterolateral profile shared by many solemyids including Solemya,
Acharax, Psiloconcha, and Dystactella as figured by Pojeta
(1988) and Ovatochoncha as figured by Cope (1996a, 2000).
In some specimens, the area between the auricular summit
and the umbo forms a weak saddle (see figures of Psiloconcha
by Pojeta, 1988: pl. 18). A similar auricule and saddle form
the brevidorsum of Mazonomya mazonensis n. gen., n. sp. (see
below).
Ligament.–Within the Solemyidae, the posterior part of
the ligament is most closely involved in articulation of the
valves and is the location of the primary hinge axis. It can be
either external, inserting at longitudinal grooves and external
nymphae (plesiomorphic) as in Ordovician Ctenodontidae,
Paleozoic Solemyidae, and extant Acharax; or it can be internal (apomorphic), supported by internal nymphae (i.e., chondrophores of other authors), structures fundamentally similar
to nymphae but wider, sunken beneath the shell margins, and
partially covered by a thin prismatic shell layer (Carter, 1990).
But the prismatic layer, as noted by Waller (1998: 19), "is
a secondary feature that is not present in the earliest ontogeny of the dissoconch … nor in the earliest members of the
Solemyoidea and Solemyidae." The essential similarities of internal nymphae and "chondrophores" are strikingly evident in
Kamenev's (2009: figs 2-6, 108) photographic comparisons
of the ligaments of various species of Holocene Solemya and
Acharax johnsoni (see also Text-fig. 2 herein). Thus, the traditional descriptor of internal nymphae as chondrophores belies
their underlying similarities to external nymphae. Carter et al.
(2000) described the solemyid ligament as parivincular wide
(PW), referring to the two conditions respectively as PWexternal and PW-internal.
Although the primary ligament is functionally opisthodetic in many living and fossil solemyids, including
Holocene Solemya (Solemya) togata, S. (Solemyarina) velesiana, S. (Zesolemya) parkinsoni, S. (Austrosolemya) australis, S.
(Petrasma) atacama Kuznetsov & Shileyko, 1984, Oligocene
Acharax dalli Clark, 1925, and Pennsylvanian A. (Nacrosolemya)
trapezoides, it can be broadly described as amphidetic because
the outer (lamellar) layer of the ligament runs the length of
the dorsal margin extending both in front of and, to a lesser
extent, behind the primary ligament (Owen, 1959; Beedham
& Owen, 1965; Pojeta, 1988; Carter, 1990; J. D. Taylor &
E. A. Glover, pers. comm., 2010). The ligamental system
consists of several contiguous components (Text-fig. 2) all
enclosed by a thick periostracum (Allen & Hannah, 1986;
Pojeta, 1988; Carter, 1990; Taylor et al., 2008; Kamenev,
2009): (1) a short, opisthodetic, primary component symmetrically attached at nymphae (either external or internal)
and consisting of an inner (fibrous) ligamental layer overlain
by an outer (lamellar) ligamental layer; and (2) a secondary
component consisting solely of the outer (lamellar) layer but
extending both anteriorly and posteriorly beyond the primary
ligament. The secondary component is subdivided into two
parts: (1) the posterior extension of the outer layer (PLE) extending above and behind the fibrous primary layer; whereas
PLEs are fairly short in S. (Petrasma), S. (Austrosolemya), and
Acharax s.s., they internally form elongated symmetrical pairs
in S. (Solemya) and S. (Zesolemya); (2) an anterior extension
of the outer layer (ALE). The ALE itself often consists of two
continuous elements: (1) a long abapical ALE, mostly external, overlain by or fused to periostracum, running along the
anterodorsal length of the shell margins, and attaching either
at the edges of the anterodorsal margins or at ligamental ridges lying just inside the anterodorsal margins; and (2) a short
adapical (subumbonal) ALE, located just in front of the beaks,
where the posterior end of the outer layer sometimes descends
internally to form laterally expanded lobes (i.e., pads, plates, or
horns of various authors) of lamellar ligament (Text-fig. 2; also
see Beedham & Owen, 1965: 408; Taylor et al., 1969: fig. 36;
Allen & Hannah, 1986: 408; Cox, 1969: N242, fig. B12b;
Bernard, 1980; Pojeta, 1988: pl. 3, fig. 4, pl. 4, fig. 3; Taylor
et al., 2008: figs 1c-e; Kamenev, 2009: figs 4-6). In their taxonomic treatments of the Solemyidae, Taylor et al. (2008) and
Kamenev (2009) regarded the lobes as an important taxobasis
for distinguishing the various subgenera of Solemya (Textfig. 2). Whereas lobes are absent in Holocene Acharax and
S. (Solemya), they are strikingly evident in three Holocene
subgenera: in S. (Solemyarina) and S. (Zesolemya), the lobes
take the form of descending narrow strips, bifurcating in the
former; but in S. (Austrosolemya), they appear as ovoid pads
(Text-fig. 2). In S. reidi, ligamental pads cover an uncalcified
area of the shell just behind the beaks (Bernard, 1980). In
S. (Petrasma) velum, pads are absent, but minor pad-like irregular expansions of the lamellar ligament have been postulated to serve as repair ligament – secondary reinforcement of
the umbonal region which, due to its exceptional thinness, is
subject to wear and and sometimes fracturing (Waller, 1990:
56, pl. 1, fig. B). Frequently in S. (Zesolemya) parkinsoni, the
descending narrow lobes correspond to dorsoventral stress
cracks that extend half the distance from the beak to the ventral margin of the shell (Carter, 1990).
Carter (1990) observed that in both Pennsylvanian
Acharax (Nacrosolemya) and Holocene Solemya (Zesolemya)
parkinsoni, the ALE is asymmetrical, attaching along the inner margin of the right valve but along edge of the left valve.
Where present, internal lobes of the ALE are symmetrically
developed in both valves, but have been reported only among
solemyids with internal primary ligaments (Bernard, 1980;
Pojeta, 1988; Taylor et al., 2008; Kamenev, 2009). However,
in at least three Paleozoic species with external primary ligaments, a lesser pad-like thickening of the posterior end of the
ALE, herein dubbed a demipad, is manifested as a longitudi-
19
nally cuneiform or almond-shaped expansion of the posterior
subumbonal terminus of the ALE. This feature is asymmetrical (lateralized), confined to right valves in A. radiata and A.
(N.) trapezoides (see below), but limited to left valves of an
unnamed solemyin from Young Island, Lower Devonian of
Nunavut, Arctic Canada (Prosh, 1988; Bailey & Prosh, 1990).
Lateralization of the demipad coupled with its unbroken continuity with the ALE supports Carter's (1990) contention
that the ALE was inherently asymmetrical in its attachment to
the left and right valves of A. (N.) trapezoides. Corresponding
configurations in the composite demipad-ALE structure in
both Devonian and Pennsylvanian solemyins demonstrate
that this asymmetry was rather widespread during the middle
to late Paleozoic.
The functional significance of the demipad is not fully determined. The demipad/ALE actually begins slightly behind
the beaks. At the same location in S. (Zesolemya) parkinsoni,
the anterior outer (lamellar) layer briefly underlies the anterior end of the inner (fibrous) layer of the primary ligament
(Beedham & Owen, 1965: 408, fig. 2). If it results from secondary deposition of ligamental repair material (cf. Waller,
1990: 56), it would presumably be variable in both outline
and texture, the observed asymmetry perhaps resulting from
repairs related to compression caused by dorsal overlap of the
opposite valve. More probably, it represents a chondrophorelike attachment site of the ligamental outer (lamellar) layer
where, extending from the primary external ligament (located
above and behind), it first descends internally in one valve before continuing its anterior run between the valves as the ALE.
In either case, it is indicative of asymmetrical attachment of
the ALE to the margins of the left and right valves. Among
protobranchs, chondrophore asymmetry is not altogether surprising; in certain species of the malletiid Palaeoneilo Hall &
Whitfield, 1869, a chondrophore is present only in the right
valve (Carter, 1990: 158-159).
Valve asymmetry/dorsal overlap.–In addition to ligamental asymmetry, lateralization of the valves has also been reported in both living and fossil solemyids. In extant Solemya
(Zesolemya) parkinsoni and S. (Austrosolemya) australis, the
aforementioned attachment asymmetry of the ALE results in
dorsal overlap of the left valve over the right valve (Carter,
1990: 174; Carter et al., 1990: 317). However, in dead specimens the natural overlap can be further exaggerated by: (1)
inherent flexibility and fragility of the shell; (2) post-mortem
sedimentary compaction; and (3) living positions like that of
S. (Petrasma) velum, which horizontally reclines at the bottom
of U-shaped or junctions of Y-shaped burrows (Stanley, 1970;
Pojeta, 1988; Carter, 1990).
Lateralization of the valves in Paleozoic solemyids has
also been reported and discussed by several authors, espe-
20
cially in regard to the problematic genus Janeia from the
Carboniferous-Permian of Northern England. After proposing the name Janeia, King (1850: 177) later withdrew it
within the same publication (p. 246). The subsequent history
of Janeia (type species Solemya primaeva Phillips, 1836, from
the lower Carboniferous of Yorkshire) is so perplexing that
Pojeta (1988: 214), in his historical review of the genus, challenged its usefulness remarking, "Janeia has been a name looking for a concept." One purported character in Janeia is the
dorsal overlap of the left valve over the right. In an obscure
comment, Meek (1876: 127) suggested that many American
Carboniferous solemyids should be placed in Janeia based on
the overlap, which he considered to be natural:
The reason for believing that the name Janeia may possibly yet have to be retained for the Paleozoic species usually
referred to Solemya (or, at any rate, for a part of them), is
that all of the American Carboniferous species of this type
that I have seen, differ from the recent species of Solemya
in not being exactly equivalve; that is, although apparently exactly like that genus in other respects, they seem
to have the beak of the left valve always lapping upon
that of the right. For a long time I thought this merely
due to accidental distortion; but after seeing numerous
specimens, some of which from the even adjustment of the
ventral margins of the valves, as well as from their general
appearance, seemed not to have suffered any distortion,
and yet showing this character, I have been led to believe
it is natural; and as the typical species of Solemya have the
beaks equal, it is not altogether improbable that there may
be a generic difference between them and the Paleozoic
species. It is true, however, that Professor King did not
mention any such inequality of the beaks as one of the
characters on which he proposed to separate Janeia; but it
is a character that might readily be overlooked, from the
supposition that it was due to accidental distortion.
Natural left-over-right overlap of the valves of Janeia was
subsequently endorsed by Beushausen (1895), who added
that Janeia also lacked shell gapes, athough this view was
later challenged by Hind (1900) who posited that both the
valve overlap, which he viewed as inconsistently developed,
and the missing gapes are post-mortem effects attributable
to sedimentary compaction. In spite of Liljedahl's (1984a,
b) compelling case for left-over-right valve overlap and lack
of gape in "Janeia" silurica from the study of 597 specimens
from the Silurian of Gotland, Pojeta (1988) concluded that,
based upon the overlap inconsistencies that he observed
among Pennsylvanian "Solemya" radiata and "S." trapezoides
Meek, 1874a, the problem remains unresolved. Nevertheless,
it seems clear from the external nymph-like ossicle present
only in the right valve, that the left-over-right attachment of
the valves in "J." siliurica is purely natural (Liljedahl, 1994b:
text-fig. 3). Based on the lateral profile, Pojeta (1988: 217)
recommended that "J." silurica be reassigned to Dystactella.
However, the ossicle, which is a unique structure among solemyids, argues for recognition of "J." silurica as a new genus.
Conjoined valves of Acharax (Nacrosolemya) trapezoides,
including both the lectotype and figure 17c of Carter (1990),
as well as A. radiata herein, also exhibit left-over-right overlap
of the valves. Carter (1990) attributed the overlap in A. (N.)
trapezoides to asymmetrical attachment of the ALE. Restriction
of the ligamental demipad to the right valve further suggests
that the left-over-right overlap in these species is natural, not
taphonomic, in origin. Interestingly, in the Lower Devonian
solemyins from Young Island, the valve asymmetry is reversed,
with the demipad placed solely in the left valve. Left-right
dominance might be a subgeneric or specific character, or,
perhaps it results from allelic or behavioral heterogeneity
within a species, reflecting individual preferences to recline
on either the left or right valve within the burrow. Although
present evidence suggests otherwise, the possibility that the
demipad represents repair ligament cannot be excluded. Shells
of many solemyins are exceedingly weak at the beaks; forces
exerted there by the overlap of the left valve would be predictably compensated by apical augmentation of the right valve
by the addition of a demipad of repair ligament.
Subfamily Solemyinae Gray, 1840
Diagnosis.–"Anteriorly elongated solemyids having barely
discernible beaks and umbones" (Pojeta, 1988: 214).
Discussion.–In addition to a secondary anterior ligamental
extension (ALE) in certain species, solemyins have a parivincular wide primary ligament (PW), described as "short to
intermediate length, barrel-shaped parivincular ligament with
wide, strongly projecting nymph in which anterior nymph
margin rises abruptly above the hinge line" by Carter et al.
(2000: 52), who recognized two categories: (1) PW-external,
as in extant Acharax s. s. and most Paleozoic solemyins, and
(2) PW-internal in which the ligament is partially covered secondarily by a thin prismatic shell layer, as in Solemya s. s.
Hypotrophy of the gut and hosting of chemosymbiotic
sulfur-oxidizing bacteria is characteristic of extant solemyins.
Whereas the gut is merely reduced in S. (Petrasma) velum, S.
(S.) togata, S. (P.) valvulus (Carpenter, 1864), S. (Zesolemya)
parkinsoni, S. (Austrosolemya) australis, and S. (Solemyarina)
velesiana, it is completely absent in S. reidi, S. (P.) borealis,
Acharax eremita Kuznetsov & Shileiko, 1984, and S. (P.) atacama (see Reid, 1998: 243).
Although Solemya and Acharax are conchologically similar, Scarlato & Starobogatov (1979), emphasized differences
in the primary ligament, dividing solemyids into two sepa-
rate families: the Acharacidae (external ligament), and the
Solemyidae (internal ligament). The same system was used by
Maxwell (1988), Amler (1999), and Zardus (2002). However,
aside from the thin prismatic shell layer partially covering the
ligament in Solemya, both genera share a PW ligament and
ligamental support structures that are fundamentally similar (see Text-fig. 2 and discussion above). Because ligamental
differences alone do not justify division of the Solemyidae,
the integrity of the family is herein conserved. However, significant disparities in 18S rRNA gene sequences of Holocene
Acharax and Solemya reported by Neulinger et al. (2006) support Cox's (1969: N243) decision to raise Acharax from subgeneric to full generic status and supports the fossil record,
which shows the two genera to have been phylogenetically
separated for a considerable span of geologic time. Yet, placing
Acharax in a separate monotypic subfamily (i.e., Acharacinae
Scarlato & Starobogatov, 1979; see Bieler et al., 2010: 115)
would perhaps overstate their differences.
Pojeta (1988) recognized two solemyid subfamilies, the
Solemyinae Gray, 1840, containing Solemya, Acharax, and
Psiloconcha, and a sister group, the Clinopisthinae Pojeta,
1988, containing Clinopistha and Dystactella. Whereas the
Solemyinae included anteriorly elongated solemyids with
weak beaks and umbones, the Clinopisthinae consisted of
"anteroventrally elongated solemyids having prominent beaks
and umbones; ligament opisthodetic, external and convex
upward" (Pojeta, 1988: 216). The short, barrel-shaped clinopisthin ligament (PW-external) is a crossover character, also
occurring among solemyins, for example, Acharax s. s., A. dalli
(see Geodert et al., 2003: pl. 42, fig. 5), A. (Nacrosolemya)
trapezoides (see Carter, 1990; Carter et al., 2000), A. radiata,
and Mazonomya mazonensis n. gen., n. sp. Thus, the main
criteria distinguishing the Clinopisthinae from other solemyids are the anteroventral elongation of the shell and the
prominent beaks and umbones. The cylindrical profile of the
Solemyinae is linked to pedal thrusts parallel to the anteroposterior axis as well a sufficient volume in the posterior part
of the mantle cavity for large gills and hypobranchial gland.
In contrast, the profile of clinopisthins is indicative of an anteroventral pedal thrust and and a variably reduced gill volume. Chemoautotrophs might have been nevertheless present
in Clinopistha levis Meek & Worthen, 1870, owing to its characteristic occurrence alongside other solemyids in the carbonaceous roof shales overlying Illinois coal seams.
Genus Mazonomya n. gen.
Type Species.–Mazonomya mazonensis n. sp.
Diagnosis.–Smoothly oval solemyins with moderately elevated umbones, weak posterodorsal auricule, well-defined co-
21
marginal growth lines of variable relief, external radii mostly
lacking. Primary ligament opisthodetic, parivincular, attached
at small external nymphae; secondary ligament consisting of
anterior ligamental extension attaching in each valve along
nymph-like lamellar ridge just inside anterodorsal margin;
periostracum extending dorsally beyond primary ligament,
and especially, beyond secondary ligament. Posterior adductor small, obscure, subdorsally placed; internal buttress lacking; pallial line consisting of weak band of radii. Composite
scar (anterior adductor/visceral/pedal musculature) weak, pyriform, subdorsally placed, tapering posteriorly toward umbones.
Etymology.–Mazon, referring to the Mazon Creek Konservat
Lagerstätte of Will, Grundy, Kankakee, and Livingston counties in northern Illinois; and mya, Latin, meaning mussel.
Systematic Placement.–The edentate hinge, longidorsally
marked by longitudinal lamelliform ridges that form grooves
in internal molds, formed the basis for Dickson's (1974)
and Sroka's (1984) placement of members of this genus in
Edmondia. Although it is true that the hinge homeomorphically recalls that of true edmondiids (see Runnegar & Newell,
1974; Johnston, 1993: fig. 68), it is likewise comparable to
Paleozoic clinopisthins, especially Clinopistha (see the partially exposed hinge in the illustration by Pojeta, 1988: pl.
12, figs 2, 4), in which the lamelliform ridges are relatively
shorter, less impressed, and closer to the valve margins than in
Mazonomya. Both Dickson (1974) and Sroka (1984) speculated that a shallow pallial sinus is present in M. mazonensis n.
gen., n. sp., although a pallial sinus is not supported by their
figures; neither was any hint of such a feature observed among
the many specimens that I have studied.
Additional evidence excluding Mazonomya n. gen. from
the Edmondiidae or related pholadomyoids lies in the location of the traces left by the foot in burrows (see below) and
in the lack of microfine ornament of the shell exterior. Among
many pholadomyoids and other Anomalodesmata, the shell
ornament is known to be finely tubercular (Hind, 1900: pl.
49,fig. 1a; Newell, 1969b: N818; Runnegar, 1974: pl. 2, fig.
16; Johnston, 1993: fig. 66; Prezant, 1998c: figs 9.9-9.10).
Several true examples of Edmondia (obtained by Harold B.
Rollins from the Pennsylvanian near Brush Creek, Athens
County, Ohio) show this distinctive structure at 20-40X using
a binocular microscope. Scanning electron photomicrographs
show this ornament to result from an outer shell layer similar to that described by Carter & Clark (1985: 55) as nondenticular composite prismatic. Although several specimens
of Mazonomya n. gen. preserve thin recrystallized or replaced
shell remnants, no comparable ornament was observed.
Widely open ("butterflied") specimens consistently show
22
Text-fig. 3. Uncoated pyrite concretion containing a "butterflied"
specimen of Acharax (Nacrosolemya) trapezoides, WIU MC 1145,
from Member 98 above the Springfield (No. 5) coal, near Canton,
Illinois. Anteroposterior axes of the articulated valves diverge by ca.
35º from the brevidorsally placed hinge axis (white dotted line), the
location of the primary (opisthodetic) ligament. Similar valve divergence is seen in other fossil solemyids. Scale bar = 10 mm.
a roughly 20º divergence of the anteroposterior shell axes
from the brevidorsum. Applying the shell orientation criteria of Bradshaw & Bradshaw (1971) for palaeotaxodonts, the
hinge axis is thereby confined to the brevidorsum. Hence, the
brevidorsum is posterior, and the longidorsum (the region
where the shells open most widely) is anterior. This agrees
with butterflied specimens of other solemyids in which the
hinge axis is also placed along the brevidorsum. For example,
the butterflied specimen of Acharax (Nacrosolemya) trapezoides
in Text-fig. 3 compares favorably with butterflied examples of
Mazonomya n. gen. in Pls 1-2. Because the valve orientation
is the reverse of that in Edmondia, Mazonomya n. gen. cannot be an edmondiid as previously supposed. Although past
authors have not reported them, organic traces of the primary
(opisthodetic) ligament along with its supporting structures
are evident in well-preserved specimens described herein. The
longidorsal portion of the ligament so commonly observed in
Mazonomya n. gen. is, in reality, the anterior ligamental extension (ALE) peculiar to amphidetic solemyids.
When the shells are butterflied, a similar divergence of
the anteroposterior shell axes from the brevidorsum is seen
in other solemyids. For example, it is evident in "Solemya" [=
Acharax] radiata and Clinopistha levis figured by Pojeta (1988:
pl. 22, fig.1, pl. 9, fig. 10), and in "Solemya" [= Acharax
(Nacrosolemya)] trapezoides figured by Kues (1992a: figs 2.9-
2.10). It is also seen in the butterflied syntypes of the probable solemyid "Carydium" elongatum Clarke, 1907 [NYSM I
8756 (Clarke, 1907: 227; 1909: pl. 5, fig. 17; Bailey, 1986a:
fig. 4.10) and NYSM I 8757 (Clarke, 1907: 227; 1909: pl. 5,
fig. 16; Bailey, 1986a: fig. 4.11)] from the Lower Devonian
Dalhousie Beds of New Brunswick. The syntypes were later
referred to Dystactella by Bailey (1986a: 322), owing to close
comparison of shell shape, sculpture, and brevidorsal placement of the hinge axis with the solemyid D. subnasuta (Hall &
Whitfield, 1872). The name is herein emended as Dystactella
elongata (Clarke, 1907) n. comb.
Because Mazonomya n. gen. uniquely combines features of
both subfamilies of the Solemyidae, it ratifies Pojeta's inclusion of the clinopisthins among the solemyids:
1. Characters in Mazonomya n. gen. that are shared by
both solemyins and clinopisthins include:
(1) anterior elongation of the shell;
(2) variably developed radii;
(3) edentate hinge;
(4) primary ligament external, parivincular, set in
grooves supported by external nymphae; and
(5) thickened longidorsal margins marked by inner
lamelliform ridges that make grooves in internal
molds.
2. A character observed in Mazonomya n. gen. that is
limited to the clinopisthins is the relative prominence
of the beaks and umbones. In Mazonomya n. gen., as
in the clinopisthins Clinopistha and Dystactella, they
are more elevated and conspicuous than in typical
solemyins like Solemya and Acharax.
3. Four characters that exclude Mazonomya n. gen. from
the Clinopsithinae, but permit it to be reasonably
placed within the Solemyinae, include:
(1) the anteroventral expansion of the shell diagnostic
of Clinospisthinae is lacking; instead, the shell is
expanded anteriorly as in the Solemyinae;
(2) the shell has overhanging periostracal extensions
taking, in this case, the form of a tent-like span
between the anterodorsal margins and a short
prolongation extending behind the primary ligament;
(3) the small posterodorsal auricule is a feature seen in
some but not all solemyins;
(4) the ligament is functionally opisthodetic although
it is, broadly speaking, amphidetic owing to the
long anterior extension of the ALE, a character
not reported in the clinopisthins, but well-known
among solemyins, including Solemya (Zesolemya)
parkinsoni, S. (Austrosolemya) australis, Acharax
dalli, A. (Nacrosolemya) trapezoides (see Vokes,
1955; Pojeta, 1988; Carter, 1990), and an un-
published solemyid from the Devonian of Arctic
Canada (Prosh, 1988; Bailey & Prosh, 1990). A
comparable ALE is herein described for the first
time both in M. mazonensis n. gen., n. sp. and A.
radiata.
Mazonomya mazonensis n. gen., n. sp.
Pls 1-2
Solenomya [nomen vanum] radiata [not Solemya radiata Meek
& Worthen]. Johnson & Richardson, 1970; Richardson &
Johnson, 1971: 1234.
Solenomya [sic]. Thompson, 1979: 467.
"common bivalve." Thompson, 1979: 468, fig 4(C).
"undetermined bivalve." Johnson & Richardson, 1970, pl. 5, fig 6.
Edmondia [sp.]. Johnson & Richardson, 1966: 627; Dickson,
1974: 15, pl. 10-20; Baird, 1979: 61; Schram, 1979: 175-176;
Yochelson & Richardson, 1979: 324; Sroka, 1984: 119, pls 3235; Baird et al., 1986: 273, 278, 286; Baird, 1990: fig 2C; Baird
& Sroka, 1990: C27, C45.
Edmondia sp. Baird et al., 1985a: 264, figs 4.3, 6.7; 1986: 92, pl.
1, fig 4; Baird, 1997a: 23-24, figs 4A.2, 4A.3c); 1997c: 43, figs
5A.7A, 5A.8a.
[?] Edmondia splendens Elias, 1957: 745, pl. 91, fig 3.
Types.–Holotype, here designated: FMNH PE 35737;
Carbondale Formation, Francis Creek Shale (Middle
Pennsylvanian – Westphalian D), Peabody Coal Company
Pit 11, Will-Kankakee counties, Illinois. Paratypes, here designated, from the same location as the holotype except as
noted: FMNH PE 18152, PE 22311, PE 22321, PE 30261,
PE 30921, and PE 47723 (with burrow; label reads, "Will
County, Baird Locality 196"), NEIU MP 110 and MP 163.
Other paratypes, here designated (probably from Pit 11, but
labeled "Mazon Creek Area, Will-Kankakee-Grundy counties, Illinois"): FMNH PE 36000, PE 35631, PE 35633, PE
35810, PE 36009, PE 36058, PE 36069, PE 36133, WIU
MC 1120. One of the paratypes, FMNH PE 18152, was previously figured by Johnson & Richardson (1970: pl. 5, fig. 6)
as "undetermined bivalve." Because their photograph is the
earliest well-known figure of the new genus and species, the
specimen would seem an obvious candidate for the holotype.
It was not chosen because the diagnostically critical primary
ligament was evidently prepared away; the dental pick traces
marking this region are unmistakable.
Diagnosis.–By monotypy, same as Mazonomya n. gen.,
above.
Description.–Shells thin, medium sized, inequiaxial,
anteriorly elongate, equivalved, with comarginal growth
23
lines, lirae and sometimes variably spaced rugae. Valves distinctly solemyiform, uniformly elliptical in lateral profile.
Opsithogyrate; umbones small and fairly low, rising slightly
above dorsal margin. Longiaxis/breviaxis ratio ca. 2.0; aspect
(length/height) ratio ca. 1.8. Brevidorsum with low, gently
convex auricule sometimes nearly reaching umbonal height;
gently concave saddle lying between auricule and beak. Hinge
edentate; in each valve, inner surface of longidorsal margin
marked by thin, lamelliform ridge flanked by weak grooves,
beginning just in front of the beaks, continuing anteriorly to
ca. ⅓ of the longidorsal length, attenuating thereafter. Hinge
axis placed between brevidorsa. Ligament strong, amphidetic
but functionally opisthodetic; primary part brevidorsally
placed, transversely striated, dorsally arched, set in narrow
grooves supported by external nymphae; behind which lies
short prolongation of periostracum or prolongation of periostracum plus outer (lamellar) ligamental layer. Anterior
ligamental extension (ALE) attaching along aforementioned
lamelliform ridge, just inside longidorsal margin of each valve.
ALE overlain by periostracum marked by longitudinal striae
and transverse wrinkles, extending anteriorly ca. ⅓ of longidorsal length, terminating in slight step roughly coinciding
with end of most conspicuous portion of lamelliform ridge.
Weakly marked by narrow curvilinear flexures, with thin periostracum continuing anteriorly beyond ALE step as tentlike span between longidorsal margins, extending to just short
of anterior termini. Soft anatomy traces visible on internal
molds as pale stains, light tan in color. Valve interior weakly
marked by few concentric rugae and by faint, radial scattering of mantle muscle scars, especially along posteroventral
margin. Adductor muscles and pallial line weakly impressed,
indistinct. Pallial line entire, obscure, consisting of broad,
comarginal band marked by series of short radial mantle
muscle tracks (cf. orbicular muscles). Anterior adductor scar
large, obscure, dorsally placed, merging with adjoining pedal/
visceral muscle scars; resultant large anterior composite scar
(ACS) pyriform, obliquely tapering and ascending to vertex
in front of umbones. Near vertex, 1 or 2 pedal muscle scars
visible on holotype. Posterior adductors very small, obscure,
feebly impressed, subdorsally placed, continuous with pallial band, their position marked by slight expansion of pallial
band. Internal buttress lacking. Original shell microstructure
and mineralogy unknown. Shell outer microsurfaces smooth;
microfine tubercular ornament (seen in many edmondiids)
lacking.
Etymology.–Mazonensis, referring to the Mazon Creek
Konservat Lagerstätte of Will, Grundy, Kankakee, and
Livingston counties in northern Illinois.
Taphonomy.–Mazonomya mazonensis n. gen., n. sp. com-
24
monly occurs in siderite concretions; most are in a natural
death pose with conjoined valves widely opened, that is, "butterflied" (Pl. 1, Figs 1-4; Pl. 2, Figs 1, 3, 5-6, 8), convex-upward, due to postmortem relaxation of the adductor muscles
and still attached by the ligament and periostracum. Although
specimens are known with the valves closed, they are rare and
normally confined within preserved burrows. That roughly
90% of the shells are widely butterflied indicates that the vast
majority of the clams were not restricted within their burrows
at the time of death, suggesting synchronous burrow-escape
response after which they perished en masse, exposed at the
sediment surface. None of the hundreds of specimens studied show any suggestion of postmortem transport, predation,
or scavenging. Taken together, the evidence suggests episodic
mass mortality events in standing stagnant water. As such, lateral distribution of a Mazonomya horizon could be useful as a
time datum within the lower Francis Creek Shale.
Soft-part Preservation and Burrowing Traces.–Within the
dark red-brown concretions, soft-part traces are marked by
light-colored (tan) stains. Stained areas often reveal muscular traces of otherwise poorly impressed features, including
the adductors, visceral integument, and pallial line (see uncoated holotype, Pl. 1, Fig. 3). The various degrees of staining and surface textures along the anterior and posterior portions of the ligament are highly suggestive of periostracum
and periostracally covered outer (lamellar) ligamental layers
in Solemya and Acharax. The probable outer ligamental layer
that forms the ALE (Pl. 1, Figs 1-6; Pl. 2, Figs 1-3, 5-6) has
a rugose, shriveled appearance, distinctly marked by a pattern of longitudinal striae and transverse crinkles (Pl. 1, Figs
4-5). Bounded by a transverse step (Pl. 1, Figs 1-2, 6; Pl. 2,
Fig. 1, ST) in several specimens, a lighter colored patch (Pl.
1, Fig. 3) marked mostly by transverse curvilinear striae (Pl.
2, Fig. 3) extends nearly to the anterior terminus of the shell.
I interpret the anteriormost patch as a thin, tent-like periostracal span extending between the valve margins (Pl. 1, Figs
1-2, PEa), and the transverse step as the limit of the anterior
outer (lamellar) ligamental layer lying directly beneath it (cf.
Beedham & Owen, 1965: fig. 1b). Speculations that these
are residual traces of the mantle and viscera (e.g., Dickson,
1974) are rejected because the regularity of the crinkled texture within the patch (Pl. 1, Figs 4-5; Pl. 2, Figs 1-3, 5-6) is
suggestive of a uniformly thin, elastic layer of organic material spanning the longidorsal margins, perhaps consisting of
uncalcified integument or, more probably, the thin outermost
layer of periostracum (Beedham & Owen, 1965). A comparable tent-like span is herein described in A. radiata, in which
it is clearly continuous with and thus part of the overhanging
periostracal frill.
The posterior ligamental traces immediately behind the
beaks have the same light coloration of the anterior lamellar
layer and are also marked by transverse crinkles, but less so
than the anterior portion (Pl. 1, Fig. 3; Pl. 2, Figs 1-4). This
marks the location where the periostracum overlies both the
outer (lamellar) and inner (fibrous) layers of the primary ligament. The distinct step (Pl. 2, Figs 2, 4, ST) probably marks
the posterior limit of the ligament and a continuing prolongation of the periostracum directly behind it (Pl. 1, Fig. 2; Pl.
2, Figs 2, 4, PEp). It is equally possible that the prolongation
(PEp) included both the periostracum as well as a posterior
extension of the outer (lamellar) ligamental layer as in extant
solemyids (cf. Text-fig. 2).
Although the band-like pallial line and internal muscle
scars are usually indistinct, they sometimes show up very well
as pale stains (Pl. 1, Fig. 3). Radial tracks within the band
mark the attachment of bundles of muscles fibers (cf. "orbicular" muscles of Beedham & Owen, 1965) like those of Solemya
(Zesolemya) parkinsoni. Their presence is suggestive of extant
solemyins in which the muscles are used to quickly infold
fused ventral margins of the mantle at the same time that the
foot is being withdrawn (Liljedahl, 1984b). The composite
outline of the anterior adductor and adjoining visceral/pedal
muscle complex emulate the distinctive pyriform shape (Pl.
1, Fig. 2, ACS; Pl. 1, Fig. 3) characteristic of extant solemyins
such as S. (Solemya) togata and S. (Austrosolemya) australis (cf.
Cox, 1969: N242, figs B1, 2b, 3). The high placement of the
posterior adductor scars suggests accommodation of a large
hypobranchial gland and, below it, large gills, perhaps for
hosting chemoautotrophic bacteria.
Although most specimens are butterflied in their surface
death pose, occasionally one or another is found preserved
within a burrow. Fine, meniscate mud laminae can occasionally be seen (Pl. 2, Fig. 9, SM). Called "sitz marks" (Frey, 1968,
1971), these are sedimentary backfill structures left by the
foot. Their placement beyond the longiterminus of the shell is
instructive, identifying the foot as longiaxial in position and
thereby confirming the shell to be anteriorly elongate as in
solemyids rather than posteriorly elongate as in edmondiids.
There is no support for the drawing of Baird et al. (1986:
fig. 5; reprinted by Baird, 1990: fig. 2C; 1997c: fig. 5A.7;
Brett et al., 1997: fig. 1.2) that inaccurately places sitz marks
in this bivalve beyond the breviterminus. The sitz marks of
Mazonomya mazonensis n. gen., n. sp., in Pl. 2, Fig. 9, show
the foot to have been relatively large with a slightly concave
plantar sole; the principal thrusts of the foot were directed
parallel to the anteroposterior axis, as in extant Solemya.
Lamellar Ridges.–The lamellar ridges, lying just inside the
thickened and edentate anterodorsal margins of the valves
in Mazonomya mazonensis n. gen., n. sp., favorably compare
with ridges seen in Acharax radiata n. comb. herein. They
likewise compare in form with the single ridge found in the
right valve of both A. (Nacrosolemya) trapezoides and extant
Solemya (Zesolemya) parkinsoni and S. (Austrosolemya) australis
in which it serves as a nymph-like analog for the attachment
of the abapical portion of the ALE (Carter, 1990: 175; Carter
et al., 1990: 317). The ridges in M. mazonensis n. gen., n. sp.,
might have served a similar function. However, because the
anterior ends of the ridges (Pl. 1, Fig. 6, LR) fall just short
of the step (Pl. 1, Fig 6, ST) that ostensibly marks the abapical limit of the ALE, P. A. Johnston (pers. comm., 2010) argued that the ridges might have alternatively functioned as
attachment sites for visceral suspensor tissue as in Goniophora
Phillips, 1848 (Johnston, 1993: 79). Nonetheless, it must be
emphasized that: (1) ridges associated with ALE attachment
have been previously documented in other living and fossil
solemyids (Carter, 1990); and (2) the ridges in Pl. 1, Fig. 6,
are mere fragments of the originals, which evidently continued well forward while gradually decreasing in relief (see Pl.
2, Fig. 8, LR).
The ridges also invite comparison to those seen in internal molds of the solemyid Clinopistha (see Pojeta, 1988: pl.
12, figs 1-4, 7), in which they are shorter and closer to the
valve margins. The similarity in strength and placement of
the ridges in the left and right valves (Pl. 1, Fig. 6; Pl. 2, Fig.
8, LR) suggests that, unlike in Acharax (Nacrosolemya) trapezoides and Solemya (Zesolemya) parkinsoni, ALE attachment
was essentially symmetrical in the left and right valves.
Occurrence.–Mazonomya mazonensis n. gen., n. sp. is the
most common bivalve species of the Essex biofaces of the
Francis Creek Shale in the Illinois counties of Grundy, Will,
Livingston, and Kankakee and elsewhere (Baird, 1979; Baird
& Anderson, 1997). Whereas ca. 7.2% of the fossils collected
from the Essex are bivalves, M. mazonensis n. gen., n. sp. alone
accounts for as high as 75.9% of the bivalve fraction, ca. 9.2%
of the invertebrate fraction, and ca. 5.5% of all Essex fossils
(Baird & Anderson, 1997: table 4B.2; see also Baird, 1979;
Sroka, 1984; Bailey & Sroka, 1997). It is endemic to Illinois
Essex localities except for one outside occurrence in an unnamed sideritic concretionary shale member containing several other Essex-type fossils and overlying the Croweburg Coal
near Windsor, Henry County, Missouri (Baird et al., 1985b:
92). Its occurrence above the Croweburg Coal, which is elsewhere distributed across a broad region of Kansas, Missouri,
and Oklahoma, supports the long-held contention that the
Croweburg and Colchester coals are stratigraphically equivalent (Wanless & Weller, 1932; Wanless, 1975) and represent
a continuous peat mire across these states exceeding 200,000
km2, the largest in Earth history (Wanless & Wright, 1978;
Greb et al., 2003). The observed endemism of M. mazonensis
n. gen., n. sp. is not altogether unexpected inasmuch as up
25
to 80% of Holocene protobranch species occurring in a basin can be endemic to that particular basin (Allen & Sanders,
1996; Zardus, 2002). Among post-Paleozoic benthic invertebrates, the dynamics of disturbed or metastable onshore tropical environments analogous to the Essex have been linked to
the origination of new taxa (Jablonski, 2005).
Occurrences of Mazonomya mazonensis n. gen., n. sp. have
been used by previous authors to mark salinity boundaries.
Johnson & Richardson (1966) posited there to be a gradual
salinity gradient between the freshwater Braidwood biofaces
and its alleged marine equivalent, the Essex. Richardson &
Johnson (1971: 1234) postulated that its occurrence "in a
band from Ottawa [Illinois] to the northern half of Pit Eleven"
marks the transition between the two environments. This
distribution was supported by Dickson (1974), who judged
M. mazonensis n. gen., n. sp. to be euryhaline, noting important occurrences in both the Essex and Braidwood biofacies.
However, Baird & Sroka (1990) considered the Braidwood
and Essex to have been separated by a relatively sharp salinity
gradient. They regarded M. mazonensis n. gen., n. sp. to be
confined to the Essex and did not mention it in connection
with mixed Braidwood-Essex assemblages.
Comparisons.–"Edmondia sp." (NEIU MCP 249), figured
by Baird et al. (1985b: pl. 1, fig. 4 ) from an unnamed shale
above the Croweburg Coal (Pennsylvanian, Henry County,
Missouri) is identical to Mazonomya mazonensis n. gen., n.
sp.
Except for the somewhat more prominent beaks and umbones, the long elliptical shells of Mazonomya mazonensis n.
gen., n. sp. recall other solemyids, especially Dystactella and
Psiloconcha. The smoothly curving posterodorsal auricule and
rounded saddle are familiarly solemyid in aspect, recalling figured examples of Acharax, Psiloconcha, Dystactella, and especially Solemya. The relatively high placement of the posterior
adductor scar recalls Psiloconcha as figured by Pojeta (1988: pl.
17, fig. 1). Besides being commonly confused with the pholadomyoid Edmondia, this species has often been mistaken for
the Pennsylvanian solemyid, "Solemya" radiata, redescribed
below as A. radiata n. comb.
Edmondia splendens Elias, 1957, from the Late Mississippian Redoak Hollow Formation of southern Oklahoma closely resembles Mazonomya mazonensis n. gen., n. sp. in shell
size, shape, and hinge morphology. However, Elias' shell is
slightly taller and the beak is placed closer to the breviterminus (longiaxis/breviaxis ratio ca. 4.0). Unfortunately, E. splendens was based solely on a single valve, now lost. Elias compared his species to one of Hind's (1897: pl. 11, figs 30, 30a)
British Carboniferous specimens of Parallelodon haimeanus
(de Koninck, 1851), noting similarities of the shell shape,
sculpture, and beak position. However, in fact, the specimen
26
in question, from Trey Cliff, Castleton, Derbyshire, shows few
similarities; it is more egg-shaped than elliptical in lateral profile, and it has a ventral sinus, lacking in Elias' specimen.
Both Dickson (1974) and Sroka (1984) believed the
Mazon Creek species to be new. Dickson (1974: 21) compared
it to Edmondia? reflexa Meek, 1872, from the Pennsylvanian
of Nebraska. The types of E. reflexa (USNM collections), in
which hinge data are lacking, were examined by Dickson
(1974) who concluded that, whereas the shells are superficially similar, the brevidorsal margin is noticeably flatter than
in the Mazon Creek species. Elias (1957: 745) compared E.
reflexa to both E. splendens and E. ovata. However, Elias dismissed the apparent similarity of E. ovata as coincidental. The
shell form of E. ovata in Meek & Worthen's (1873: pl. 26, fig.
13) figure is much shorter than in Mazonomya mazonensis n.
gen., n. sp. Although Meek (1874a: 580) was uncertain of the
taxonomic status of E. ovata, it is clearly an edmondiid (see
Bailey & Sroka, 1997: 111, figs 8A.5G-H) with strong similarities to both E. aspinwallensis Meek, 1871, and E. oblonga
M'Coy, 1855 (see Runnegar & Newell, 1974: figs 3-4).
Genus Acharax Dall, 1908[a]
Type Species.–By original designation, Solemya johnsoni
Dall, 1891. Holocene, NE Pacific.
Diagnosis.–"Solemyins having the main ligament external, set in elongate longitudinal grooves and supported by
nymphs; buttresses anterior to the posterior adductor muscle
are absent or highly reduced" (Pojeta, 1988: 214).
Discussion.–Overall shell morphologies of Acharax and
Solemya are strikingly similar, but in Acharax, the parivincular
primary ligament and nymphae are external rather than internal. Despite the evident similarities, analysis of 18S rRNA
gene sequences indicates extant Acharax to be phylogenetically more distant from Solemya than the morphology would
suggest (Neulinger et al., 2006). Among Paleozoic solemyins,
an external parivincular ligament similar to that in Acharax
is a persistent trait. Internalization of the ligament seen in
Solemya thus appears to be a post-Paleozoic apomorphic condition. A possible, although doubtful, exception is "Janeia"
truncata (Goldfuss, 1840) (Middle Devonian, Germany). A
transverse section through the beak shows an L-shaped ossicle alleged to be a chondrophore supporting an internal ligament (Quenstedt, 1930: pl. 1, fig. 4; Cox, 1969: fig. B1, 1b).
However, the strong angle and upright prolongation of the
ossicle, lacking in the chondrophore of Quenstedt's (1930:
pl. 1, fig. 5) transverse section of S. (Zesolemya) parkinsoni,
are consistent with the erect form of a nymph supporting an
external, parivincular ligament (cf. Beede & Rogers, 1899: pl.
34, fig. 2b; Carter, 1990: fig. 17D).
Dall's (1908a: 2) original characterization of the ligament
of Acharax as "opisthodetic, wholly external" was subsequently emended by Vokes (1955: 540) who described the ligamental condition as amphidetic in the large species A. dalli
from the Keasey Formation (upper Eocene-lower Oligocene,
Oregon). In addition to the prominent external opisthodetic
ligament, there is also a considerable anterior extension of the
ligament (ALE) that is at least partly internal in its attachment
(Vokes, 1955). However, in a well-preserved specimen of A.
dalli from the Whiskey Creek methane-seep deposits(Eocene,
Washington State), the conspicuous ALE appears to be largely
external, beginning just in front of the umbones and, as in
Solemya (Solemya), S. (Zesolemya), S. (Austrosolemya), and S.
(Solemyarina), continuing nearly the full length of the anterodorsal margins (Goedert et al., 2003: pl. 42, fig. 5). A comparable ligamental arrangement was described by Carter (1990)
in A. (Nacrosolemya) trapezoides and is herein described in A.
radiata n. comb.
The reduction or absence of internal buttresses seen in
the type species, Acharax johnsoni, is by no means universal
in Acharax; neither is it a character useful in distinguishing
it from all Solemya. Although buttresses are developed in S.
(Petrasma), S. (Solemyarina), S. (Zesolemya), and especially in
S. (Austrosolemya), they are no better developed in the type
species of Solemya, i.e., S. (S.) togata, than they are in A. johnsoni (Text-fig. 2; Table 3). In fossil Acharax, buttresses are
variably developed from species to species and, in some cases,
from individual to individual within the same species. For
example, Vokes (1955) observed that A. dalli has a well-developed internal buttress very similar to that of S. (Zesolemya)
parkinsoni.
Ligamental Demipads.–Internal molds of both species of
Acharax treated herein show the scar of a small internal ligamental pad located at the adapical (subumbonal) end of the
ALE and limited to the right valve. A similar scar, confined
to the left valve, has been observed in internal molds of an
unpublished probable Acharax from the Lower Devonian of
Young Island, Nunavut, Arctic Canada (Prosh, 1988; Bailey
& Prosh, 1990), in which, marked by radial crenulations and
striae, it appears as a low cupola signifying a shallow, chondrophore-like depression on the internal surface of the left
umbonal summit (Pl. 3, Fig. 7). A similar structure also appears in "Solemya sp." from the Pennsylvanian of Kansas, in
which it is, once again, restricted to the right valve (Pl. 3, Fig.
8). The crenulations clearly are not true hinge teeth – they are
too weak and distant from the hinge line, and they continue
as striae beyond the limits of the depression into the valve
interior where many of them appear to visually merge with
radial mantle muscle tracks. Such structures are not typical of
Text-fig. 4. Vertical sections through the umbonal region in two
Paleozoic solemyins, showing differences in asymmetrical attachment of the proximal part of the anterior ligamental extension (ALE)
(lamellar outer layer); in both cases, the ALE attaches to the outer
edge of the left valve (LV): (1) "Janeia" silurica, showing external attachment of ALE in a groove of a nymph-like ossicle (OS) attached
outside umbo of right valve (RV) (after Liljedahl, 1984b: text-fig.
3). (2) Acharax radiata, showing internal attachment of ALE as a
demipad (DP) occupying a shallow, chondrophore-like depression
beneath umbo of right valve.
chondrophores because they are placed in front of the beaks
and limited to only one valve.
In internal molds of both Acharax radiata and the Young
Island solemyins, the outline of the cupola is almond-shaped,
gradually tapering forward and merging with the narrow trace
of the ALE. This suggests that the calcified part of the right
valve had a shallow, chondrophore-like recess supporting an
internal pad-like thickening of the adapical (subumbonal)
component of the ALE. Dubbed a ligamental demipad, it was
first reported by Bailey & Prosh (1990). In placement, it coincides with the prominent internal lobes that characterize the
adapical part of the ALE in extant Solemya (Solemyarina), S.
(Zesolemya), and S. (Austrosolemya) (Text-figs 2.4-2.6, Table 3;
see also Cox, 1969: fig. B1, 2b; Bernard, 1980: fig. 1; Pojeta,
1988: 202-203, pl. 3, fig. 4, pl. 4, figs 2-3; Waller, 1990: pl. 1,
fig. E), as well as the lesser, irregular expansions of the adapical
ALE in S. (Petrasma) velum (Text-fig. 2.3; Waller, 1990: pl. 1,
fig. B). In Young Island solemyins, the posterior part of the
demipad is radially incised by the internal buttress. Each of
the internal pads of extant S. (Austrosolemya) australis is similarly incised by the anterior confluence of the internal nymph
and buttress (Cox, 1969: fig. B1, 2b; Taylor et al., 2008: fig.
1D; Kamenev, 2009: fig. 5). However, the pads of S. (A.) australis are symmetrically developed in both the left and right
valves, whereas demipads are much smaller, narrower, and
asymmetrical, being present in one valve but missing in the
other. It seems probable that this asymmetry, as well as the
overlap of the valves, is related to the asymmetry of attachment of the ALE described by Carter (1990).
In "Janeia" silurica, Liljedahl (1984a: figs 14h-i; 1984b:
text-fig. 1.4) described a small chondrophore-like ossicle externally attached to the beak of the right valve but missing in
27
the left valve. The ossicle, which has the form of a miniature
nymph, occupies an apical location similar to the demipad.
Because the primary ligament is parivincular, attaching at external nymphae behind the beaks (Liljedahl, 1984a: fig. 14c;
1984b: text-fig 1.1), the ossicle was interpreted to function in
the asymmetrical attachment of an ALE that extended from
an insertional groove in the ossicle of the right valve to an attachment site along the dorsal edge of the left valve. Liljedahl
(1984b: text-fig. 3) attributed the asymmetry to be caused by
the natural dorsal overlap of the left valve. The demipad in A.
radiata n. comb. and ossicle in "J." silurica could be plausibly
judged to be functionally analogous structures for asymmetrical attachment of the right valve, the former serving to attach
it internally, and the latter externally (Text-fig. 4).
Stratigraphic Range.–?Lower Devonian-Holocene. Pojeta
(1988: 214) gave the range of Acharax as Permian (Leonardian)
to Holocene, with possible occurrences in the Middle Devonian
(Eifelian) and Pennsylvanian (Atokan). Carter (1990) and
Carter et al. (1990) confirmed A. (Nacrosolemya) in the
Atokan Magoffin Member (Breathitt Formation), southeastern Kentucky. Carboniferous solemyins from Europe, including Solemya [Janeia] primaeva, S. costellata M'Coy, 1844, S.
excisa de Koninck, 1885, S. saginata Ryckholt, 1853 [1854],
and S. puzosiana de Koninck, 1842, and unpublished solemyins from the Lower Devonian (Emsian) Disappointment
Bay Formation of the Canadian Arctic (Prosh, 1988; Bailey
& Prosh, 1990) are all probable Acharax.
Acharax radiata (Meek & Worthen, 1860) n. comb.
Pl. 3, Figs 1-6, 9; Pl. 4, Figs 1-7
Solemya [also, Solenomya, nomen vanum] radiata Meek & Worthen,
1860: 457; Meek & Worthen, 1866: 349, pl. 26, figs 10a-b;
Miller, 1877: 284; 1889: 512; Beede, 1900: 160, pl. 22, figs 5,
5a; Mark, 1912: 264, etc.; Price, 1918: 784, etc.; Morningstar,
1922: 193; Chow, 195: 12; Lintz, 1958: 100; Hoare, 1961: 94, pl. 12, fig. 12; Hoare et al., 1979: 29, pl. 2, figs 12-13; Pojeta,
1988: 215, pl. 22, figs 1-7, 10-11, pl. 23, figs 1-4; Kues, 1992a:
91, figs 2.7-2.8; Cope, 1997, pl. 1, fig. 10.
[?] Solenomya [nomen vanum] costellata (M'Coy, 1844). Hind, 1900:
442, pl. 50, figs 8-10.
Remarks.–Beginning with Children (1823), the widely
used archaic spelling, Solenomya [nomen vanum] and other
variations departed from Lamarck's (1818: 488) original
spelling, Solemya. As noted by Miller (1877: 204), Lamarck's
orthography was first conserved by Menke (1828: 72).
Solemya radiata is herein reassigned to Acharax. Dall
(1908b: 366) first noted the ligament and hinge similarities
of extant Acharax and Pennsylvanian S. radiata, but hesitated
28
to ally the two because of the apparent lack in the fossil species of shell gapes and lack of a frill of periostracum extending
beyond the calcified margins of the valves. However, besides
confirming ligament and hinge similarities, the remarkable
Essex preservation reveals the presence of a periostracal frill
very similar to that in extant Acharax. Although unequivocal shell gapes were not observed in Essex examples, gapes
are nonetheless evident in other closely related Carboniferous
species (see below). Moreover, the functional significance of
the frill in extant solemyins argues that shell gapes in Acharax
radiata were probable.
Diagnosis.–Small to medium-sized solemyins with anteriorly elongated narrow shells radially marked by weak,
flattened plicae and striae. Internal buttress weak. Primary
ligament opisthodetic, parivincular, set in grooves supported
by external nymphae; anterior ligamental extension asymmetrical, beginning with small demipad on inner subapical
surface of right valve, then narrowing anteriorly, attaching at
longitudinal lamellar ridges and grooves along anterodorsal
valve margins; left valve often dorsally overlapping right valve.
Periostracal frill along anterior and ventral margins and between anterodorsal margins. Dorsal and ventral margins subparallel as in Solemya parallela Beede & Rogers, 1899, but
more elliptically rounded in profile.
Materials.–Originally described from "Coal Measures,"
Grayville, Illinois (Meek & Worthen, 1860: 457). Materials
herein from the Carbondale Formation, Francis Creek
Shale (Middle Pennsylvanian-Westphalian D) Peabody Coal
Company Pit 11, Will-Kankakee counties, Illinois, except
as noted. Partial listing: FMNH PE 10984, PE 20958, PE
22329, PE 25419, PE 29875 (labeled Pit, 11, Will County),
PE 39388, PE 52187; NEIU MCP 162; ISM lot 2994 (quillpenned label reads "Coal Measures, Fulton County, Illinois"; a
second but older label adds "Fulton and Schuyler counties").
Description.–Shell small to medium-sized, inequiaxial,
anteriorly elongate, laterally compressed, subequivalved; left
valve margins often dorsally overlapping those of right valve.
Strong anterior elongation with longiaxis/breviaxis ratio 4-5;
aspect (length/height) ratio ca. 2.0. Beaks weak, opisthogyrate; umbones low, subdued. Shell profile subelliptical, anteriorly expanding; dorsal and ventral margins subparallel.
Longidorsum straight to weakly convex; brevidorsal margin
oblique, short, gently concave; ventral margin either straight
or gently convex to weakly concave. Valves very thin, smooth,
marked on outer (and inner) surfaces by faint, comarginal
growth lines, and sometimes even fainter comarginal rugae;
radial plicae of very low relief, flattened, rather regularly clustered anteriorly and posteriorly but separated by gaps (here-
in dubbed diastemata) behind middle of valves. Inner shell
surface faintly marked by few obsolescent comarginal rugae,
but abundantly marked by faint radial striae with stippled appearance. Hinge axis and primary ligament placed between
brevidorsa. Primary ligament external, parivincular (opisthodetic, dorsally arched), deltoid in dorsal outline, supported
by small nymphae, inserting at short, longitudinal grooves.
Hinge edentate; anterodorsal margins with weak lamellar
ridges. Anisomyarian; internal musculature feebly marked;
anterior adductor scar large, faint, more or less bilobate in
outline, expanding dorsally, visually merging with adjoining
visceral/pedal muscle complex to form large composite scar
(ACS), pyriform in shape, tapering radially to vertex well in
front of umbones. Posterior adductor scar small, subdorsally
placed, subtrigonal in outline, bordered anteriorly by weak
buttress that is best defined dorsally. Pallial line simple, obscure, consisting of broad comarginal band marked by series
of short, radial tracks (cf. orbicular muscles); band widening
anteriorly as it approaches anterior adductor scar, narrowing
posteriorly, merging with posterior adductor scar. In each
valve, periostracum projecting beyond shell margins forming short, wrinkled, frill-like selvage, especially along ventral
margin and anterior terminus, merging between longidorsa
to form tent-like span. Surface texture of periostracal span
smooth, with anterior margin gently embayed; at posterior
limit lies weak, transverse, step-like demarcation where periostracal span overlies anterior ligamental extension (ALE) as
it continues toward beaks. Behind demarcation, where weakly
marked by longitudinal striae, ALE laterally attaches at longitudinal groove and lamellar ridge running along length of
inner dorsal margin of each valve. Approaching beaks, ALE
asymmetrically forms internal, almond-shaped demipad scar
in right valve only.
Discussion.–The primary ligament, herein shown to be
parivincular and external (PW-external category of Carter et
al., 2000), requires this species to be reassigned from Solemya
to Acharax. The weak lamellar ridges along the longidorsum
of A. radiata are similar in position but lower in relief than
those of Mazonomya mazonensis n. gen., n. sp. The restriction
of both the demipad and trace of the ALE to the right valve
(Pl. 3, Figs 5-6), coupled with the dorsal overlap of the left
valve, implies probable asymmetry in the left-right valve attachment comparable to that described by Carter (1990) in
A. (Nacrosolemya) trapezoides (see below).
The weak step (Pl. 3, Fig. 1; Pl. 4, Fig. 1, ST), possibly
marking the boundary between the anterior end of the ALE
and the overlying periostracal span in Acharax radiata, is also
evident in Holocene A. johnsoni (see Kamenev, 2009: fig.
103).
Two of three diagnostic criteria, the PW-external structure
of the primary ligament and the asymmetry of the ALE, argue for transferal of A. radiata to subgenus A. (Nacrosolemya).
However, the third and arguably most important criterion,
shell microstructure, is presently unavailable for study.
As in extant solemyins, the posterior adductor is subdorsally placed, but its slightly larger relative size coupled with
the more obliquely sloping posterodorsal margin are suggestive of a slightly reduced volume in the posterior part of the
mantle cavity for the gills and hypobranchial gland. As in extant species, the exhalant aperture of the mantle was probably
located beneath the posterior adductor.
Although the medially placed diastemata in the radial plicae were emphasized by Meek & Worthen (1860: 457; 1866:
349) as diagnostically unique characters, similar diastemata
are observed in plical arrangements of many extant solemyids, including Solemya reidi, S. (Austrosolemya) australis, S.
(Petrasma) valvulus, A. johnsoni, and A. japonica Dunker,
1882.
The radial tracks composing the obscure band composing
the pallial line are probably scars left by small bundles of muscle fibers ("orbicular" muscles of Beedham & Owen, 1965)
that extend from the pallial line to the mantle margin and beyond to the flexible frill of periostracum where they function
to fold the frill between the valve margins when the adductors are contracted (Yonge, 1939; Beedham & Owen, 1965).
Radial orbicular muscle tracks can be taken as evidence for
fused mantle lobes (Liljedahl, 1984a, b).
Preservation of Ligament, Periostracum, and Burrowing
Traces.–The exquisite preservation of the Essex examples significantly add to data on this species supplied in the redescriptions of other authors, including Hoare et al. (1979), Pojeta
(1988), and Kues (1992a). Several features have not been previously described:
1. preserved external (primary) ligament that is parivincular wide (PW-external of Carter et al., 2000) in
character;
2. the pallial line and adductor musculature (but partially visible in the illustration by Pojeta (1988: pl.
23, figs 1-4); and
3. moldic facsimiles of both the ALE and the frilled periostracum.
As in extant solemyins, the periostracum shows natural
stress cracks; on two specimens, a comarginal arc of cracked
periostracum (Pl. 3, Fig. 2; Pl. 4, Fig. 5, PC) on the outside
of the valve corresponds to stress points exerted by the radial
(orbicular) muscles that form the pallial line on the inside. As
in extant Solemya and Acharax, there was a conspicuous frill
of periostracum extending as a selvage along the ventral and
anterior margins (Pl. 3, Figs 1-3; Pl. 4, Figs 1-2, 5, PF) and
forming an embayed span between the anterodorsal margins
29
of the valves (Pl. 3, Figs 1-2; Pl. 4, Figs 1-2, 4, 6, PE). On
composite molds and casts, the narrow line marking comarginal attachment of the overhanging frill to the shell (Pl. 3,
Fig. 7.3; Pl. 4, Fig. 5, PFL) can be easily confused with a
pallial line.
Although overhanging frills (i.e., extensions, fringes, or
lappets of various authors) of periostracum are distinctive features of living Solemya and Acharax (hence the common name,
"awning shells"), this is the first direct evidence that frills were
present among solemyins as early as the Pennsylvanian. As
discussed above, the frill is part of a unique suite of characters
associated with sulfide pumping, possibly serving to anchor
and seal the anterior shell margin during pumping movements (Seilacher, 1990). If so, the presence of the frill strongly
suggests that A. radiata, like extant solemyins, hosted chemosymbionts in their gills and probably had a reduced gut.
Like Mazonomya mazonensis n. gen., n. sp., specimens of
Acharax radiata are occasionally preserved within portions of
the burrow. The forms of the burrows are short and simple
arcuate tubes (Pl. 4, Fig. 7), but none is complete enough to
determine if the burrow had the Y- or U-shape of other solemyids. Backfill ("sitz") marks have not been observed.
Richardson & Johnson (1971) suggested that variegated
staining indicates preservation of a color pattern radiating
from the beak. However, no specimens examined in the preparation of this study show convincing indications of color,
and the whereabouts of the specimen(s) cited by these authors
is unknown. The alleged color pattern probably arises not so
much by differences in color as differences as in the thickness
of the original periostracum. Beedham & Owen (1965) noted
that in the periostracum of extant Solemya (Zesolemya) parkinsoni, the radial plicae, representing narrow pleats of the periostracum, consist only of the thin, outermost layer, whereas
the interplicate periostracum becomes much thicker with
the addition of bulky inner layers. In many living solemyids,
these differences visually result in the characteristic alternating pattern of light brown pleats, at which the periostracum
is thinnest, with the dark brown interplicae at which the periostracum is thickest. Preservation of shading differences in
the Essex specimens of Richardson & Johnson (1971) thus
record traces of an original periostracal structure in Acharax
radiata fundamentally similar to that of living solemyids.
Although they secondarily become overprinted onto the thin
calcified shell as plicae, the distinctive radial pleats represent
a pattern of divisions within the periostracal frill that function to enhance its flexibility. Even though frill preservation is
rare, the occurrence of radial plicae on the shell of any fossil
solemyid can thereby be taken as evidence that a frill of periostracum was originally present.
Comparisons.–Acharax radiata occurs at Essex localities
30
but is much less common than Mazonomya n. gen. and is
distinguished by its lateral profile and radial ornament. Essex
examples of A. radiata are similar to specimens from the
Pennsylvanian midcontinent called Solemya radiata by Pojeta
(1988: pl. 22, figs 1-7, 10-11, pl. 23, figs 1-4) and Hoare et al.
(1979: pl. 2, figs 12-13[?]), but two of these (Pojeta, 1988: pl.
23, figs 2-4, 6) are referable to A. (Nacrosolemya) trapezoides
(see below). Pojeta's figures show left valves dorsally overlapping right valves. Conjoined valves of Essex and other specimens studied herein show similar overlap.
Hind (1900: 443) unhesitatingly regarded Acharax radiata as a junior synonym of Solemya costellata from the
Carboniferous of England and Northern Ireland. Whereas
the size, profile, and ornament are all in close agreement
(see especially Hind, 1900: pl. 50, figs 8-10; but his fig. 7
resembles A. (Nacrosolemya) trapezoides described below), the
internal buttress of S. costellata, described by Hind as "wellmarked," seems more prominent than in A. radiata, and plical
diastemata are not apparent; the ligament of S. costellata is
unknown. Thus, conspecificity cannot confidently be demonstrated, and M'Coy's (1844: pl. 8, fig. 5, as Sanguinolites
costellatus) figured original from the Carboniferous of Ireland,
is only fragmentary and otherwise lacking in useful data.
A series of other British shells assigned by Hind (1900: 310,
pl. 35, figs 2-4) to Edmondia arcuata (Phillips, 1836), from
the Lower Carboniferous (Dinantian) Redesdale Ironstone of
Northumberland are, in reality, solemyids (probably Acharax)
somewhat resembling A. radiata in their size, oval profile,
small internal buttress, and large elevated anterior adductor.
Yet, they probably represent a separate species because of their
conspicuous comarginal rugae, small brevidorsal auricule and
weaker radii. The occurrence of solemyids in the Redesdale
Ironstone is particularly interesting, because it is a sideritic,
concretionary shale overlying a thin coal, stratigraphically
analogous to the Francis Creek Shale in Illinois.
Solemya parallela Beede & Rogers, 1899, has been reported from roof shales above coal seams in several Pennsylvanian
cyclothems (Wanless, 1958), although I have not seen
it within the Essex. It must not be confused with a senior
homonym S. parallela Ryckholt, 1853 [1854] from the Early
Carboniferous (Tournaisian) of Belgium. The latter (probably
Acharax) is a very small and distinctive solemyin with a parivincular external ligament, dorsal and ventral margins that are
parallel, a very narrow cross section, and ornament lacking
radii (de Koninck, 1885: pl. 23, figs 35, 37). The former has
similar dorsal and ventral margins, but, unlike Ryckholt's
species, has much weaker umbones and is externally well
marked with radial plicae lacking diastemata. It now seems
probable that S. parallela Beede & Rogers is conspecific with
S. [Janeia] primaeva Phillips, 1836, sensu Hind (1900) from
the Carboniferous of northern England, Scotland, and Ulster.
Although Phillips' (1836: 209, pl. 5, fig 6) meager description and and rough figure of the Northumberland type (now
lost) of S. primaeva are not instructive, the materials of Hind
(1900: 438, pl. 50, figs 1-6) are essentially indistinguishable
from the Kansas specimen, showing:
1. similar lateral profile;
2. comparable aspect ratio and longiaxis/breviaxis
ratio;
3. elevated adductor muscles that are nearly isomyarian;
4. a narrow but conspicuous internal buttress; and
5. narrow radial plicae without diastemata.
The parivincular external ligament in Hind's (1900) fig. 1 justifies transfer of S. primaeva to the genus Acharax. Thus, S.
parallela Beede & Rogers becomes a probable junior synonym
of Acharax primaeva (Phillips, 1836) n. comb. Zhang & Pojeta
(1986: fig. 5.14) reported S. parallela Beede & Rogers from
the coal-bearing Ceshui Formation (Lower Carboniferous)
of China. Interestingly, another Chinese specimen from the
same strata called S. (Janeia) primaeva (Phillips) by Zhang &
Pojeta (1986: fig. 5.4) strongly resembles the original specimen of S. parallela Beede & Rogers.
Because of their similar size, well-defined radii and narrowly elongate profiles, Acharax radiata and A. primaeva are
easily confused. Although the longiaxis/breviaxis ratios are
fairly comparable (4-5), the aspect ratio (length/height) is ca.
3.0 in the specimen of Beede & Rogers, but averages only ca.
2.0 in eight specimens of A. radiata herein. Unlike that of A.
primaeva, the profile of A. radiata is more smoothly elliptical
in shape with weakly convex dorsal and ventral margins, the
internal buttress is not as conspicuous, and most specimens I
have seen have plical diastemata.
Acharax radiata also compares with two solemyin species
(probable Acharax), S. saginata and S. puzosiana from the
Lower Carboniferous (Tournaisian) of Belgium. In the figures
of de Koninck (1885: pl. 23, figs 31-32), S. saginata is distinguished by the lack of plical diastemata, and the less extreme
placement of the umbones (longiaxis/breviaxis averaging 2.7
in two specimens). Solemya puzosiana (de Koninck, 1885: pl.
23, figs 29, 33-34, 41) seems composed of more than one
species:
1. de Koninck's fig. 29 has ratios comparable to A. radiata but lacks radii;
2. the conjoined specimen (his figs 33-34) shows leftover-right dorsal overlap of the valves, nymphs indicative of a parivincular external ligament, a possible
(?) posterodorsal gape, lower longiaxis/breviaxis ratio
than A. radiata, and weak radii; and
3. his fig. 41 has a parivincular external ligament, fairly
conspicuous radial plicae like A. radiata, but the umbones are more prominent and, like the specimen in
his figs 33-34, less extreme in their posterior place-
ment (longidorsum/brevidorsum ca. 2.9).
Subgenus Nacrosolemya Carter, 1990
Type Species.–By original designation of Carter (1990:
174), Solemya trapezoides Meek, 1874a (Upper Carboniferous,
Illinois).
Diagnosis.–"Shell as in Acharax s. s., with a posterior, external parivincular ligament [PW-external of Carter et al., 2000]
supported by nymphae, and an elongate, anterior, asymmetrical, external to submarginal, lamellar ligament, but with nacreous as well as porcelaneous structure in the middle and
inner shell layers, and with a slightly stronger (but still weak)
posterior, radial internal buttress" (Carter, 1990: 174).
Remarks.–Both Acharax (Nacrosolemya) and Acharax s. s.
have aragonitic shells with an outer prismatic layer, but unlike
in A. (Nacrosolemya), the inner shell layers of Acharax s. s. are
non-nacreous. In addition, internal buttresses of Acharax s. s.
are either absent or very weak (Carter, 1990).
Discussion.–Although Carter (1990) designated Solemya
trapezoides as the type species, his diagnosis of A. (Nacrosolemya)
is based on three nontopotypic specimens from the Magoffin
Member, Breathitt Formation of eastern Kentucky. One
relatively complete specimen (Carter, 1990: fig. 17C, UNC
13659b) is the right size (large, ca. 7 cm in length; J. G.
Carter, pers. comm., 2007), and strongly resembles Meek's
original (the lectotype; see below) as well as other published
examples, aside from a concave longidorsum and dilation of
the anterior extremity which, Carter (pers. comm., 2007) asserted, are taphonomic artifacts resulting from sedimentary
compaction.
In the Magoffin specimens, Carter (1990) noted a marked
asymmetry in the attachment of the anterior extension of
the outer (lamellar) ligamental layer (ALE) that attaches the
longidorsa together in front of the umbones. In the right
valve, the abapical part of the ALE attaches to a longitudinal lamellar ridge lying just beneath the longidorsal margin,
whereas, in the left valve, it attaches to the valve edge, and the
ridge is absent. He compared this asymmetry to that of extant
Solemya (Zesolemya) parkinsoni and S. (Austrosolemya) australis
but noted that, unlike in these two species, the adapical part
of the ALE in the Magoffin specimens does not extend internally and laterally below the beaks to form subumbonal lobes
or pads (see Pojeta, 1988: pl. 3, fig. 4). However, Carter mentioned crenulations just beneath the beak in at least one of his
specimens. A crenulated area, interpreted as a chondrophore
by Hoare et al. (1979: 30), is also present in the lectotype of
Acharax (Nacrosolemya) trapezoides. Study herein suggests that
31
the crenulated area is not a chondrophore in the conventional
sense, but a striated attachment scar of a small internal ligamental demipad that forms at the adapical extremity of the
ALE (see below).
Acharax (Nacrosolemya) trapezoides (Meek, 1874a).
Text-fig. 3; Pl. 5, Figs 3, 1-6
Solenomya [nomen vanum] (sp. undet.) Meek & Worthen, 1873: pl.
27, figs 1a-b.
Solenomya (Janeia) trapezoides Meek, 1874a: 582.
Solenomya trapezoides Meek. Beede & Rogers, 1899: 132, pl. 34, figs
2a-b; Beede, 1900: 159, pl. 21, figs 2a-b.
Solemya trapezoides Meek. [?] Hoare, 1961: 96, pl. 12, fig. 14; Hoare
et al., 1979: 30, pl. 2, figs 14-17; Pojeta, 1988: 215, pl. 22, figs
8-9, pl. 24, figs 1, 5, 7-8; [?] Kues, 1992a: 91, figs 2.9-2.10;
Kues et al., 2002: 129, fig. 4Q; non Krainer, et al., 2003: fig.
7L.
Solemya radiata Meek & Worthen. Pojeta, 1988: pl. 24, figs 2-4, 6.
Acharax (Nacrosolemya) trapezoides Meek. Carter, 1990: 174, figs 1718.
Acharax? sp. Pojeta, 1988: pl. 20, fig. 9.
[?] Solenomya costellata (M'Coy, 1844). Hind, 1900: pl. 50, fig. 7.
Diagnosis.–Medium to large solemyin with subquadrate
or trapezoidal valves with obliquely sloping brevidorsa; shell
ornament of comarginal lirae and rugae with intercalated elements; external radii few to none. Valve interior marked by
broad, comarginal rugae and fine, obsolescent radii. Posterior
adductors scars moderately large, well impressed, each anterodorsally bordered by broad buttress. In internal molds, anterior adductor scars large, faint, each bordered by weak groove
("rib" in internal molds) ascending radially from below anterior adductor to dorsal margin in front of umbo. Anterior
gape large; possible posterior gape small, above posterior adductors. Left valve typically overlapping right valve dorsally.
Primary ligament parivincular, short, dorsally arched, attached
at external nymphae; asymmetrical anterior extension of outer
ligamental layer forming grooved (i.e., crenulated) demipad
scar on inner subapical surface of right valve, then narrowing
anteriorly, joining longidorsa by attachment to valve edge in
left valve and to longitudinal lamellar ridge just inside margin
of right valve. Shell microstructure with nacreous middle and
inner layers (Carter, 1990; Carter et al., 1990).
Lectotype.–Lectotype of Acharax trapezoides here designated, USNM 36315 (Pl. 5, Figs 1-5), a large calcareous internal
mold with conjoined valves. One quill-penned museum label reads: "Solenomya Vol. V Geol. Surv. of Illin. Plate 27 fig.
1 Coal Meas. Illinois?"; a second quill-penned label marked
"U.S.N.M. C Rominger Coll" also reads "36315 Solenomya
32
Pennsylvanian Coal Measures [crossed out] Illinois?" Pojeta
(1988: pl. 24, figs 2-3) identified this specimen as the one
figured by Meek & Worthen (1873: pl. 27, figs 1a-b) as
"Solenomya sp. indet.," but not discussed in their text. Meek
(1874a: 582-583) subsequently cited the same specimen as
the material basis of "Solenomya" trapezoides.
Description of Lectotype.–Large, articulated internal mold,
subquadrate, trapeziform in lateral profile, strongly inequiaxial, anteriorly elongate, laterally compressed, measuring ca.
73.3 mm in length, 31.6 mm in height, 20.2 mm in total
depth (both valves conjoined). Longiaxis/breviaxis ratio ca.
3.0; aspect ratio (length/height) 2.3. Shell slightly expanded
anteroventrally and anterodorsally; maximium shell height
near anterior (longiterminal) margin, gently tapering posteriorly. Beaks small, opisthogyrous; umbones very low; dorsal
margin straight or weakly convex; ventral margin straight or
weakly concave, running subparallel to longidorsa. Anterior
margin gently convex with slight angle at anterodorsal extremity; posterior margin short, strongly convex, subrostrate.
Brevidorsum oblique, concave, sloping steeply upward (ca.
55º) from breviterminus. Left valve taller than right valve,
conspicuously overlapping it dorsally. Ventral margins closed,
but anterior-anteroventral margin gaping (pedal gape), and
posterodorsal margins weakly gaping (possible exhalant gape)
behind nymphae. Shell marked by comarginal rugae with intercalations irregularly spaced, variably defined. Faint radii
of low relief, barely visible over much of valve interior, most
evident ventrally, especially near anterior and posterior shell
extremities. Anisomyarian; in each valve, posterior adductor
scar moderately large, conspicuous, subtrigonal, ventrally expanded, overprinted by radial striae extending from umbo to
posteroventral shell margin; bounded anteriorly and dorsally
by broad radial buttress continuing and best defined toward
umbo. In each valve, anterior adductor scar large, elevated, indistinct, incompletely preserved, approximately bounded anteroventrally by weak radial "rib" (AR; i.e., groove in original
valve), overprinted by radial striae extending from umbo to
anteroventral shell margin. Above AR, behind and adjoining
anterior adductor scar lies long trigonal scar marking attachment of visceral/pedal musculature, ascending posterodorsally
and narrowing to vertex in front of umbo, visually merging
with anterior adductor to form large composite scar (ACS),
pyriform in outline. Pallial line simple, obscure, consisting
of broad comarginal band weakly marked by faint radii (cf.
orbicular muscle scars). Hinge edentate. Primary ligament
opisthodetic, parivincular, attaching behind beaks at short
external nymphae with erect medial edges and posteriorly
bounded by small step (S). Attachment of abapical part of
anterior ligamental extension (ALE) marked by longitudinal
"furrow" (marking lamellar ridge in original valve) lying just
inside anterodorsal margin of right valve and extending anteriorly to half or more of longidorsum length. Beneath right
umbo, internal attachment of adapical part of ALE denoted
by elongated demipad scar marked by series of transverse-toradial grooves (crenulations). Anteriorly, scar gradually narrowing, joining "furrow" (= ridge) of abapical ALE; grooves
gradually trending parallel to dorsal margin such that furrow
of abapical ALE becomes longitudinally grooved. Shell not
preserved.
Discussion.–Because the mold of the left nymph (broken
away) was partly superimposed over the right one, it is clear
that the dorsal overlap of the left valve in the lectotype has
been taphonomically amplified. Although external ornament
is not preserved in the lectotype, in other specimens (Pojeta,
1988: pl. 22, fig. 9; Text-fig. 3), well-marked comarginal lirae
and rugae are visible, but external radii are inconspicuous. The
relatively large and ventrally expanded posterior adductors
and the apparently high (posterodorsal) location of the exhalant gape are distinctive features. Among extant solemyids (S.
reidi), the condition is reversed – the posterior adductor is
relatively smaller and posterodorsally confined with the exhalant gape located posteroventrally (see Reid, 1998: fig. 5.8E).
Coupled with the obliquely sloping brevidorsum (implying
posterior restriction of the mantle cavity), these features suggest gills in Acharax (Nacrosolemya) trapezoides that were either
relatively smaller or less expanded posteriorly than in extant
solemyids.
Aside from the postmortem distortion of the shells, the
similarities of Carter's (1990: fig. 17c) specimens from the
Magoffin Member, Breathitt Formation (PennsylvanianAtokan), eastern Kentucky, to the lectotype are sufficiently
strong to confidently apply the subgenus Nacrosolemya to
both. In the Magoffin specimens, unlike in the lectotype, the
posterior (primary) ligament, left-right mode of attachment
of the ALE, and shell microstructure were preserved. Carter
(1990: 174, figs 17-18) described it as aragonitic with a simple prismatic-to-homogenous outer layer and interstratified
nacreous and porcelaneous structure in the middle and inner
layers, in contrast to extant solemyids in which the inner layers are typically homogeneous and non-nacreous.
Carter (1990: 175) described the posterior ligament as
"short, thick, and dorsally arched… Its outer, lamellar sublayer
is physically continuous with the anterior, lamellar ligament.
The thick, inner, fibrous sublayer inserts onto the ligamental
nymph in each valve, and extends continuously from valve
to valve." In the lectotype, the short mold of the underside
of the right nymph (posteriorly bounded by the small step,
Pl. 5, Fig. 4, S) is comparable in length and placement to the
dorsally arched ligament in the Magoffin specimen (Carter,
1990: fig. 17c).
Carter (1990: 175; see also Carter et al., 1990: 317) also
compared the lateral asymmetry of the ALE with that of extant Solemya (Zesolemya) parkinsoni, and S. (Austrosolemya)
australis, attaching at a longitudinal lamellar ridge that runs
along the full length of the inner edge of the anterodorsal
margin of the right valve, but attaching directly to the edge
of the left valve where the ridge is lacking. The ridge, Carter
noted, was thickest immediately in front of the beaks, narrowing anteriorly. The grooved "furrow" (Pl. 5, Fig. 4) in the right
valve of the lectoype (an internal mold) represents the lamellar
ridge described by Carter (1990).
The weak radial "rib" in the internal mold (Pl. 5, Figs 1-2,
AR) represents a shallow groove in the original shell that marks
the approximate anteroventral limit of the pyriform composite scar of the anterior adductor muscle and adjoining visceral/
pedal muscle complex (Pl. 5, Figs 1-2, ACS). A similar AR is
visible in the internal mold of Acharax? sp. figured by Pojeta
(1988: pl. 20, fig. 9; USNM 32987) from the Permian near
Wymore, Nebraska, and in the internal mold of "Solenomya"
trapezoides from the Pennsylvanian of Kansas figured by Beede
& Rogers (1899: pl. 34, figs 2a-b). Ascending diagonally toward the umbones, the AR also marks the approximate axis
of extension of the foot and posterior pedal retractor musculature observed in extant Solemya (Zesolemya) parkinsoni (cf.
Owen, 1961: fig. 3; Cox, 1969: fig. B1, 2b, 3).
The Problem of Hinge Crenulations.–Directly in front of
the beak of the right valve (Pl. 5, Figs 3-4), the lectotype bears
a series of internal crenulations that are also evident in other
specimens attributable to Acharax (Nacrosolemya) trapezoides.
For example, Carter (1990: 177) described "tooth-like crenulations" in one of the Magoffin examples and noted a comparably marked area in front of the beak of a partial "left" (=
right) valve described by Hoare et al., (1979) from Putnam
Hill Shale (Pennsylvanian of Ohio). Similar crenulations are
also visible in the right valve of a possible A. (N.) trapezoides
(= "Solemya sp.," USNM 415967, Upper Pennsylvanian,
Missourian Series, Erie, Kansas) figured by Pojeta (1988: pl.
23, fig. 7) (Pl. 3, Fig. 8). Clearly, however, a crenulated area
is not a character limited to A. (N.) trapezoides, because it
also is visible in left valves of an unpublished solemyin (Pl. 3,
Fig. 7) from the Lower Devonian of Young Island, Nunavut,
Arctic Canada (Prosh, 1988; Bailey & Prosh, 1990). At first
glance, the crenulations suggest either pseudotaxodont or
taxodont hinge teeth that are functionally vestigial. However,
in the Putnam Hill specimen the crenulations take the form
of an inverted "V" that Hoare et al. (1979: 30) interpreted
as a chondrophore. Superficially, the outline of the feature
(Hoare et al., 1979: pl. 2, fig. 15) recalls the deltoid shape
of the chondrophore below the beaks in Devonian nuculids,
such as Nuculoidea corbuliformis (Hall and Whitfield, 1869)
33
and N. deceptriformis Bailey, 1983 (see Bailey, 1983: figs 26BC; 1986b: figs 1.3-1.4). However, the crenulated area in the
lectotype of A. (N.) trapezoides takes the shape of a narrow
band instead of an inverted "V." In each case, placement of
the crenulated area is coincident with the internal scar of the
ligamental demipad at the adapical end of the ALE. Among
Pennsylvanian examples, the crenate demipad appears to be
confined to the right valve; in the Devonian example, the condition is reversed. In the demipad of A. radiata, crenulations
are apparently lacking. As discussed above, demipad lateralization is probably related to left-right asymmetry in the attachment of the ALE (Text-fig. 4).
A possible analogue of this structure is observed in the
malletiid Palaeoneilo in which a chondrophore is sometimes
present, but in only the right valve. In Devonian P. filosa
(Conrad, 1842), the crenate texture flooring a small trigonal
chondrophore possibly results from ontogenetic superposition
over a small group of radiating hinge teeth and sockets (Bailey,
1983: 280). In Pennsylvanian P. oweni (McChesney, 1860),
Permian P. tebagaensis (Termier & Termier, 1959), and Late
Triassic P. elliptica (Goldfuss, 1837), asymmetrical attachment
of the valves is the outcome of fibrous ligament sublayers superposed over taxodont teeth in the left valve that attach to
a small chondrophore in the right valve (Carter, 1990: 158159). By analogy, the crenulations in Acharax (Nacrosolemya)
trapezoides could be a case of atavism – pseudotaxodont toothlike structures recalling functional taxodont dentition of ancestral ctenodontids, reappearing here in a functionally vestigial state, but limited to one valve in which, by exaptation,
they have become secondarily employed as the attachment
site for the adapical end of the ALE.
Occurrence.–Acharax (Nacrosolemya) trapezoides is fairly
common in Pennsylvanian (Atokan-Virgillian) strata in
Illinois, Missouri, Kentucky, Ohio, Kansas, and New Mexico.
The internal mold of Acharax? sp. (Pojeta, 1988: pl. 20, fig.
9; USNM 32987) from the Lower Permian (Wolfcampian),
near Wymore, Nebraska, probably belongs to this species.
Essex Material.–A single specimen from the Carbondale
Formation, Francis Creek Shale (Middle Pennsylvanian –
Westphalian D), FMNH PE 27728, Eureka Coal Company
Mine "E," Will County, Illinois (Pl. 5, Fig. 6) consists of a
left composite cast with replaced shell fragments near the
valve margins. Traces of the periostracum are visible, but the
ligament and other features are not preserved. Although it is
relatively small and somewhat flattened, it nevertheless has
the distinctive trapezoidal profile, sloping brevidorsum, and
broad comarginal rugae. One important difference is the plica-like radial elements in the shell ornament, which, although
subtle, are nevertheless more conspicuous than usual for this
34
species. However, these are not shell plicae in the conventional sense, but preserved traces of radial pleats in the overlying
periostracum that are taphonomically overprinted onto the
calcified part of the shell by sedimentary compaction. The
rare preservation of Essex bivalves accounts for the absence of
these features elsewhere.
In Acharax (Nacrosolemya) trapezoides, Carter (1990) described: (1) large anterior and small posterior gapes; and (2)
slight overlap of the left valve near the apical portion of the
hingeline of the right valve. Although these features are confirmed in the lectotype and other materials, the Mazon Creek
specimen provides data neither on overlap nor gapes.
Although Acharax (Nacrosolemya) trapezoides is rare in the
Essex biofacies, this distinctive and often large species is abundant in analogous roof strata overlying coal seams in other
Illinois cyclothems. For example, in Member 98 above the
Springfield (No. 5) Coal Member, individuals can reach up
to 15 cm in length, are sometimes found "butterflied," like
Mazonomya mazonensis n. gen., n. sp., with their anteroposterior axes diverging from the brevidosal attachment site of the
primary ligament (Text-fig. 3).
Comparisons.–The specimen figured by Beede & Rogers
(1899: pl. 34, figs 2a-b, as "Solenomya" trapezoides) shows external nymphae behind the umbones for the attachment of
a parivincular ligament as well as an apparent anterior gape.
The comarginal rugae, weak radii, and radial "rib" (cf. Pl. 5,
Figs 1-2, AR) are consistent with Acharax (Nacrosolemya) trapezoides, but the posterior adductor is placed higher. However,
this might not be accurate inasmuch as other internal features
(anterior adductor and pallial line) are, by Beede & Rogers’
own admission, "exaggerated."
The "left" (= right) valve of "Solemya trapezoides" (UM
13331) from the concretionary shales above the WeirPittsburg Coal (Desmoinesian of Missouri) figured by Hoare
(1961: pl. 12, fig. 14) has the size and general form of Acharax
(Nacrosolemya) trapezoides, but the shell is more elliptical
than usual. An internal mold (Solemya sp., USNM 415967)
from the Hertha Limestone (Missourian of Kansas) figured
by Pojeta (1988: pl. 23, figs 5-8) is problematic. It closely
compares to A. (N.) trapezoides in its profile, moderately weak
internal buttress, and distinct radial "rib" (cf. Pl. 5, Figs 1-2,
AR); like A. (N.) trapezoides, it also has a striated scar from
an internal demipad in the right (but not left) valve (Pojeta,
1988: pl. 23, fig. 7). The same specimen possesses paired upright plates interpreted by Pojeta (1988: pl. 23, figs 7-8) to be
chondrophores to support an internal ligament. However, it
is possible these are erect nymphae to support an external ligament (cf. Beede & Rogers, 1899: pl. 34, fig. 2b; Carter, 1990:
fig. 17D). The aforementioned internal mold of Acharax? sp.
(Permian near Wymore, Nebraska) figured by Pojeta (1988:
pl. 20, fig. 9; USNM 32987), is nearly identical to the lectotype of A. (N.) trapezoides.
An internal mold from the Carboniferous of Yorkshire attributed by Hind (1900: pl. 50, fig. 7) to Solemya costellata
could be a British example of Acharax (Nacrosolemya) trapezoides owing to the trapezoidal profile, large size, obsolescent
radii, comarginal rugae, and overlap of the left valve.
De Koninck (1885: 121, pl. 19, figs 13-14, pl. 33, figs 4243) compared his species Solemya excisa (Early Carboniferous,
Tournaisian, of Belgium) to Meek & Worthen's (1873: pl.
31, fig. 1) Illinois specimen (the lectotype) that formed the
material basis of Acharax (Nacrosolemya) trapezoides. Features
similar to those of A. (N.) trapezoides include:
1. lateral profile with sloping posterodorsal margin;
2. greatest height near anterior terminus;
3. external parivincular ligament;
4. well-impressed posterior adductor scar and subdorsal
anterior adductor scar;
5. shell ornament consisting mostly of comarginal rugae
with intercalations; and
6. a narrow radial "rib" along the posterior margin of the
anterior adductor (cf. Pl. 5, Fig. 2, AR).
Dissimilarities with A. (N.) trapezoides include:
1. relatively shorter shell (length/height ca. 1.9);
2. umbones more centrally placed (longiaxis/breviaxis
ca. 1.4);
3. more smoothly curving anterior margin;
4. strong narrow buttress in front of the posterior adductor; and
5. anterior adductor scar that is well defined.
With respect to the lateral profile and deeply impressed adductors, S. excisa is intermediate between "Janeia" silurica
(Silurian of Gotland) and A. (N.) trapezoides. Based on the
external ligament, S. excisa should be reassigned to Acharax.
The lateral profiles of specimens from the Late Pennsylvanian of New Mexico identified as Solemya trapezoides
(UNM 11134 and 11135) by Kues (1992a: fig. 2.9) are so
strikingly similar to de Koninck's (1885: pl. 33, figs 42-43) figures of S. excisa that conspecificity seems plausible, inasmuch
as the two specimen groups more nearly resemble each other
than either does to the lectotype of Acharax (Nacrosolemya)
trapezoides. However, further study is needed; both might yet
prove to fall within the range of shell variation of A. (N.) trapezoides.
The right valve (NMMNH 39021) from the Oso Ridge
Member of the Abo Formation (Upper Pennsylvanian, Zuni
Mountains, New Mexico), misdentified as Solemya trapezoides by Krainer et al. (2003: fig. 7L), is a typical example
of Dystactella. The anteroventrally elongated shell profile, the
shell ornament, and prominent barrel-shaped opisthodetic
ligament are in agreement with the type species, D. subna-
35
suta. Pojeta (1988) gave the range of Dystactella as Upper
Whiterockian (lower Middle Ordovician) to lower Givetian
(upper Middle Devonian). The presence of Dystactella in the
Oso Ridge Member extends the biostratigraphic range of this
genus into the Upper Pennsylvanian.
The large size and trapezoidal profile of Acharax (Nacrosolemya) trapezoides are obviously iterative traits that recall
those of a much later species, A. dalli from the late Eocene
Whiskey Creek methane-seep deposit of western Washington
(see Goedert et al., 2003: pl. 42, fig. 8; Goedert & Benham,
2003: fig. 2F).
Comparisons.–Although the small size and oval profile is
somewhat suggestive of Acharax radiata, there is a conspicuous
brevidorsal auricule (lacking in A. radiata in which the brevidorsum is obliquely sloping), the longiterminus is straighter,
and radii are lacking.
Acharax? sp. "A"
Pl. 5, Figs 7-8
Acharax? sp. "B"
Pl. 5, Figs 9-10
Materials.–An external cast in matrix (WIU MC-1121) and
a free internal mold (WIU MC-1122) from the Carbondale
Formation, Francis Creek Shale (Middle Pennsylvanian
– Westphalian D) Peabody Coal Company Pit 11, WillKankakee counties, Illinois.
Materials.–A composite cast with matching external
mold with shell remnants, FMNH PE 10982, Carbondale
Formation, Francis Creek Shale (Middle Pennsylvanian
– Westphalian D), Peabody Coal Company Pit 11, WillKankakee counties, Illinois.
Description.–Shell small, inequiaxial, equivalve, edentate,
elliptical in profile. Umbones low rounded; beaks brevigyrous;
brevidorsal auricule prominent. Longiaxis/breviaxis ratio ca.
2.7; aspect ratio (length/height) ca. 2.0. Shell ornament with
very weak radii and comarginal growth lines of variable relief. Internal mold with weak comarginal rugae; radii lacking.
Dorsal and ventral margins straight, subparallel. Dorsal margin of internal mold weakly concave; ventral margin gently
convex. Longidorsal inner margins faintly marked by longitudinal ridge; small weakly impressed adductor muscle placed
near breviterminus and bordered by weak shell buttress; valves
naturally gaping at anterior and posterior extremities; pallial
line simple. Other features not observed.
Description.–Shells medium sized, smoothly oval in profile; solemyiform. Longiaxis/breviaxis ratio ca. 1.6; aspect
ratio (length/height) ca. 1.9. Beaks small, brevigyrous; umbones low, with broad saddle and auricule along brevidorsum.
Longiterminal margin slightly flattened along its upward
curve. Sculpture consisting of concentric growth lines and
lirae of variable relief. Composite cast with shell remnants
suggests placement of fairly strong internal buttress bordering posterior (?) adductor. Ligament and other features unknown.
Remarks.–Due to the limitations of the material, these bivalves remain in open nomenclature. The weak internal buttress of the free internal mold (Pl. 5, Fig. 8, left, BT) is suggestive of Acharax. The beveled extremes of the anterior and
posterior termini of the same specimen connote a gape in the
valves at each location. Because the ventral and dorsal margins
are otherwise tightly appressed, I judge the gapes (Pl. 5, Fig.
8, right, PG, AG) to be natural. The brevidorsal auricule of
the valve exterior (Pl. 5, Fig. 7) is suggestive of a solemyid; the
subparallel dorsal and ventral margins especially recall those
of Solemya parallela Beede & Rogers [= Acharax primaeva
(Phillips, 1836) n. comb.], although the squarish profile of
the longiterminus does not. Whereas the umbo seems narrower than expected for a solemyid, this appearance is exaggerated by the fortuitous overlap of bits of unprepared matrix
directly behind the beak (Pl. 5, Fig. 7) and attachment (prob-
ably postmortem) by a cornulitid. Inasmuch as one specimen
shows only the interior whereas the other shows only the exterior, precise taxonomic relations of the two are unresolved.
Nevertheless, their dimensions, profile, low umbones, and
beak placement are in general agreement.
Comparisons.–Due to limitations of the material, this bivalve remains in open nomenclature; taxonomic placement
is questionable. The figured specimen somewhat resembles
Acharax (Nacrosolemya) trapezoides in size and overall appearance, but the beaks are more centrally placed, the sculpture
is more like that of Mazonomya n. gen., and radial elements
are lacking. The solemyiform shell, buttress, low umbones,
and broad saddle and auricule suggest a possible solemyid; the
strength of the buttress and lack of anteroventral expansion
are somewhat suggestive of certain Solemyinae. It is provisionally placed in Acharax because primary ligaments in Paleozoic
solemyids appear to be external.
PEAT SWAMP-INDUCED EUTROPHICATION
Essex Eutrophication
The distinctive composition of the Essex biofacies invites
comparison with communities associated with modern coastal
36
environments altered through anthropogenic eutrophication.
The Essex is suggestive of eutrophication in several ways:
1. the most abundant invertebrates are jellyfish;
2. the second most abundant invertebrates are solemyid
bivalves, a group relatively rare in marine deposits and
absent in freshwater deposits; the remainder of the
bivalve fauna is unusual when compared with coeval
marine assemblages;
3. a significant fraction of the Essex consists of small
shrimp and other crustaceans;
4. fossil plant debris and coprolites together account for
as much as a third of the total sample in some collections; and
5. the faunal content of the Essex is consistent with
a Type 2 oxygen-deficient paleoenvironment of
Sageman et al. (1991), characterized by dysoxic/anoxic substrata and oxic to dysoxic bottom waters.
Essex jellyfish include one cubomedusan (Anthracomedusa
turnbulli Johnson & Richardson, 1968), and several scyphozoans, including Lascoa mesostuarata Foster, 1979, Octomedusa
pieckorum Johnson & Richardson, 1968, Reticulomedusa greenei Foster, 1979, and especially the superabundant Essexella
asherae Foster, 1979, which singly could account for nearly
70% of invertebrate fraction of the Essex assemblage (Baird
& Anderson, 1997). In modern environmentally degraded
conditions, oxygen depletion of the water in estuaries and
restricted bays caused by athropogenic eutrophication often
results in dramatic blooms in jellyfish populations, typically
dominated by only a single, usually non-native jellyfish species (Arai, 2001; Mills, 2001; Andersen & Nielsen, 2003;
Purcell et al., 2007; Lo et al., 2008).
Estuarine ecostystems altered by anthropogenic eutrophication tend to host thin-shelled bivalve guilds dominated by
organic-loving solemyids (Howes et al., 2003; Kidwell, 2007;
2008; Toledo-Rivera et al., 2009) the ability of which to actively pump sulfides to supply the chemosymbiotic bacteria in
their gills accounts for their success in sewage-contaminated
estuaries and polluted harbors (Seilacher, 1990; Reid, 1998).
In anthropogenic eutrophic communities, as in the Essex fauna, shells of bivalve taxa in general tend to be relatively fragile,
poorly calcified, and rich in organic content in contrast to
those of noneutrophied communities. In eutrophic waters,
carbonate precipitation can be inhibited by reduced pH and
by random occupation of carbonate lattice sites by excess
organic molecules (Clark, 1976). The thick periostracum of
the solemyids inhibits corrosion of the shell in acidic habitats
(Seilacher, 1990).
There are more than 22 species of crustaceans in the
Mazon Creek fauna (Schram et al., 1997), and many, including cycloids as well as eocarid and syncarid shrimp, are very
abundant in the Essex fauna, accounting for as much as 3%
in some samples (Baird & Anderson, 1997). Among the WIU
collections, fossil shrimp roughly account for 15% of the total Essex assemblage. Today, eutrophication of Dutch coastal
estuaries by runoff from the Rhine and Meuse rivers has been
cited as a causal factor in the increase in overall density of
crustaceans in general and the enhancement of shrimp populations in particular (Boddeke, 1996). Elsewhere in dysoxic
conditions, certain shrimp (Bresiliidae and Alvinocaridae)
gain nutrition from chemosynthetic bacteria growing on the
body surfaces (Little & Vrijenhoek, 2003) and within the
branchial cavity (Schmidt et al., 2008).
A significant percentage of the Essex fauna consists of annelid worms and sea cucumbers. Eurytopicity observed among
modern examples of these groups accounts for their success in
Holocene stressed environments near hydrothermal vents and
areas affected by anthropogenic eutrophism. Eutrophication
reportedly increases the densities of holothurian populations
(Mangion et al., 2004).
In Baird & Anderson's (1997: table 4B.2) study of relative
abundances of selected Essex fossils, coprolites ranked fifth
(4.8%), solemyid bivalves (mostly Mazonomya mazonensis n.
gen., n. sp.) ranked fourth (5.5%), and plants ranked second
(29%), exceeded only by the jellyfish Essexella (42%). The
Mazonian coal swamp supported a rich diversity of plants
that have been the subject of numerous studies (Peppers &
Pfeffekorn, 1970; Schopf, 1979; Pfefferkorn, 1979; Wittry,
2006). Occasionally, wood fragments have euryhaline myalinid bivalves still byssally tethered as in life (Bailey, 1992).
The conditions of deltaic drowned swamps of the Pennsylvanian are beguilingly suggestive of effluent-induced eutrophication of modern "booming grounds" near pulp mills
along the western coast of Vancouver Island. In muds beneath
the booms, Reid (1980) and Shepard (1986) located dense
populations of Solemya, and noted that other marine bivalve
species were relatively rare where the solemyids were abundant. Reid believed the oxygen tension within the solemyid
burrows to be low because the mud beneath the booms was
littered with tree bark and wood debris. These conditions are
mimicked in the Francis Creek Shale, in which the rich flora
of the drowned coal swamp of the Mazonian Delta is distributed as numerous wood- and leaf-strewn bedding-plane horizons within the Braidwood and Essex. Eutrophication might
have resulted from effluence or leaching of underlying peat
deposit (antecedent of the Colchester No. 2 Coal) or river
runoff. Roof strata of other Illniois cyclothems are similarly
strewn with wood debris and iterative solemyid assemblages
suggestive of natural cycles of coal swamp-induced eutrophication.
Recurring Solemyid Guilds as Markers of Iterative
Eutrophication Events
It should be emphasized that the Essex fauna occurs within
the basal shales of the Francis Creek; these are the roof shales
directly overlying the Colchester (No. 2) Coal Member of
the Liverpool Cyclothem and associated with the onset of a
marine transgressive event. Furthermore, Essex bivalve assemblages best compare not with those of other marine shales,
but rather with carbonaceous, concretion-rich roof shales and
black limestones directly above other coal seams [members 6-8
of Weller's (1957) idealized cyclothem]. A good example is
the fissile, pyritic shale (Member 98 of Wanless, 1958) overlying the Springfield (No. 5) Coal of the St. David's Cyclothem
(Carbondale Formation), near Wyoming, Stark County,
Illinois. In this unit, as in the lower Francis Creek, solemyids
are the most common bivalves; Acharax (Nacrosolemya) trapezoides and A. radiata reappear in profusion, whereas a different solemyid, Clinopistha levis, replaces Mazonomya mazonensis n. gen., n. sp. Euryhaline pectinoids and myalinids likewise
reoccur, but nuculoids and nuculanoids, commonly indicative
of euhaline salinities and only marginally dysaerobic condtions, are lacking. Roof shales like these probably represent
euryhaline, oxygen-depleted mudflats or salt marshes formed
essentially by sedimentary entrapment within the drowned
forests. Today, tidal salt marshes form along drowned coastlines at the beginning of transgressive cycles where the tangle
of vegetation (modern salt-marsh grasses) entraps organic debris, thereby initiating and assisting in the accumulation of
peat. As such, these unstable environments are temporary and
transitional rather than true marine.
In addition to Member 98, solemyids have also been reported from Desmoinesian roof strata above several other coal
seams in west-central Illinois (Text-fig. 5; see also Meek &
Worthen, 1873; Wanless, 1958; Johnson, 1962). Examples
include: the Rock Island (No. 1) Coal [Acharax (Nacrosolemya)
trapezoides]; the Seahorne Coal (A. radiata); the Kerton Creek
Coal [A. parallela (Beede & Rogers); probable junior synonym of A. primaeva n. comb.; not A. parallela (Ryckholt)];
the Houchin Creek (= Summum) (No. 4) Coal [A. (N.)
trapezoides, A. parallela, Clinopistha levis], and the Sparland
(= Danville) (No. 7) Coal [A. (N.) trapezoides]. Roof strata
communities like the "Orbiculoidea Association" of Johnson
(1962) were interpreted as inhabiting carbonaceous substrata
"probably derived from a prolific growth of seaweed which
prevented circulation of the water and whose decaying remains exhausted the oxygen adjacent to the bottom" (Weller,
1957: 351).
Analogous recurrences of solemyid biofacies in Pennsylvanian roof strata are widespread. For example, they have elsewhere been reported in concretionary roof shales and limestones overlying the Weir-Pittsburg Coal (Desmoinesian of
37
Text-fig. 5. Recurrence of solemyid guilds in roof strata above coal
members in Pennsylvanian cyclothems of Illinois. Stratigraphic
occurrences based largely on reports of Meek & Worthen (1873),
Wanless (1958), and Johnson (1962). Note: here, Acharax parallela
= Solemya parallela Beede & Rogers (1899), not S. parallela Ryckholt
(1853) [1854]); the Beede & Rogers species is probably a junior
synyonym of Acharax primaeva (Phillips) n. comb.
Missouri; Hoare, 1961), Putnam Hill shales and limestones
overlying the Brookville (No. 4) Coal (Desmoinesian of
Ohio; Hoare et al., 1979), and dark argillaceous limestones of
the Pine Shadow Member, Wild Cow Formation (Virgillian
of New Mexico; Kues, 1992a). The unnamed shale overlying the Croweburg Coal, Henry County, Missouri, contains
38
both Braidwood- and Essex-type fossils (Baird et al., 1985a,
b), including a second occurrence of Mazonomya mazonensis n. gen., n. sp. Each solemyid biofacies is characterized by
elevated input of plant- and algal-based organics (suggestive
of eutrophication), fluctuating salinities, reduced circulation,
and depleted oxygen of either drowned or nearby deltaic coal
swamps. However, Kues et al. (2002) also reported a middleouter shelf marine occurrence of Acharax (Nacrosolemya) trapezoides and A. radiata from the shales of the Pennsylvanian
(Missourian) Bar B Formation of the Derry Hills, New
Mexico. Although this biofacies is farther offshore than expected for this species, Kues et al. (2002: 123) noted that
these shales were not representative of most marine biofacies,
being, like the aforementioned roof shales, low in stenotopic
biodiversity and rich in unoxidized organics indicative of low
oxygen and poor circulation. The rather consistent association
of both A. radiata and A. (N.) trapezoides with oxygen-depleted eutrophic biofacies strongly suggests that, like their extant
counterparts, they hosted chemosymbiotic bacteria.
A seeming exception to the general pattern is the occurrence of Acharax (Nacrosolemya) trapezoides in Magoffin
Member (Breathitt Formation, Atokan of eastern Kentucky),
a marine transgressive zone suggestive of a large open bay
(Rice et al., 1979; Rice, 1986). However, solemyid fossils and
burrows in seemingly well-aerated sediments probably represent paleoenvironments that were lower in oxygen than they
appear (Seilacher, 1990). Moreover, Bennington (1996) concluded that the bivalve-bearing shales overlying the coals and
coaly shales of the Magoffin member were probably paleoenvironmentally analogous to those of other mid-continent
Pennsylvanian cyclothems.
ACKNOWLEDGMENTS
Partial funding for the publication of this study was provided
by the Department of Geology, Western Illinois University.
I am grateful to John Pojeta, Jr. (United States Geological
Survey) whose instructive comments helped to launch this
study. I am indebted to Richard D. Hoare (Department of
Geology, Bowling Green State University) and to Joseph G.
Carter (Department of Geosciences, University of North
Carolina at Chapel Hill) for comments on an early draft of
the manuscript, to Paul A. Johnston (Department of Earth
Sciences, Mount Royal University, Calgary, Alberta) and John
Pojeta, Jr., for reviewing the revisions, and to Paula Mikkelsen
(Paleontological Research Institution, Ithaca, New York) for
editing the final draft. My gratitude is also extended to Scott
Lidgard and Glen Buckley (Field Museum of Natural History)
and to Andrew Hay and Chris Ledvina (Northeastern Illinois
University) for the loan of specimens. I am grateful to John
D. Taylor and Emily A. Glover (Zoology Department, The
Natural History Museum, London), Charles W. Shabica
(Shabica & Associates), Steven D. Sroka (Utah Field House
of Natural History State Park Museum), Barry S. Kues
(Department of Earth & Planetary Sciences, University of
New Mexico), and Joseph G. Carter for enlightening communications and/or materials, and I thank Leslie Melim
(Department of Geology, Western Illinois University) for
helpful comments on sedimentological portions of the manuscript. Finally, I thank members of the Earth Science Club of
Northern Illinois (ESCONI), and especially Jack Wittry for
valuable assistance in the location fossil materials and for continuing encouragement during the completion of this study.
Photos and drawings are by the author except as noted.
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69-154. H. W. Rokker, Springfield, Illinois.
Wright, C. R. 1965. Environmental Mapping of the Beds of the
Liverpool Cyclothem in the Illinois Basin and Equivalent Strata in
the Northern Mid-continent Region. Unpublished Ph.D. dissertation, University of Illinois, Urbana, 100 pp.
Yochelson, E. L., & E. S. Richardson. 1979. Polyplacophoran molluscs of the Essex Fauna (Middle Pennsylvanian, Illinois). Pp
321-332, in: Mazon Creek Fossils, M. H. Nitecki (ed.), Academic
Press, New York.
Yonge, C. M. 1939. The protobranchiate Mollusca: a functional
interpretation of their structure and evolution. Philosophical
Transactions of the Royal Society (of London) B, 230: 79-147.
Yonge, C. M. 1959. The status of the Protobranchia in the bivalve
Mollusca. Proceedings of the Malacological Society of London, 33:
210-214.
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Lower Carboniferous, Guangdong Province, China. Journal of
PLATES
51
52
Plate 1
Figure
Page
Mazonomya mazonensis n. gen., n. sp., Middle Pennsylvanian (Westphalian D) Carbondale Formation, Francis Creek Shale, Peabody Coal Pit 11, Will-Kankakee counties, Illinois. All specimens coated with NH4Cl (except Fig. 3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1-2. Reconstructed external and internal morphology showing attachment of ligament and
periostracum and "butterflied" divergence of valves from the brevidorsally positioned hinge axis.
Abbreviations: AA, anterior adductor; ACS, pyriform anterior composite scar marking muscular
attachment sites of the anterior adductor, and integument of the visceral mass and foot; ALE,
anterior ligamental extension; AU, auricule; BD, brevidorsum; HA, hinge axis; LD, longidorsum;
LG, primary ligament; LR, lamellar ridge; N, position of ligamental nymph; PA, posterior
adductor (inferred placement); PEa, anterior periostracal extension; PEp, posterior periostracal
extension [possibly consisting of outer (lamellar) ligamental layer overlain by periostracum]; PL,
band of radial (cf. orbicular) muscles forming the pallial line; ST, step separating PE from ALE.
1. Reconstructed external morphology.
2. Same drawing as Fig. 1, modified to show placement of internal features (bounded by dashed
lines) based on organic traces preserved in the holotype.
3. Holotype, FMNH PE 35737, dorsal view of uncoated composite (external/internal) cast in
siderite concretion; lighter hues mark organic traces of primary ligament, anterior ligamental
extension, periostracum, adductor musculature, muscles of visceral integument and foot, and
band of radial muscles forming the pallial line. Scale bar = 5 mm.
4-5. Paratype, FMNH PE 18152, external cast in siderite concretion; this specimen was called "undetermined bivalve" by Johnson & Richardson (1970: pl. 5, fig. 6). Notice that the primary
ligament had previously been prepared away.
4. Dorsal view. Scale bar = 5 mm.
5. Detail of anterior ligamental extension. Scale bar = 3 mm.
6. Paratype, FMNH PE 36000, hinge detail of articulated internal mold with partial nymph (N),
and adapical fragments of lamellar ridges (LR) still attached; ligament mold (LGM) shows
inverted U-shape of the parivincular ligament; other abbreviations as above. Scale bar = 3 mm.
53
54
Plate 2
Figure
55
Page
Mazonomya mazonensis n. gen., n. sp., Francis Creek Shale, Mazon Creek area, Will-Grundy-Kankakee
counties, Illinois. All specimens coated with NH4Cl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1-8. Dorsal views of external and internal casts in siderite concretions; valves gaping widely ("butterflied") in natural death pose.
1-2. Holotype, FMNH PE 35737.
1. Articulated valves showing brevidorsal placement of preserved primary ligament; anterior
ligamental extension (ALE) and overhanging tent-like extension of periostracum (PE)
are separated by step (ST). Scale bar = 5 mm.
2. Detail of parivincular primary ligament (LG); step (ST) marks boundary between
primary ligament and posteriorly overhanging extension of periostracum (PE). Scale
bar = 2 mm.
3-4. Paratype, FMNH PE 36069.
3. Articulated valves, showing preserved primary ligament, anterior ligamental extension,
and periostracum. Scale bar = 5 mm.
4. Detail of preserved primary parivincular ligament (LG), step (ST), and overhanging
periostracal extension (PE). Scale bar = 2 mm.
5. Paratype, FMNH PE 35631, articulated valves with preserved primary ligament, anterior
ligamental extension and periostracum. Scale bar = 5 mm.
6-7. Paratype, FMNH PE 35810.
6. Articulated valves showing inverted U-shaped mold of primary ligament, mold of insertional groove and nymph, anterior ligamental extension, and periostracum. Scale bar
= 5 mm.
7. Brevidorsal detail (rotated view) showing sedimentary mold of parivincular ligament
(LGM), insertional groove and nymph (N). Scale bar = 2 mm.
8. Paratype, WIU MC 1120, internal mold showing thickened hinge margins, grooves marking
lamellar ridges (LR) for probable attachment of anterior ligamental extension; also shown
are primary ligament, anterior ligamental extension/periostracum, and faint traces of pallial
musculature; cubic features (PS) are sideritic pseudomorphs after pyrite. Scale bar = 5 mm.
9. Paratype, FMNH PE 47723, left lateral view of articulated individual in burrow with closed
valves; note sedimentary backfill laminae – "sitz marks" (SM) produced by repeated thrusts of
large foot parallel to the anteroposterior axis; curved shape of sitz marks suggest that sole of foot
was slightly concave. Scale bar = 10 mm.
56
Plate 3
Figure
Page
1-6, 9. Acharax radiata (Meek & Worthen, 1860), Middle Pennsylvanian (Westphalian D), Carbondale
Formation, Francis Creek Shale of Illinois. All specimens coated with NH4Cl. . . . . . . . . . . . . . . 27
1. Reconstruction of external morphology showing attachment of ligament and periostracal
frill; placement of internal features enclosed by dashed lines; because shell is thin, internal
buttresses and lamellar ridges are sometimes faintly visible on exterior; note divergence of
shells from brevidorsally placed hinge axis. Abbreviations: AA, anterior adductor scar (inferred placement); ACS, pyriform anterior composite scar marking muscular attachment
of the anterior adductor, integument of the visceral mass, and foot; ALE, anterior ligamental extension; BT, internal buttress; D, diastemata (plical gaps); GR, insertional groove of
primary ligament; LG, primary (parivincular) ligament; LR, lamellar ridge; PA, posterior
adductor scar; PE, overhanging tent-like extension of periostracum; PF, overhanging frill of
periostracum; PFL, junction of periostracal frill with shell; PL, band or radial muscles composing pallial line; ST, step separating ALE from PE.
2. FMNH PE 39388, left lateral view of articulated specimen; high contrast lighting showing
radial plicae, faint outline of anterior adductor scar (AA), small buttress (BT), and wrinkled
remnants of the anterior portion of periostracal frill (PF); note external splitting and cracking
of original periostracum (PC) coinciding with internal placement of pallial line. Scale bar =
5 mm.
3. FMNH PE 29875, right lateral view (cropped) of articulated specimen (composite mold);
high contrast lighting shows radial plicae, weak buttress (BT), faint outlines of anterior (AA)
and posterior adductor scars, junction of periostracal frill with shell (PFL), and mold of frill
selvage (PF). Scale bar = 5 mm.
4-6, 9. ISM lot 2994, "Coal Measures," Fulton County (older label reads "Fulton and Schuyler
Counties"); three free articulated specimens. Internal molds with shell remnants.
4. Left valve in matrix. Scale bar = 5 mm.
5. Dorsal view of articulated specimen showing left valve overlap (LVO). Scale bar = 5
mm.
6. Detail of umbonal region of specimen in Fig. 5 showing mold of anterior ligamental
extension (ALE) with subumbonal demipad (DP). Scale bar = 1 mm.
9. Right lateral view of a specimen showing dorsal overlap of the left valve (LVO) and pair
of gaps (diastemata) (D) in radial plicae. Scale bar = 3 mm.
7. 8..
Unpublished solemyid, Lower Devonian, Disappointment Bay Formation, Young Island,
Nunavut, Canada. Umbonal and preumbonal detail of internal mold of left
valve showing longitudinally grooved mold of attachment area of the anterior
ligamental
extension
(ALE)
and
crenulated
ligamental
demipad (DP) (from Prosh, 1988; Bailey & Prosh, 1990). Scale bar = 3 mm. . . . . . . . . . . . . . . 33
Solemya sp., USNM 415967, Hertha Limestone, Upper Pennsylvanian, Missourian Series, Erie,
Kansas. Articulated internal mold with shell fragments; umbonal detail of figure by Pojeta (1988:
pl. 23, fig. 7) showing crenulated scar of ligamental demipad, restricted to left valve. Scale
bar = ca. 2 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
57
58
Plate 4
Figure
59
Page
Acharax radiata (Meek & Worthen, 1860), Carbondale Formation, Francis Creek Shale, Peabody Coal
Co. Pit 11, Will-Kankakee counties, Illinois. External and composite casts in sideritic concretions of
conjoined valves with relict shell and periostracum. All specimens coated with NH4Cl. . . . . . . . . . . . . . 27
1-2. FMNH PE 22329, composite cast with conjoined valves. Scale bars = 5 mm.
1. Left dorsolateral view, showing radial plicae and anterior ligamental extension; preserved
elements of periostracum include a periostracal frill (PF) along anterior margins and a tentlike periostracal extension (PE) stretched dorsally between valves and separated from anterior
ligamental extension by a step (ST).
2. Left lateral view showing radial plicae, preserved wrinkles in periostracal frill (PF), and
posterolateral diastemata (D) in radial plicae; note preserved periostracal cracks (PC)
occurring along trace of pallial line.
3-4. FMNH PE 52187. Scale bars = 5 mm.
3. Left dorsolateral view; note fracturing of periostracum near ventral margin.
4. Dorsal view showing anterodorsal lamellar ridges (LR) for attachment of anterior ligamental
extension, tent-like anterior periostracal extension (PE), weak internal buttresses (BT),
and preserved parivincular primary ligament with insertional grooves (LG); large but faint
anterior adductor muscle scars (AA) visible in both valves.
5.
FMNH PE 29875, external cast with articulated valves; right dorsolateral view showing preserved parivincular primary ligament (LG); note preserved selvage of periostracal frill (PF) along
ventral margin, and junction of periostracal frill (PFL) with shell; also note preserved periostracal cracks (PC). Scale bar = 5 mm.
6.
FMNH PE 10984, dorsal view of composite cast showing tent-like anterior periostracal extension (PE), grooved hinge margin with traces of lamellar ridge (LR), and preserved parivincular
primary ligament (LG) with insertional grooves; small cubes (PS) are sideritic pseudomorphs
after pyrite. Scale bar = 5 mm.
7. NEIU MCP 162, left lateral view of specimen with burrow trace. Scale bar = 10 mm.
60
Plate 5
Figure
Page
Pennsylvanian Solemyidae of Illinois. All specimens coated with NH4Cl.
1-5. 6-10. Acharax (Nacrosolemya) trapezoides (Meek, 1874a). Lectotype, USNM 36315 (museum label
reads "Pennsylvanian, Illinois?"), a large internal mold figured by Meek & Worthen (1873: pl.
27, figs 1a-b) as "Solenomya sp. indet." and cited by Meek (1874a) as the material basis for
Solemya trapezoides Meek, 1874a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
1. Left lateral view, showing comarginal rugae, weak radii, internal buttress (BT), posterior
adductor scar (PA), pyriform anterior composite scar (poorly preserved) marking muscular
attachment of the anterior adductor, integument of the visceral mass, and foot (ACS) and
anterodorsal "rib" (AR) representing shallow groove in original valve. Scale bar = 10 mm.
2. Right lateral view additionally showing dorsal overlap of left valve, and visceral retractor scar
(VR). Scale bar = 10 mm.
3. Dorsal view, tilted slightly toward left valve to show hinge detail and strongly impressed
posterior adductor scar (PA). Scale bar = 10 mm.
4. Hinge detail (enlarged and enhanced) showing left valve overlap (LVO), internal traces of anterior ligamental extension (ALE) with crenulated ligamental demipad (DP), mold of underside of right nymph (N), possible posterior shell gape (PG), and step (S) marking posterior
limit of primary external ligament. Scale bar = 5 mm.
5. Anterior view showing mold of anterior (pedal/inhalant) gape (AG). Scale bar = 10 mm.
Essex Solemyidae, Francis Creek Shale, Carbondale Formation (Middle Pennsylvanian – Westphalian D), Illinois.
6. Acharax (Nacrosolemya) trapezoides, left lateral view of small valve in sideritic concretion
(FMNH PE 27728, Eureka Coal Company, Mine "E," Will County) showing weak
comarginal rugae and radii; weak radial ribs are not calcareous plicae, but are preserved traces
of radial pleats of original periostracum overprinted onto shell by sedimentary compaction.
Scale bar = 5 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7-8. Acharax? sp. "A." . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
7. Left lateral view of WIU MC 1121, Pit 11, a solemyiform shell in sideritic concretion
with low umbones and brevidorsal auricule; umbones are brevidorsally overlapped by
matrix and encrusted (probably postmortem) by a cornulitid (C), exaggerating their
prominence. Scale bar = 2 mm.
8. WIU MC 1122, Pit 11, a free internal mold; left, lateral right view showing impression
of a weak buttress (BT); right, ventral view showing anterior gape (AG) and posterior
gape (PG). Scale bar = 5 mm.
9-10. Acharax? sp. "B," FMNH PE 10982, Pit 11, Kankakee County. Scale bar = 5 mm. . . . . . . 35
9. Left lateral view of composite cast in sideritic concretion with relict shell and buttress
(BT).
10. Left lateral view of external mold with relict shell; this is the other half of the same
specimen.
61
62
INDEX
Note: Page numbers are in regular font; plate numbers are in bold font; pages numbers denoting definitions or principal discussions
are in italics. Major headings and subheadings are in small capital font.
18S rDNA 12
18S rRNA 12, 21, 26
28S rRNA 12
abapical, definition of 3
Abo Formation [of New Mexico] 34
abbreviations, repository 2
Abstract 1
Acanthopecten sp. 6
Acharachoidea 14
Acharacidae 21
Acharacinae 21
Acharax 1, 5, 8, 14, 16, 18-22, 24-25, 26-27, 28-30, 34-35
agassizii 18
dalli 18, 21-22, 26, 35
eremita 20
johnsoni 15-18, 26, 28-29
mikasaensis 14
parallela 37
primaeva 1, 30, 35, 37
radiata 1, 6, 8, 19-26, 27-31, 33, 35, 37-38, 56, 59; 3-4
(Nacrosolemya) 5, 17, 19, 27, 29, 31
trapezoides 1, 6, 13-14, 18-22, 25-26, 28, 30, 31-35, 37-38,
60; 5
Acharax s.s. 16-17, 19-21, 31
Acharax?
sp. 31, 33-34
sp. "A" 6, 35, 60; 5
sp. "B" 6, 35, 60; 5
(Acharax), Solemya 15
Acila castrensis 13
Acknowledgments 38
ACS [see anterior composite scar]
Adamkewicz et al. (1997) 12
Adams, H. & A. (1858) 10-12
adapical, definition of 3
adoral sense organ 12
Adulomya 16, 17
aerobic tier 8
Afghanodesmatoidea 14
affinis, Schizodus 6
agassizi, Acharax 18
A House Divided? 12-14
Aitken et al. (2002) 9
Albany [New York] 2
Albuquerque [New Mexico] 2
ALE [see anterior ligamental extension]
Allen
(1978) 11
(1985) 11
& Hannah (1986) 9-11, 14-16, 18-19
& Sanders (1969) 11
& Sanders (1973) 14
& Sanders (1996) 25
Allison (1988) 3
Allmon (1985) 4
Alvinocaridae 36
Amano et al. (2007) 8
Ambonychioidea 7
Ames marine zone, Biofacies-3 3
Amler (1999) 11-12, 21
Andersen & Nielsen (2003) 36
Anderson et al. (1987) 8
angustia, Paleyoldia 13
Anomalodesmata 6, 21
anomalodesmatan 12
anterior composite scar (ACS) 12, 18, 23-24, 28, 32-33, 52, 56,
60
anterior ligamental extension (ALE) 16, 19, 20, 22-29, 31-33, 52,
55-56, 60
Anthracomedusa turnbulli 36
Anthraconaia 2, 5
ohioense 6
Anthraconauta 2, 5
sp. 6
Antipleuroida 11
Aplacophora 13
aplacophorans 13
archaeogastropods 10
AR [see radial "rib"]
Arai (2001) 36
Arctic Canada 19, 23, 26-27, 33
Arkansas 7
asherae, Essexella 2, 36
aspect ratio 18, 30, 32, 35
aspinwallensis, Edmondia 6, 26
Astartella 7
atacama, Solemya (Petrasma) 18, 20
Athens County, Ohio 21
auricule 18, 21-23, 25, 30, 35, 52, 60
australis
Solemya 15-17
Solemya (Austrosolemya) 16, 18, 20, 22, 24-25, 27, 29, 31, 33
(Austrosolemya), Solemya 16, 17, 19, 26-27
australis 16-18, 20, 22, 24-25, 27, 29, 31, 33
Autobranchia 6, 12
Autolamellibranchiata 11-12
Aviculopecten 7
janewiedlini 6
mazonensis 6
Aviculopectinidae 6
Babin (1982) 12
backfill, sedimentary 1, 5, 24, 29, 55
Bailey
(1983) 33
(1986a) 22
(1986b) 33
(1992) 1-3, 5-6, 36
(2009) 2
& Prosh (1990) 19, 23, 26-27, 33, 56
& Sandberg (1985) 13
& Sroka (1997) 1-3, 5-7, 25-26
Baird
(1979) 1-2, 23, 25
(1990) 1, 23-24
(1997a) 1-3, 23
(1997b) 2
(1997c) 2-4, 23-24
& Anderson (1997) 2, 25, 36
& Sroka (1990) 1-3, 23, 25
et al. (1985a) 1-3, 5, 23, 38
et al. (1985b) 1-3, 25, 38
et al. (1986) 1-4, 23-24
Bar B Formation [of New Mexico] 38
Beede
(1900) 27, 31
& Rogers (1899) 1, 13, 26, 28, 30-31, 33-35, 37
Beedham & Owen (1965) 9, 18-19, 24, 29
Beil-type (Pulchratia) assemblage 3
Belgium 30, 34
bellistriata, Phestia 13
Bennington (1996) 38
Bernard (1980) 8-9, 11, 16, 19, 27
Berner (1981) 4
Beurlen (1944) 11
Beushausen (1895) 12, 20
Bieler
& Mikkelsen (2006) 11
et al. (2010) 21
Biofacies 3 association, Ames marine zone 3
Bivalve Taphonomy 3-5
"blobs" 2
Boddeke (1996) 36
booming grounds 8, 36
borealis
Solemya 15, 17
Solemya (Petrasma) 20
Boss (1982) 11
Bradshaw & Bradshaw (1971) 22
Braidwood [fauna/biota/biofacies] 2-3, 5-6, 25, 36, 38
Breathitt Formation [of Kentucky] 27, 31-32, 38
Bresiliidae 36
Brett et al. (1997) 5, 24
breviaxis/breviaxial, definition of 3
brevidorsum/brevidorsal, definition of 3
breviterminus/breviterminal, definition of 3
Brookville (No. 4) Coal [of Ohio] 37
Brush Creek [Athens County, Ohio] 21
63
Burgess Shale 1
burrows 1, 4-5, 8-9, 19, 21, 24, 29, 36, 38
"butterflied" valves 2, 4-5, 21-22, 24, 34, 38, 52, 55
buttress, shelly internal 15-16, 17, 21, 23, 26-28, 30-32, 34-35,
56, 59, 60
Byrne & Dietz (1997) 5
Callender & Powell (1999) 9-10
Campbell, D., et al. (1998) 11
Campbell, K. (1992) 9
Carbondale Formation 1, 7, 23, 28, 33, 35, 37, 52, 56, 59-60
cardinal teeth 14
Carditoidea 7
Carpenter (1864) 20
Carter
(1990) 2, 5, 10-11, 13-22, 25-28, 31-34
& Clark (1985) 21
& Tevesz (1978) 13
et al. (1990) 11, 13, 15, 19, 25, 27, 31, 33
et al. (2000) 11-14, 18, 20-21, 28-29, 31
pers. comm. (2007) 31
Carydium elongatum 1, 22
Castagna & Chanley (1973) 5, 8
Castleton, Derbyshire [England] 26
Cavanaugh (1983) 8
Chapel Hill [North Carolina] 2
chemoautotrophic bacteria [see also endosymbionts] 1, 3, 5, 8, 10,
24
Chicago [Illinois] 2
Children (1823) 15, 27
chondrophores [see also nymphae, internal] 10, 14-19, 26-27, 31,
33-34
Chow (1951) 27
"Chowder Flats" 2
Chronic (1952) 11, 15
"clam-clam" 2
Clark, B. (1925) 18
Clark, G. (1976) 5, 36
Clarke
(1907) 1, 22
(1909) 22
clavicle [see buttress]
Clinopistha 16, 18, 21-22, 25
levis 21-22, 37
Clinopisthinae 18, 21
Coalgate Quadrangle, Oklahoma 7
Coal Measures 28, 32, 56
Coan et al. (2000) 3, 10-11, 15
Colchester (No. 2) Coal Member 1, 3, 25, 36-37
Colchester peat swamp 3
cold seeps 8
Coleman et al. (1993) 3-4
color 4, 23-24, 29
Columbia [Missouri] 2
concretions/concretionary 1-4, 7, 22, 24-25, 30, 34, 37, 39, 52,
55, 59-60
Conrad (1842) 33
Conway et al. (1989) 8
64
Cope
(1996a) 8, 10, 12, 15, 17-18
(1996b) 13
(1997) 10-11, 27
(2000) 10, 12, 18
(2002) 14
& Babin (1999) 11
coprolites 36
corbuliformis, Nuculoidea 33
cornulitid 35, 60
corrugata, Phestia 13
costellata, Solemya 1, 27, 30-31, 34
costellatus, Sanguinolites 30
Cox
(1959) 13
(1960) 10-11, 14
(1969) 14, 16, 18-19, 21, 24, 26-27, 33
Cragg (1996) 13
Crassatelloidea 7
crenulations [see hinge crenulations]
Croweburg Coal [of Missouri] 25, 37
crustaceans 36
Cryptodontida 11
Ctenodonta 12, 14
ctenodontid 11, 33
Ctenodontidae 11, 12, 18
Cyclothem, Liverpool 1, 37
cyclothems 1, 3, 30, 34, 36-38
Dacromya ovum 13
Dalhousie Beds [of New Brunswick] 22
Dall
(1889) 11, 14
(1891) 15-17, 26
(1908a) 5, 15, 17-18, 26
(1908b) 15, 27
& Simpson (1901) 17
dalli, Acharax 18, 21-22, 26, 35
Danville (No. 7) Coal [of Illinois] 37
deceptriformis, Nuculoidea 33
Decker et al. (2007) 13
de Koninck
(1841) 1-2
(1842) 1, 27
(1851) 25
(1885) 1, 27, 30, 34
delphinodonta, Nucula 13
demipad 19, 20, 26-28, 31-34, 56, 60
Derbyshire [England] 26
Derry Hills [New Mexico] 38
Diamond [Grundy County, Illinois] 2
diastemata, plical 28, 29-30, 56, 59
Dickson (1974) 2, 21, 23-26
digestion, intracellular 13
digestive
diverticula 11, 13, 15
gland 12, 18
Disappointment Bay Formation [of Nunavut, Canada] 27, 56
dominichnia 5
d'Orbigny (1847) 7
Drew
(1897) 13
(1899a) 13
(1899b) 13
Driscoll & Brandon (1973) 7
Duan et al. (1996) 4
Duff (1975) 9
Dufour & Felbeck (2003) 9
Dunbar (1924) 6
Dunbarella 7
striata 6
sp. 6
Dunker (1882) 29
dysaerobic
-anaerobic interface 5, 9
tier 8
Dystactella 16, 18, 20-22, 25, 34-35
elongata 1, 22
subnasuta 22
Drum-type (Euconospira) assemblage 3
Eagar (1970) 2
East Anglia [England] 4
Ectenagena 17
edentate hinge 2, 5, 6, 11, 15, 21-24, 28, 32, 35
edentulous space 14
Edmondia 1-4, 21-22, 25
aspinwallensis 6, 26
oblonga 6, 26
ovata 6, 7, 26
reflexa 26
splendens 23, 25, 26
sp. 23, 25
"Edmondia" 2, 4
edmondiid 5, 21-24, 26
Edmondiidae 6, 21
Effingham County [Illinois] 7
Eh-pH fence diagram 4
Ehrlich & Newman (2008) 3-4
Elias (1957) 23, 25-26
elliptica, Palaeoneilo 33
elongata, Dystactella 1, 22
elongatum, Carydium 1, 22
endosymbionts (endosymbiotic bacteria) 5, 8-9
England 4, 9, 20, 30
eocarid 36
eremita, Acharax 20
Erie, Kansas 33, 56
Essex [fauna/biota/biofacies] 1-8, 25, 28-30, 33-38, 60
Essexella 36
asherae 2, 36
Euchondria 7
pellucida 6
Eureka Coal Company Mine "E," Will County [Illinois] 33, 60
eutrophication [see Peat Swamp-Induced Eutrophication]
eutrophic cycles 5, 36-37
exaerobic tier 5, 9
extension, periostracal 16, 22, 52, 55, 59
excisa, Solemya 1, 27, 34
exemplarius, Heteropecten 6
Felbeck
(1983) 8
et al. (1984) 8
fibrous ligamental layer [see ligamental layers]
filibranch gills 11
fjords [of Vancouver Island] 8
filosa, Palaeoneilo 33
FMNH [Field Museum of Natural History, Chicago, Illinois]
catalog numbers:
PE 10982 35, 60, 5
PE 10984 28, 59, 4
PE 18152 23, 52, 1
PE 20958 28
PE 22311 23
PE 22321 23
PE 22329 28, 59, 4
PE 25419 28
PE 27728 33, 60, 5
PE 29875 28, 56, 59, 3-4
PE 30261 23
PE 30921 23
PE 35631 23, 55, 2
PE 35633 23
PE 35737 23, 52, 55, 1-2
PE 35810 23, 55, 2
PE 36000 23, 52, 1
PE 36009 23
PE 36058 23
PE 36069 23, 55, 2
PE 36133 23
PE 39388 28, 56, 3
PE 47723 23, 55, 2
PE 52187 28, 59, 4
Foster (1979) 1-2, 36
fracta, Posidonia 6
fragilis, Ovatoconcha 10-12
Franc (1960) 11
Francis Creek Shale 1, 3, 7, 23-25, 28, 30, 33, 35-37, 52, 55-56,
59-60
Frey
(1968) 24
(1971) 24
frill, periostracal 1, 9-10, 15, 24, 28-29, 56, 59
fringe, periostracal [see frill, periostracal]
fugichnia 4-5
Fujiwara et al. (2007) 8
Fulton County [Illinois] 7, 28, 56
gape, shell 10, 14, 17, 20, 28, 30-32, 34-35, 60
Germany 9, 26
Geyer & Streng (1998) 13
gill buds 13
Giribet
(2008) 12-13
& Distel (2004) 12
& Wheeler (2002) 12-13
Girty
(1909) 11
(1915) 7
glabra, Paleyoldia 13
Glover, E. A., pers. comm. (2010) 16, 18
Goedert
& Benham (2003) 35
et al. (2003) 9, 26, 35
Goldfuss
(1837) 33
(1840) 26
Goldring (1962) 9
gonad 12, 18
Goniophora 25
Gotland 8, 10, 20, 34
gradidentate taxodont dentition 11
graphica, Ryderia 13
Grammysiidae 6
Grammysioidea hayi 6
grandis, Psiloconcha 12
Gray
(1824) 10, 12
(1840) 2, 10, 12, 15, 18, 20-21
(1854) 10
Grayville, Illinois 28
Greb et al. (2003) 3, 25
greenei, Reticulomedusa 36
Grobben (1894) 11-12
Grundy County [Illinois] 2, 7, 21, 23, 25, 55
Gustafson
& Lutz (1992) 13
& Reid (1986) 13
& Reid (1988) 13
haimeanus, Parallelodon 25
Hall
(1858) 7
& Whitfield (1869) 19, 33
& Whitfield (1872) 16, 22
Halla beds [of Möllbos, Gotland] 8
Haszprunar et al. (1995) 13
hayi, Grammysioidea 6
hemocyanin 12
Hertha Limestone [of Kansas] 34, 56
Herrick (1887) 6
heteroconch 5, 7-8, 17
Heteroconchia 6
Heteropecten exemplarius 6
Hind
(1897) 25
(1900) 1, 20-21, 27, 30-31, 34
Hinds (1843) 13
hinge
axis 1, 9, 18, 22, 23, 28, 52, 56
crenulations 26, 31-33
Hoare
65
66
(1961) 27, 31, 34, 37
et al. (1989) 2, 7, 13-14
et al. (1979) 8, 27, 29-31, 33, 37
Hoeh et al. (1998) 12
holothurian 36
holotype [of Mazonomya mazonensis n. gen., n. sp.] 23-24, 52, 55,
1-2
Holzmaden 9
Houchin Creek (No. 4) Coal [of Illinois] 37
Howes
& Goehringer (1996) 7
et al. (2003) 36
human hair 8
Huxleyioidea 14
hydrocarbon seeps 9
hydrothermal vents 8, 9, 36
hypertrophy
gills 9, 14-15
hypobranchial gland 9, 15
hypobranchial gland 9, 12, 15, 21, 24, 29
hypotrophy
gut 9-11, 20
labial palps 9
Illinois 1-2, 7, 21-23, 25, 28, 30-35, 37-38, 52, 55-56, 59-60
Imo Formation [of Arkansas] 7
inequiaxial, definition of 3
inner ligamental layer [see ligamental layers]
integument (of the visceral mass) 15, 18, 24, 52, 56, 60
internal ligamental layer [see ligamental layers]
interplicae 29
Introduction 1-2
ISM [Illinois State Museum, Springfield] catalog number: lot 2994
28, 56, 3
Iredale
(1931) 8, 15-17
(1939) 11, 16
Ireland 30
Jablonski
(2005) 25
& Lutz (1983) 13
Janeia 16, 20
primaeva 1, 20
"Janeia"
silurica 11, 17, 20, 27, 34
truncata 26
(Janeia), Solemya
primaeva 30
trapezoides 31
janewiedlini, Aviculopecten 6
Jell (1980) 13
jellyfish 36
Johnson
(1962) 1, 7-8, 37
& Richardson (1966) 1-3, 5, 23, 25
& Richardson (1968) 36
& Richardson (1970) 1-5, 23, 52
johnsoni, Acharax 15-18, 26, 28-29
Johnston
(1993) 6, 21, 25
& Collom (1998) 11
pers. comm. (2010) 25
Kamenev (2009) 16-19, 27-28
Kammer & Lake (2001) 7
Kanie & Nishida (2000) 9
Kankakee County [Illinois] 7, 21, 23, 25, 28, 35, 52, 55, 59-60
Kanno et al. (1998) 17
Kansas 1, 3, 25-26, 30, 33-34, 56
Kentucky 27, 31-33, 38
Kerton Creek Coal [of Illinois] 37
Keasey Formation [of Oregon] 26
Kidwell
(2007) 36
(2008) 36
Kiel
(2007) 17
et al. (2008) 8-9, 14
King (1850) 16, 20
Korobkov (1954) 10-11
Krainer et al. (2003) 31, 34
Kříž (2007) 11
Krueger et al. (1992) 8
Krumbein & Garrels (1952) 4
Kues
(1992a) 5, 8, 22, 27, 29, 31, 34, 37
(1992b) 7
et al. (2002) 31, 38
Kuroda (1931) 16-17
Kuznetsov & Shileiko (1984) 8, 18, 20
labial palps 8-9, 11, 14-15
Lamarck
(1799) 12
(1818) 1-2, 15-17
lamellar ligamental layer [see ligamental layers]
lamellar/lamelliform ridges 21, 24, 28, 31-33, 52, 55, 56, 59
Lametilidae 14
lappets, periostracal [see frill, periostracal]
Lascoa mesostuarata 36
lateralization (of valves) 19, 33
Lebold & Kammer (2006) 3
lectotype [of Acharax trapezoides] 1, 20, 31-32, 33-34, 60, 5
Ledidae 10
Leptodesma ohioense 6
Lesquereux (1870) 4
levis, Clinopistha 21-22, 37
ligament
primary 14, 16-29, 31-35, 52, 55-56, 59
secondary 14, 21 [see also anterior ligamental extension
(ALE)]
ligamental layers
fibrous (inner) layer 10, 16, 18-19, 24, 32-33
lamellar (outer) ligamental layer 16, 18-19, 23-24, 27, 31-32,
52
Liljedahl
(1984a) 9-12, 17, 27, 29
(1984b) 9-12, 20, 24, 27, 29
(1991) 8, 10
limatula, Yoldia 13
Limoidea 7
Link (1807) 12
Linnaeus (1758) 10
Lintz (1958) 27
Lipodonta 11
Lipodontida 11
Literature Cited 38-50
Little & Vrijenhoek (2003) 36
Liverpool Cyclothem 1, 37
Livingston County [Illinois] 21, 23, 25
Llangynog Inlier [of Wales] 8
Lo et al. (2008) 36
lobes
ligamental [see pads, ligamental]
mantle 9, 14-15
Locality 22 [of Baird] 2
Locality 196 [of Baird] 23
longiaxis/breviaxis ratio 18, 23, 25, 28, 30, 35
longiaxis/longiaxial, definition of 3
longidorsum/longidorsal, definition of 3
longiterminus/longiterminal, definition of 3
Los Angeles [California] 8
Lovley (1995) 4
lumen (of hind gut) 15
Macomb [Illinois] 1-2
Magoffin Member [Breathitt Formation of Kentucky] 27, 31-33,
38
malletiid 19, 33
Malletiidae 11
Manzanella 11, 14
Manzanellidae 8, 11, 15
manzanelloids 10
Manzanelloidea 14-15
Marginifera 7
Mangion et al. (2004) 36
mantle fusion 1, 9-10, 13, 15
Mark (1912) 27
mass mortality 5, 24
Martin (1999) 2-3
Materials and Terminology 2-3
Fossil Repositories 2
Shell Orientation Terms 2-3
Mattoon Formation [of Illinois] 7
Maxwell (1988) 11, 21
Mazon Creek Bivalve Assemblages 5-8
Mazon Creek [Fauna/Lagerstätte] 1-7, 21, 23, 26, 34, 36, 55
Braidwood [fauna/biota/biofacies] 2-3, 5-6, 25, 36, 38
Essex [fauna/biofacies] 1-8, 25, 28-30, 33-38, 60
Mazon Creek Project 2, 7
mazonensis
Aviculopecten 6
n. sp., Mazonomya n. gen. 1-2, 6, 8, 18, 21, 23-26, 28-29, 34,
36-38, 52, 55; 1-2
Mazonian coal swamp 36
67
Mazonian delta complex 3, 36
Mazonomya n. gen. 1-6, 8, 18, 21-23, 24-26, 28-30, 34-38, 52, 55
mazonensis n. sp. 1-2, 6, 8, 18, 21, 23-26, 28-29, 34, 36-38,
52, 55; 1-2
McAlester (1969) 12
McChesney (1860) 33
McMahon & Reid (1984) 8
M’Coy
(1844) 1, 27, 30, 31
(1851) 7
(1855) 6, 26
mediterranea, Solemya 15
Meek
(1871) 6, 26
(1872) 26
(1874a) 1, 6, 13, 20, 26, 31-32, 60
(1874b) 7
(1875) 6
(1876) 20
& Worthen (1860) 1-2, 6, 8, 27-29, 56, 59
& Worthen (1866) 27
& Worthen (1870) 16, 21
& Worthen (1873) 6-7, 26, 31-32, 34, 37, 60
meeki, Myalinella 6
Member 98, Springfield (No. 5) Coal [of Illinois] 7, 22, 34, 37
Menke (1828) 27
mesostuarata, Lascoa 36
Mighels & Adams (1842) 13
mikasaensis, Acharax 14
Miller
(1877) 27
(1889) 27
Mills (2001) 36
Missouri 1, 2, 25, 33-34, 37
Mizzaro-Wimmer & Salvini-Plawen (2001) 13
Möllbos [Gotland] 8
Möller (1842) 12
molluscan cross 13
Moore (1964) 3
Morningstar (1922) 6, 27
Morse & Meyhöfer (1990) 13
Morton
(1977) 9
(1996) 11
Myalinella 5, 7
meeki 6
sp. 6
Myalinidae 6
Mytiloidea 7
Mytilus solen 15
(Nacrosolemya), Acharax 5, 17, 19, 27, 29, 31
trapezoides 1, 6, 13-14, 18-22, 25-26, 28, 30, 31-35, 37-38, 60;
5
Nagel-Myers et al. (2008) 11
Nebraska 26, 33-34
NEIU [Northeastern Illinois University, Chicago] catalog numbers:
MCP 162 28, 59; 4
68
MCP 249 25
MP 110 23
MP 163 23
Nepiomorphia 11
Neulinger et al. (2006) 8, 21, 26
Neumayr (1884) 11
New Brunswick [Canada] 1, 22
Newell
(1937) 6-7
(1942) 5, 7
(1965) 11
(1969a) 11, 14
(1969b) 21
New Mexico 2, 33-34, 37-38
NMMNH [New Mexico Museum of Natural History, Albuquerque],
catalog number: 39021 34
Northumberland [England] 30
Nucinella 11
Nucinellacea 14-15
Nucinellidae 8, 11, 14-15
Nucinellina 14
Nucula 12
delphinodonta 13
proxima 13
Nuculana 12
Nuculaniformii 12
Nuculanoida 12-13
Nuculanoidea 7, 12
nuculanoid 2, 5, 8, 10, 12-14, 37
Nuculidae 10-12
Nuculiformii 6, 12
Nuculoida 11-12, 15
Nuculoidea 7, 12
nuculoid 5, 8, 10-13, 37
Nuculoidea
deceptriformis 33
corbuliformis 33
Nunavut [Arctic Canada] 19, 26, 33, 56
nymphae
external 14, 17-18, 21-23, 26-28, 31-32, 34
internal 14, 17-18, 26
NYSM [New York State Museum, Albany] catalog numbers:
I 8756 22
I 8757 22
Oak Grove Limestone [of Illinois] 7
oblonga, Edmondia 6, 26
obrution 4
Octomedusa pieckorum 36
Ohio 21, 33, 37
ohioense
Anthraconaia 6
Leptodesma 6
Oklahoma 7, 25
Økland & Økland (1986) 5
Okusu (2002) 13
Opponobranchia 12-13
orbicular muscles, definition of 23-24, 28-29, 32, 52
Orbiculoidea 7, 37
Orbiculoidea Association 37
Oregon 26
Orientation Terms, Shell 2-3
Oschmann (1991) 5
Oso Ridge Member, Abo Formation [of New Mexico] 34-35
Ottawa [Illinois] 25
outer ligamental layer [see ligamental layers]
Ovatoconcha 17
fragilis 10-12
ovata, Edmondia 6-7, 26
Owen
(1959) 11, 13, 15, 18
(1961) 8, 13-14, 33
oweni, Palaeoneilo 33
overlap (of valves) 17, 19-20, 27-28, 30, 32, 56, 60
ovum, Dacromya 13
Oxford Clay [of England] 9
pads, ligamental 19, 27, 31
Palaeolima retifera 6
Palaeoneilo 19, 33
elliptica 33
filosa 33
oweni 33
tebagaensis 33
Palaeotaxodonta 10-11
Palaeotaxodontida 11
Paleosolemya 16
Paleyoldia
angustia 13
glabra 13
Paleoenvironmental Setting 3
pallial band 9, 14, 21, 23-24, 28-29, 32, 52, 56
palp proboscides 11, 14
papillae
foot 10
mantle 15
siphonal 13
parallela [of Ryckholt, 1853 (1854)]
Acharax 37
Solemya 1, 30, 37
parallela [of Beede & Rogers, 1899]
Acharax 37
Solemya 1, 28, 30, 35, 37
Parallelodon haimeanus 25
paratypes [of Mazonomya mazonensis n. gen., n. sp.] 23, 52, 55,
1-2
parivincular wide (PW) ligament 18, 20, 29
PW-external 18, 20-21, 28-29, 31
PW-internal 18, 20
parkinsoni
Solemya 15-17
Solemya (Zesolemya) 13, 18-20, 22, 24-26, 29, 31, 33
partitioning (of oxygen and sulfide) 8
Peabody Coal Company Pit 11 23, 28, 35, 52, 59-60
peat mire 3, 25
Peat Swamp-Induced Eutrophication 35-38
Essex Eutrophication 35-36
Recurring Solemyid Guilds as Markers of Recurrent Eutrophication 37-38
Peck et al. (2009) 14
Peckmann et al. (2001) 9
Pectinoidea 7
pedal/visceral muscle complex 10, 12, 21, 24, 28, 32-33
pellucida, Euchondria 6
Pelseneer
(1889) 10
(1891) 8
(1911) 10
Peppers
(1996) 1
& Pfefferkorn (1970) 36
pericalymma 13, 15
periostracum/periostracal
extension 16, 22, 52, 55, 59
frill 1, 9-10, 15, 24, 28-29, 56, 59
pleats 9-10, 29, 34, 60
selvage 28-29, 56, 59
span 22-24, 28-29
Periploma 12
Pfefferkorn (1979) 36
Phillips
(1836) 1, 20, 30, 35
(1848) 25
pieckorum, Octomedusa 36
Permophoridae 6
Permophorus
spinulosa 6
sp. 6
(Petrasma), Solemya 15-17, 19, 26
atacama 18, 20
borealis 15, 17, 20
valvulus 20, 29
velum 8, 13, 16, 19-20, 27
Phestia
bellistriata 13
corrugata 13
pholadomyoid 1-3, 8, 21, 25
Pholadomyoidea 7
Pine Shadow Member [Wild Cow Formation of New Mexico] 37
Pit Eleven/Pit 11 [see Peabody Coal Company Pit 11]
plants 36
plates, ligamental [see pads, ligamental]
Plates 51-61
pleats, periostracal 9-10, 29, 34, 60
plicae, radial 9-10, 17, 28-30, 34, 56, 59-60
poikiloaerobic/aerobic episodes 5
Pojeta
(1978) 11
(1988) 2, 8, 10-12, 14-16, 18-22, 25-27, 29-35, 56
& Runnegar (1985) 11, 16
Pojetaia runnegari 13
Poli (1795) 15-17
polychaetes 13
Posidonia fracta 6
Posidonienschiefer 9
posterior (exhalant) aperture [of the mantle] 9, 13-14, 18, 29
Powell & Somero (1986) 8
Praecardioida 11
Praenuculidae 12
Prezant
(1998a) 3
(1998b) 11
(1998c) 21
Price (1918) 27
primary ligament [see ligament]
primaeva
Acharax 1, 30, 35, 37
Janeia 1, 20
Solemya 1, 20, 27, 30
Solemya (Janeia) 30
prop [see buttress]
Prosh (1988) 19, 23, 26-27, 33, 56
Proteobacteria 8
Protobranchia 1, 6, 8, 10-12
protobranch 2, 8, 10-14, 19, 25
protobranch gills 10-12
proxima, Nucula 13
Pruvost (1930) 2
pseudomorphs [after pyrite] 4, 8, 22, 55, 59
Pterineidae 6
Pterioidea 7
Pterinopectinidae 6
Pteriomorphia 6, 11
Psiloconcha 11-12, 16, 18, 21, 25
grandis 12
Purcell et al. (2007) 36
Purchon
(1956) 11
(1959) 11
(1987) 11
Putnam Hill shales and limestones [of Ohio] 33, 37
puzosiana, Solemya 1, 27, 30
PW ligament [see parivincular wide ligament]
Pye (1984) 4
pyrite 4, 8, 22, 55, 59
Quenstedt (1930) 11, 26
radiata
Acharax 1, 8, 19-26, 27-31, 33, 35, 37-38, 56, 59; 3-4
Solemya 1-2, 20, 23, 25, 27, 30-31
radial "rib" (AR) 32, 33-34, 60
radii 9, 17, 21-22, 30-32, 34-35, 60
Redoak Hollow Formation [of Oklahoma] 25
Redesdale Ironstone [of Northumberland, England] 30
reflexa, Edmondia 26
reidi, Solemya 8, 13, 19-20, 29, 32
Reid
(1980) 8-9, 18, 36
(1986) 14
(1989) 8
(1990) 8
69
70
(1998) 4, 8-11, 14-15, 20, 32, 36
& Bernard (1980) 8-9
repair ligament 19-20
repositories, abbreviations 2
resilifer 10, 14
resilium 10
Reticulomedusa greenei 36
retifera, Palaeolima 6
Rhoads & Morse (1971) 5
rib (of Dall) [see buttress]
Rice
(1986) 38
et al. (1979) 38
Richardson & Johnson (1971) 1, 3-4, 23, 25, 29
Riding et al. (1998) 8
ridge (of Dall) [see buttress]
Robin Hood's Bay [Yorkshire, England] 4
Rock Island (No. 1) Coal [of Illinois] 37
roof shales 1, 3, 21, 30, 34, 36-37
Rollins, Harold B. 21
Runnegar
(1974) 21
& Bentley (1983) 13
& Newell (1974) 21, 26
runnegari, Pojetaia 13
Ryckholt, (1853 [1854]) 1, 27, 30, 37
Ryderia graphica 13
Sageman et al. (1991) 36
saginata, Solemya 1, 27, 30
Salis-Marschlins (1793) 15
Salter (1852) 12
salt marsh(es) 3, 8, 37
Salvini-Plawen
(1973) 13
(1980) 13
& Steiner (1996) 11-12
Sanders & Allen (1973) 11, 14
Sanguinolites costellatus 30
Savrda & Bottjer
(1987) 5, 9
(1991) 4
Say
(1822) 8, 13, 16
(1831) 13
Scaphopoda 13
scaphopods 13
Scarlato & Starobogatov
(1978) 13
(1979) 11, 14, 20-21
Schaefer (2000) 12
Schellenberg (2002) 3-4
Schizodidae 6
Schizodus
cf. affinis 6
cf. wheeleri 6
Schmidt et al. (2008) 36
Schneider (2001) 11
Schopf (1979) 4, 36
Schram
(1979) 1-3, 23
et al. (1997) 36
Schumacher (1817) 12
Schuyler County [Illinois] 28, 56
Scott
(2005) 15
& Cavanaugh (2007) 4
Seahorne Coal [of Illinois] 37
Searcy County [Arkansas] 7
secondary ligament [see ligament]
Sedgwickia? 6
Seilacher
(1990) 9-10, 29, 36, 38
et al. (1985) 9
Sellwood (1971) 3-4
selvage, periostracal 28-29, 56, 59
Sepkoski (1981) 1
Septimyalina 7
sewage 8, 36
Shabica
(1970) 3-4
(1971) 4
(1979) 3
Shepard (1986) 8, 36
shell composition and microstructure
aragonitic 10, 13, 31-32
crossed lamellar (CL) 10
crossed acicular 10
complex crossed lamellar (CCL) 10
homogeneous 10, 32
nacreous 10, 13, 31-32
nacreous films 13
nondenticular composite prismatic 21
non-nacreous 13, 31-32
porcelaneous 31-32
prismatic 10, 13, 15, 18, 20-21, 31-32
shrimp 36
Shumard (1858) 6
Shumway Limestone [of Illinois] 7
siderite/sideritic
concretions 1-4, 7, 24-25, 30, 37, 52, 55, 59-60
concretionary diagenesis 4
microbial mediation 4
pseudomorphs 4, 8, 55, 59
Siliculidae 14
silurica, "Janeia" 11, 17, 20, 27, 34
siphon development 13-14
sipunculids 13
sitz marks 24, 29
span, periostracal 22-24, 28-29
sphalerite 4, 8
Solemya [also Solenomya nom. van.] 1-2, 8, 12, 14-22, 24-29, 36
borealis 15, 17
costellata 1, 27, 30-31, 34
excisa 1, 27, 34
johnsoni 15, 17, 26
mediterranea 15
parallela
Ryckholt, 1853 (1854) 1, 30, 37
Beede & Rogers, 1899 1, 28, 30, 35, 37
parkinsoni 15-17
primaeva 1, 20, 27, 30
puzosiana 1, 27, 30
radiata 1-2, 20, 23, 25, 27, 30-31
reidi 8, 13, 19-20, 29, 32
saginata 1, 27, 30
sp. 26, 33-34, 56
sp. indet. 31-32, 60
togata 15, 17
trapezoides 31-34, 60
(Austrosolemya) 16-17, 19, 26, 27
australis 16-18, 20, 22, 24-25, 27, 29, 31, 33
(Janeia)
primaeva 30
trapezoides 31
(Petrasma) 15-17, 19, 26
atacama 18, 20
borealis 20
valvulus 20, 29
velum 8, 13, 16, 19-20, 27
(Solemya) 15-16, 19, 26
togata 16, 18, 20, 24, 26
(Solemyarina) 16-17, 19, 26-27
velesiana 8, 16-18, 20
(Zesolemya) 16-17, 19, 26-27
parkinsoni 13, 15-20, 22, 24-26, 29, 31, 33
Solemya s.s. 16-17, 20
"Solemya" 1
radiata 20, 22, 25
trapezoides 20, 22
(Solemya), Solemya 15-16, 19, 26
togata 16, 18, 20, 24, 26
Solemyarina 15-17
(Solemyarina), Solemya 16-17, 19, 26-27
velesiana 8, 16-18, 20
Solemyatuba 9
Solemyid Autecology and Functional Morphology 8-10
Autecology 8-9
Functional Morphology 9-10
Solemyidae 1-2, 6, 8-11, 14, 15-20, 21-22
solemyin[s] 19-22, 24, 26-31, 33
Solemyina 14
Solemyinae 2, 18, 20-21, 22, 35
Solemyoida 8, 11-12, 14-15
Solemyoidea 7, 12, 14, 15-20
solen
Mytilus 15
Solemya 15
Solenomya nom. van. [see Solemya]
Sowerby (1825) 13
Sparland Coal [of Illinois] 37
Spathelopsis 2, 14
71
spinulosa, Permophorus 6
splendens, Edmondia 23, 25-26
Springfield (No. 5) Coal Member [of Illinois] 7, 22, 34, 37
Springfield, Illinois 2
Sroka (1984) 2, 6, 23, 25-26
Smith, E. (1874) 13-17
Smith, W. (1970) 1
St. David's Cyclothem 37
stagnation deposit Lagerstätten 9
Stanley (1970) 8, 10, 19
Stark County [Illinois] 7, 37
Starobogatov (1992) 12
Stasek (1961) 11
Steiner & Hammer (2000) 11
Stempell
(1898a) 11
(1898b) 11
(1899) 11
stenocalymma 13
Stevens (1858) 6, 13
Stewart & Cavanaugh (2006) 8
striata, Dunbarella 6
subnasuta, Dystactella 22
sulfate-reducing bacteria 3-4
sulfide conduit 9
sulfur-oxidizing
bacteria 14-15, 20
mitochondria 8
Summum (No. 4) Coal [of Illinois] 37
Swallow (1863) 6
symbiosis, bacterial 8, 10, 15
syncarid 36
Systematic Paleontology 10-35
Tables 6, 7, 17
Tancrediopsis 12
taxodont [dentition] 8, 10-12, 14-15, 33
Taylor
et al. (1969) 13, 15, 19
et al. (2008) 12-13, 16-19, 27
pers. comm. (2010) 16, 18
tebagaensis, Palaeoneilo 33
Tellina togata 15
Termier & Termier (1959) 33
Text figures 2, 16, 22, 27, 37
tier
aerobic 8
dysaerobic 8
exaerobic 5, 9
Tironuculidae 12
togata
Solemya 15, 17
Solemya (Solemya) 16, 18, 20, 24, 26
Tellina 15
Toledo-Rivera et al. (2009) 36
Totten (1834) 15, 17
trapezoides
Acharax (Nacrosolemya) 1, 6, 13-14, 18-22, 25-26, 28, 30, 31-
72
35, 37-38, 60; 5
(Janeia), Solemya 31
Solemya 31-34, 60
Trey Cliff [Derbyshire, England] 26
Trigonioidea 7
Trueman & Weir (1946) 2
truncata, "Janeia" 26
tubercular shell ornament 21, 23
turnbulli, Anthracomedusa 36
Type I stomach 11
Type 2 oxygen-deficient paleoenvironment 36
Ulrich (1894) 11, 16
Ulster [Northern Ireland] 30
UM [University of Missouri, Columbia] catalog number: 13331
34
UNC [University of North Carolina, Chapel Hill] catalog number:
13659b 31
UNM [University of New Mexico, Albuquerque] catalog numbers:
11134 34
11135 34
Urbana [Illinois] 2
U-shape(d) [burrow] 9, 19, 29
USNM [National Museum of Natural History, Washington, DC]
catalog numbers:
32987 33-34
36315 31, 60, 5
415967 33-34, 56, 3
valve asymmetry 17, 19-20, 27-29, 31, 33
visceral/pedal muscles 10, 12, 21, 24, 28, 32-33
valvulus, Solemya (Petrasma) 20, 29
Van Buren County [Arkansas] 7
Vancouver Island [British Columbia, Canada] 8, 36
velesiana, Solemya (Solemyarina) 8, 16-18, 20
velum, Solemya (Petrasma) 8, 13, 16, 19-20, 27
Vesicomyidae 17
Vokes
(1955) 16, 22, 26
(1956) 11, 15
(1980) 11
Waagen (1884) 7
Wales 8, 10
Waller
(1990) 11, 13, 19, 27
(1998) 8, 10-12, 15, 18
Wanless
(1958) 7-8, 30, 37
(1975) 25
& Weller (1932) 25
& Wright (1978) 3, 25
Washington (state ) 26
Washington, DC 2
Waterhouse (1969) 3
Weir-Pittsburg Coal [of Missouri] 34, 37
Weller (1957) 37
Westphalian D 1, 23, 28, 33, 35, 52, 56, 60
Wewoka
Formation [of Oklahoma] 7
Quadrangle [Oklahoma] 7
whale carcasses 8
wheeleri, Schizodus 6
Whiskey Creek methane-seep deposit 26, 35
White (1993) 4
Wignall & Hallam (1991) 5
Wilbur (1983) 5
Wild (1990) 9
Wild Cow Formation [of New Mexico] 37
Will County, Illinois 7, 21, 23, 25, 28, 33, 35, 52, 55, 59-60
Windsor [Missouri] 25
Wittry (2006) 36
WIU [Western Illinois University, Macomb] catalog numbers:
MC 1120 23, 55; 2
MC 1121 35, 60; 5
MC 1122 35, 60; 5
MC 1145 22
Wöhrmann (1893) 11
Wolf Covered Bridge [Knox County, Illinois] 7
Wood (1851) 11
Woodland & Stenstrom (1979) 4
Woodring (1938) 17
Worthen (1890) 6
Wright (1965) 3, 25
Wymore [Nebraska] 33-34
Wyoming [Illinois] 37
Yochelson & Richardson (1979) 23
Yoldia 12
limatula 13
Yonge
(1939) 11, 29
(1959) 11
Yorkshire [England] 4, 20, 34
Young Island [Nunavut, Arctic Canada] 19, 20, 26-27, 33, 56
Y-shape(d) [burrow] 8-9, 19
Zardus
(2002) 8, 10-13, 21, 25
& Morse (1998) 13
Zesolemya 16-17
(Zesolemya), Solemya 16-17, 19, 26-27
parkinsoni 13, 18-20, 22, 24-26, 29, 31, 33
Zhang & Pojeta (1986) 30
Zuni Mountains [of New Mexico] 34
73
74
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