09-Jenks (Churkites).p65

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09-Jenks (Churkites).p65
Lucas, S.G. and Spielmann, J.A., eds., 2007, Triassic of the American West. New Mexico Museum of Natural History and Science Bulletin 40.
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SMITHIAN (EARLY TRIASSIC) AMMONOID BIOSTRATIGRAPHY AT
CRITTENDEN SPRINGS, ELKO COUNTY, NEVADA AND
A NEW AMMONOID FROM THE MEEKOCERAS GRACILITATIS ZONE
JAMES F. JENKS
1134 Johnson Ridge Lane, West Jordan, UT 84084; [email protected]
Abstract—Crittenden Springs in northeastern Nevada yields M. gracilitatis Zone ammonoids renowned for the
finely detailed, exquisite preservation of the test, which sometimes even displays relict color patterns. Evidence
reveals most of the fossiliferous Lower Triassic beds in the Crittenden Springs area are overturned and this
conclusion places the Meekoceras gracilitatis Zone portion of the section and the immediately adjacent, but
condensed Anasibirites fauna in the proper biostratigraphic sequence. The paleoequatorial affinity of the western
US Smithian ammonoid fauna is affirmed, and the co-occurrence of certain ammonoids from western US and
Tethyan low paleolatitude localities demonstrates faunal exchange between both sides of Panthalassa during the
Early Triassic. One of these co-occurring ammonoids, Churkites noblei n. sp. from the Meekoceras gracilitatis
Zone is herein described and compared to two species of Churkites from the Anasibirites nevolini Zone of the
Russian Far East.
INTRODUCTION
Early Triassic ammonoids from Crittenden Springs have long been
recognized for their exceptional quality of preservation. Kummel and
Steele (1962) stated that these ammonoids represent “by all odds, the
finest preserved Meekoceras fauna known” from any of the other Early
Triassic localities in the western US. Many ammonoids are complete and
retain all or nearly the entire test. Although the test is re-crystallized, its
preservation is such that the finest of details are preserved on numerous
specimens, including the “wrinkle” layer and associated “black layer”
and in rarer cases, relict color patterns (R.H. Mapes, personal
commun.1987). Evidence of sub-lethal shell damage due to various types
of predators and stresses is also present on some exceptionally well
preserved specimens (H. Keupp, personal commun. 1997). In the mid1980’s, an extensive investigation of color patterns preserved on
Crittenden Springs ammonoids was undertaken by Royal H. Mapes of
the Ohio University. He was later joined by graduate student G.E.
Gardner. Their study revealed that many different genera and species
retain transverse or longitudinal color patterns, while others are simply
monochromatic (Mapes and Sneck 1987; Gardner and Mapes 2000; and
Gardner 2000). Well preserved ammonoids representative of the
Anasibirites kingianus Zone also occur at Crittenden Springs, but they
are not as plentiful as those from the Meekoceras gracilitatis Zone.
(Note: The Anasibirites kingianus Zone is used throughout this paper in
the context of a worldwide ammonoid zone even though the fauna from
the Anasibirites beds of the western U.S. has not been formally assigned
to an ammonoid zone.) Intensive collecting at Crittenden Springs by the
writer over a period of 30 years has revealed the existence of at least
seven probable new ammonoid taxa. Five of these taxa belong to the
Meekoceras gracilitatis Zone, while two are assignable to the Anasibirites
kingianus Zone. Most of the different taxa are represented by several
specimens, whereas two very rare forms consist of only one or two
specimens. One of the new taxa, Churkites noblei n. sp., from the
Meekoceras gracilitatis Zone is herein described and compared with two
very similar ammonoids described from the Anasibirites nevolini Zone of
Southern Primorye, Churkites egregius Zharnikova & Okuneva, 1990
and Churkites syaskoi Zakharov & Shigeta, 2004.
LOCALITY AND GEOLOGICAL CONTEXT
The Crittenden Springs ammonoid site, situated approximately
29 km north of Montello in northeastern Elko County, Nevada, is located about two km northeast of Crittenden Springs on the north side of
Long Canyon. Although several intermittent fossiliferous exposures exist in the hills to the northeast, the main collecting locality is situated on
a south facing hillside immediately north of the Long Canyon road in the
SW1/4SW1/4 sec. 34, T43N, R69E, W1/2SW1/4 sec.3, T42N, R69E
(Kummel and Steele, 1962).
Lower Triassic sediments are exposed for several km in the hills
N-NE of the Long Canyon road (Clark, 1957). Although highly faulted,
these beds, which generally strike N-NE and dip at varying angles to the
W-NW, consist of portions of the Dinwoody Formation and the overlying Thaynes Formation (Mullen, 1985). The ammonoid-bearing beds
occupy the lowermost portion of the Thaynes Formation and are, in
turn, overlain by an upper shale and limestone unit, which includes a thin
black calcareous shale interval at its base (Clark, 1957).
Considerable confusion has existed for many years regarding the
geology and biostratigraphy of the fossil bearing beds at this locality.
Kummel and Steele (1962) reported that ammonoids of the Meekoceras
gracilitatis Zone occur in each of three distinct beds over a stratigraphic
distance of 175 feet (designated as units “a”, “d” and “g”, in descending
order, with unit “a” supposedly containing the better preserved and
more diverse fauna). Silberling and Tozer (1968) recognized that these
beds were actually a repetition of the same bed by faulting. Furthermore,
they found Anasibirites and Wasatchites near the bottom of unit “g”.
Since the Anasibirites fauna normally occurs above the Meekoceras fauna,
this occurrence was in clear disagreement with the accepted biostratigraphic sequence.
Throughout the latter half of the 1970’s and early 1980’s, the
writer and other collectors collected ammonoids from several isolated,
discontinuous beds or blocks exposed on the south-southeast facing
hillside (easternmost unit of Kummel & Steele’s three fossiliferous units)
at the main Crittenden Springs locality. In nearly all cases, each of these
generally N-NE striking, W-NW dipping blocks consisted of the entire
thickness (106 to 122 cm) of a typical bed as depicted in Figure 1. Most
blocks had been sheared across strike at fairly high angles, and although
variable in length, a couple of the longer blocks were followed along
strike for approximately 3 to 4 m before either the end was reached or the
amount of overburden became prohibitive. The blocks were obviously
bedded and they could usually be broken along bedding planes, which
greatly facilitated excavation. By the mid 1980’s, these exposures were
either exhausted or reduced to the point that it was nearly impossible to
remove fossiliferous limestone without removing great quantities of overburden. Consequently, a bulldozer was utilized in 1988 in conjunction
with Royal Mapes’ study to better expose the fossiliferous blocks on
82
tice was acknowledged by Orchard and Tozer (1997), and followed by
Gardner, (2000) and Gardner and Mapes (2000). In retrospect, although
the Canadian zones are more or less contemporaneous with those of the
western US, they represent ammonoid faunas from the mid and higher
paleolatitudes, i.e., northeastern British Columbia and the Canadian Arctic, whereas the somewhat more diverse faunas from the western US,
represented by the Meekoceras gracilitatis and Anasibirites kingianus
Zones, are representative of the low paleolatitudes (Nichols and Silberling,
1979). Although Anasibirites is not restricted to the low paleolatitudes,
it is a rather negligible component of the Anawasatchites tardus Zone of
mid-paleolatitude, northeastern British Columbia (Tozer, 1994), whereas
in the western US, it is a near dominant member of the Anasibirites
kingianus Zone.
PANTHALASSIC CORRELATION
The paleoequatorial nature of the Meekoceras gracilitatis assemblage is illustrated by the common occurrence of such ammonoid genera
as Aspenites, Lanceolites, Inyoites and Owenites within a narrow intertropical band between the Tethys on the west and the Panthalassic
basins (California, Nevada, Idaho) on the east (Brayard et al., 2006). One
particular Tethyan paleoequatorial locality, the South China Block, has
recently been shown to contain a few additional ammonoid genera that
are common to each side of the Panthalassic Ocean (Brayard and Bucher,
unpubl. 2006). The Russian South Primorye has also been shown to
contain many ammonoids with paleoequatorial affinities (Zakharov et
al., 2002). One of these forms, an acute-ventered arctoceratid, Churkites
syaskoi Zakharov & Shigeta, 2004 described from Primorye, is very
similar to the slightly older, new taxa herein described. This occurrence
further demonstrates equatorial faunal exchange between both sides of
Panthalassa during the Early Triassic.
BIOSTRATIGRAPHY
FIGURE 1. Stratigraphic section of a typical ammonoid bearing block at
Crittenden Springs. As indicated a few M. gracilitatis Zone ammonoid taxa
occur only at certain levels. All other Gracilitatis Zone taxa occur throughout
the M. gracilitatis Zone portion of the block.
the same hillside. This excavation not only made possible the collection
of hundreds of ammonoids with preserved color patterns, but it also
uncovered three hitherto unknown blocks and provided a much better
understanding of the geology and biostratigraphy of the site. Foremost,
the highly faulted nature of the fossiliferous beds became even more
readily apparent. Prior to the bulldozer excavation, it had been noted that
with the exception of one block, all of the generally N-NE striking isolated blocks from which ammonoids had been collected were oriented
with the Anasibirites fauna at the bottom. This abnormality was also
found to be true for two of the three newly excavated blocks. Most of
these highly fractured, discontinuous blocks were found to be complete
and nearly identical with respect to fossil content and lithology. Only
one block appeared to have been sheared laterally. No significant difference in faunal makeup was noted at corresponding levels within each bed
and, furthermore, a variance in the lithology of the limestone (color, grain
size, etc.) at different levels in each block was apparent, but this variance
appeared to be quite similar from block to block. Correspondingly, the
quality of fossil preservation varied with lithology, but was also similar
in most blocks. However, in a few cases fossil preservation and lithology
were found to vary laterally in the same bed. Most of the blocks were
bounded by obvious “fault gouge” material.
PALEOLATITUDINAL CORRELATION
Although the Meekoceras fauna from the various western US
localities had long been assigned to the Meekoceras gracilitatis Zone, the
writer, in the early 1990’s, began referring Crittenden Springs ammonoids
and those from various Bear Lake Valley, Idaho localities to the Canadian
Euflemingites romunderi and Anawasatchites tardus Zones. This prac-
Figure 1 illustrates a stratigraphic section of a typical fossiliferous
limestone block, but it is shown in a biostratigraphically upright position
rather than the upside down orientation in which most of the blocks are
found. Obviously, the relationship between the Meekoceras and
Anasibirites faunas is very unusual since no stratigraphic separation
exists between them, and the total thickness of the horizon containing
the Anasibirites fauna is usually only about 5 cm. Despite this close
proximity, the author has never found any sign of intermixing of the two
faunas. As shown in Figure 1, most of the Meekoceras gracilitatis Zone
ammonoids occur throughout the typical 115 cm thick M. gracilitatis
Zone portion of the bed, but others are found only at certain horizons.
The uppermost 5.0 to 7.5 cm portion of the M. gracilitatis Zone is
unusual, not just because certain ammonoids are found only at this level,
but on infrequent occasions, other species that are rare at Crittenden
Springs are found only within this interval in unusually high numbers
and in very close proximity. This unusual accumulation of rare species
was depicted as occurring within a lense in Gardner and Mapes (2000),
but in actuality, no lense structures are apparent within this interval. A
plausible explanation of the mechanism responsible for this unusual
occurrence remains elusive.
The bed containing the Anasibirites assemblage consists of a 5.0
to 7.5 cm thick brown-black limestone packed with bivalves and ammonoids that include abundant Xenoceltites youngi Kummel and Steel,
Pseudosageceras multilobatum Noetling, rare specimens very similar to
Xenoceltites subevolutus Spath, and at least two new ammonoid taxa
represented by only a few specimens, as well as orthoconic nautiloids.
In addition, lenses containing Anasibirites kingianus, Wasatchites sp.
and various other prionitids occur infrequently between the topmost bed
of the M. gracilitatis Zone and the 5.0 -7.5 cm thick brown-black limestone bed. Most of these lenses are only 10 cm or so in diameter and 3 to
5 cm thick, but they can range in size up to about 30 cm in diameter and
12 cm thick.
83
At the Georgetown, Idaho and Cephalopod Gulch (Salt Lake City)
sections, the M. gracilitatis and A. kingianus Zone are in a normal succession, but they are separated by a few meters of unfossiliferous limestone. At Crittenden Springs, this portion of the column is obviously
condensed. However, the uppermost surface of the M. gracilitatis Zone
and the condensed interval representing the A. kingianus Zone contain
evidence that seems to indicate the ammonoid succession is indeed normal, and that the blocks with the Anasibirites fauna on the bottom are
actually overturned. As depicted in Figure 1, the uppermost surface of
the bed containing the Meekoceras gracilitatis Zone ammonoids is always very heavily leached, as are the upper surfaces of ammonoids
preserved at this level. In some cases, this leaching or dissolution is so
severe that up to one quarter of the ammonoid thickness has been corroded away. In addition, similar evidence exists at the contact with the
upper surface of the lenses mentioned above and the brown-black limestone bed containing the abundant specimens of Xenoceltites youngi.
Again, the upper surfaces of ammonoids of the Anasibirites kingianus
Zone preserved at the top (Fig. 1) of the lenses are very heavily leached,
but sometimes even more so than ammonoids at the top of the M.
gracilitatis Zone. It seems that this phenomenon is what one might
expect to find on the upper side of ammonoid shells in a condensed
deposit in which deposition rates were extremely low.
SYSTEMATIC DESCRIPTION
The classification of Tozer (1981, 1994) is utilized for all systematic descriptions. All specimens of the type series of Churkites noblei n.
sp. are reposited in the New Mexico Museum of Natural History and
Science (NMMNHS).
Order CERATITIDA Hyatt, 1884
Superfamily Meekocerataceae Waagen, 1895
Family Arctoceratidae Arthaber, 1911
Genus Churkites Okuneva, 1990
Type species: Churkites egregius Zharnikova & Okuneva
FIGURE 2. Whorl sections of Churkites noblei n. sp. Note variation in
shape of venter. Clockwise from top left: NMMNH P-55141, Holotype;
55142, Paratype; 55144, Paratype, phragmocone of adolescent shell;
55143, Paratype; 400C, 1777C and 1776C. Scale bar = 1 cm.
Churkites noblei n. sp.
Figures 7-9.
2000 Arctoceras sp. – Gardner and Mapes, pl. 2, fig. 1
Diagnosis: Large-sized arctoceratid with an ovoid whorl section,
umbilical shoulders characterized by tuberculation, and a venter that
varies from narrowly rounded on juvenile and most adolescent whorls to
distinctively acute on mature whorls. Differs from Churkites egregius in
that it never develops a trapezoidal whorl section, and its ribbing (when
present) is not as dense or as strong and does not project forward at the
ventral shoulder.
Type series: Type series, five specimens: Holotype, specimen
NMMNH P-55141, Figure 7; Paratypes, four specimens: 55142, Figure
7; 55143, 55144 and 55145, Figure 8; Type series reposited in
NMMNHS. Other specimens: five specimens in writer’s private collection (400C, Figure 9; 1772C, 1774C, 1775C and 1776C) included in
species, but excluded from the type series.
Etymology: Species named in honor of the late Ed Noble of El
Cajon, California.
Description: Very large sized, moderately involute, compressed,
high whorled arctoceratid with gently convex flanks slowly converging
to very narrowly rounded venter on juvenile and adolescent whorls, but
becoming distinctively acute or angular with barely perceptible ventral
shoulders on adult whorls. Ovoid whorl section (Fig. 2) ranges from
highly compressed (whorl height greater than two times width) to more
robust forms (H/W = 1.64). Maximum whorl width occurs about midway between venter and umbilicus. Wide umbilicus with moderately
high, inclined wall and abruptly rounded shoulders. Umbilical wall quite
FIGURE 3. Whorl sections of Arctoceras tuberculatum (Smith). Note
variation in shape of venter. Clockwise from top left: 280C, 1779C, 688C,
1786C & 1778C. Specimens from writer’s private collection. Scale bar = 1
cm.
84
FIGURE 6. Suture lines of Churkites syaskoi Zakharov & Shigeta: Quarry,
village of Smolyaninovo. a , 3/830, H = 91 mm. b, holotype, 1/830, H = 71
mm. c, 2/830, H = 35 mm. d, 2/830, H = 12 mm. (Zakharov and Shigeta,
2004).
FIGURE 4. Suture lines of Churkites noblei n. sp. (A, B, C & D) and
Arctoceras tuberculatum (Smith) (E & F). A, NMMNH P-55141, Holotype,
H = 57 mm. B, NMMNH P-55142, Paratype, H = 64 mm. C, NMMNH P55143, Paratype, H = 37 mm. D, NMMNH P-55144, Paratype, H = 47
mm. E, 1770C, H = 44 mm. F, 1771C, H = 51 mm. Scale bar = 1 cm.
FIGURE 5. Whorl sections and suture lines of Churkites egregius Zharnikova
& Okuneva. Whorl height (H) is depicted as “B” in Okuneva’s caption
(Okuneva, 1990).
steep on early whorls, but gradually becoming more inclined throughout
ontogeny. Maximum inclination of about 32° on largest specimens.
Ontogenetic point of transition to acute venter is not size dependent and
varies from phragmocone on some specimens to body chamber on oth-
ers. Length of body chamber unknown, but at least ½ whorl. Ornamentation consists of weak to strong tuberculation on umbilical shoulders
varying in number from 8 to 14 per whorl on adolescent whorls, becoming more dense (30+ per whorl) on adult whorls. As these tubercles
become very dense on the last quarter or third of adult body chamber of
some specimens, they gradually become more bullate-like and culminate
in weak ridges on the umbilical wall. Flanks of most adult shells are
relatively smooth, whereas a few others bear highly variable, radial, but
ever so slightly sinuous fold-like ribs (Fig. 9DE), which vary greatly in
strength, width and frequency. In most cases, origination of ribs tends to
be associated with tuberculation on umbilical shoulder. Ribs rapidly fade
away on ventral shoulders. Well preserved shells exhibit very fine, narrow, barely perceptible strigate bands (Fig. 8F) consisting of fine, dense
radial lirae. Suture line (Fig. 4A-D) ceratitic, consisting of well-denticulated ventral, first lateral and second lateral lobes, as well as short auxiliary series. Width of lobes gradually decreases towards umbilicus. First
lateral saddle wide and well rounded, while second lateral saddle is somewhat narrower, but still well-rounded.
Measurements: See Table 1.
Discussion: The genus Churkites (type species, Churkites
egregius Zharnikova and Okuneva) was erected in 1990 by Okuneva for
an acute-ventered arctoceratid (Fig. 10A-D) found in the Anasibirites
nevolini Zone in the Artiomovka River area in Southern Primorye and the
Bolshie Churki Range in Southern Khabarovsk. The specimens shown in
Figure10 are paratypes. Apparently, the holotype has never been illustrated. In 2004, Zakharov and Shigeta described a similar acute-ventered
arctoceratid (Churkites syaskoi, Fig. 10, E-F), which was also found in
the A. nevolini Zone, but near the village of Smolyaninovo in Southern
Primorye.
Churkites noblei n. sp., C. egregius Zharnikova and Okuneva and
C. syaskoi Zakharov and Shigeta are similar in that each species is characterized by large sized shells (±300 mm), a narrowly rounded venter on
early whorls that gradually becomes acute with growth and the presence
of umbilical tuberculation.
Churkites noblei n. sp. differs from C. egregius in that its whorl
section remains ovoid throughout ontogeny, whereas that of C. egregius
varies from ovoid on smaller specimens to trapezoidal (Fig. 5) on larger
specimens (Okuneva, 1990: Y. Shigeta, personal commun., 2007). The
venter of C. egregius appears to be slightly more acute than C. noblei n.
sp. and it apparently becomes acute at a much smaller diameter. The
umbilical wall of C. egregius is quite steep, even on larger shells, while
that of C. noblei n. sp. gradually becomes significantly inclined. The
style of ribbing on each species is significantly different. Churkites
egregius bears radial ribs (Fig. 10A) that are consistently denser and
85
FIGURE 7. Churkites noblei n. sp. A-C, NMMNH P-55141, holotype in A, lateral, B, ventral and C, apertural views. D-F, NMMNH P-55142, paratype
in D, lateral, E, ventral and F, apertural views.
stronger than C. noblei n. sp. (Y. Shigeta, personal commun., 2007). As
with C. noblei n. sp., they originate with the tuberculation on the umbilical shoulder, but they become stronger and narrower as they approach
the ventral shoulder, where they are strongly projected forward and then
rapidly fade away before reaching the carina (Y. Shigeta, personal commun.,
2007). As the aperture is approached, the ribs tend to become even
denser, but less intense. It is not known if the shell of C. egregius bears
strigate ornamentation as does C. noblei n. sp. The lobes (Fig. 5) of C.
egregius appear to be slightly more denticulated than C. noblei n. sp.
However, for all practical purposes the suture lines of each species are
close enough so as not to be diagnostic.
Churkites noblei n. sp. differs significantly from Churkites syaskoi
Zakharov and Shigeta in that C. syaskoi also develops a trapezoidal
whorl section on larger shells, and its venter is more acute and becomes
86
FIGURE 8. Churkites noblei n. sp. A-B, NMMNH P-55143, paratype in A, lateral and B, ventral views. C-D, 1776C, specimen in C, lateral and D, ventral
views. E, NMMNH P-55145, paratype in lateral view. F, NMMNH P-55141, close-up of strigate ornamentation (horizontal) on holotype. G-I, NMMNH
P-55144, paratype in G, ventral, H, lateral and I, apertural views.
acute at a significantly smaller diameter (10 cm vs. 14.6 cm, respectively)
(Y. Shigeta, personal commun., 2007). As with C. egregius, the umbilical
wall of C. syaskoi remains quite steep throughout ontogeny. As demonstrated by shell diameter (D) measurements and the ratio U/D in Tables
1 and 2, Churkites noblei n. sp. appears to be slightly more involute than
C. syaskoi at larger shell diameters, but this difference does not seem to
hold true for smaller diameter shells. The ribbing on C. syaskoi is very
similar to C. egregius, but again, significantly different than C. noblei n.
87
FIGURE 9. Churkites noblei n. sp. A-C, 400C, specimen in A, lateral, B, ventral and C, apertural views. D, NMMNH P-55142, close-up of ribbing on
paratype. E, NMMNH P-55145, close-up of ribbing on paratype.
sp. C. syaskoi bears strong, dense radial ribs (Fig. 10E-F) that originate
with the tuberculation on the umbilical shoulder and project forward on
the ventral shoulder, fading away near the carina (Y. Shigeta, personal
commun., 2007). These ribs tend to become even denser as the aperture
is approached. Preservation of the shell of C. syaskoi is such that it
cannot be determined whether it bears strigate ornamentation (Y. Shigeta,
personal commun., 2007). The suture line (Fig. 6) of C. syaskoi is somewhat different than C. egregius as well as C. noblei n. sp. in that the lobes
are more denticulated, and the unusual auxiliary series is more complex
and also more denticulated. However, these differences are not necessar-
88
FIGURE 10. Churkites egregius Zharnikova and Okuneva (A-D) and Churkites syaskoi Zakharov and Shigeta (E-F). A, 1/12605, (x 0.33), paratype in
lateral view. B-C, 2/12605, (x 0.43), paratype in B, lateral and C, ventral views. D, 3/12605, (x ?), paratype in lateral view (Okuneva, 1990). E-F, 1/830,
holotype in E, lateral and F, ventral views (Zakharov and Shigeta, 2004).
ily diagnostic and they can be a function of preservation, preparation,
shape of the whorl and size of the specimen (A. Brayard, personal
commun., 2007).
Since Churkites noblei n. sp. is found only in the Meekoceras
gracilitatis Zone at Crittenden Springs, it is obviously slightly older than
C. egregius Zharnikova and Okuneva as well as C. syaskoi Zakharov and
Shigeta, both of which occur only in the Anasibirites nevolini Zone of the
Russian Far East. Indeed, aside from its acute venter on larger shells, the
89
TABLE 1. Measurements of Churkites noblei n. sp.
TABLE 2. Measurements of Churkites syaskoi Zakharov & Shigeta (Zakharov
and Shigeta, 2004).
morphology of C. noblei n. sp. seems to be closer to Arctoceras
tuberculatum (Smith) than to the other two species of Churkites. For all
practical purposes, its ornamentation (variable ribbing and strigation) is
identical to that of A. tuberculatum, whereas both of the Russian species
of Churkites are significantly different. Although not necessarily diagnostic in arctoceratids, the suture lines of C. noblei n. sp. (Fig. 4A-D)
and A. tuberculatum (Fig. 4E-F) are nearly identical. At first glance, it can
be somewhat difficult to differentiate between juvenile and adolescent
specimens of C. noblei n. sp. and A. tuberculatum, especially if the
particular A. tuberculatum specimen happens to be one of the more
compressed variants. However, upon close scrutiny the somewhat ovoid
whorl section (Fig. 2) of C. noblei n. sp. is usually readily apparent,
whereas A. tuberculatum is subquadrate (Fig. 3).
In summary, the acute venter on mature shells of C. noblei n. sp.
distinguishes it from A. tuberculatum, and its significantly different whorl
section and style of ribbing set it apart from the two Russian species of
Churkites. These differences are diagnostic and provide sufficient justification for the erection of a new species.
Given the probable faunal exchange between opposite sides of
Panthalassa during the Early Triassic (Brayard et al., 2006) and the close
affinity of C. noblei n. sp. to Arctoceras tuberculatum, it is plausible that
Churkites noblei n. sp. is an ancestor of the two Russian species of
Churkites. These considerations serve to illustrate the dilemma in determining whether to assign the new Crittenden species to Arctoceras or
Churkites. Since Churkites had been erected to represent an acute-ventered
arctoceratid, the decision was ultimately made to assign the new species
to Churkites in spite of its apparent relative closeness to Arctoceras
tuberculatum.
Occurrence: Churkites noblei n. sp. is rare at Crittenden Springs
and has been found only in the uppermost 5 to 8 cm (Fig. 1) of the M.
gracilitatis Zone portion of the bed. According to the web site (K. Bylund,
www.ammonoid.com/inyoites.htm), an acute-ventered arctoceratid,
which may be synonymous with C. noblei n. sp., also occurs in the
Meekoceras gracilitatis Zone of the Thaynes Formation in the Confusion Range of west-central Utah.
ACKNOWLEDGMENTS
The author is deeply indebted and especially grateful to Marco
Balini (University of Milano) for creating the stratigraphic log depicted
in Fig. 1 and for patiently reviewing two versions of this paper and
offering much needed constructive criticism. The writer generously thanks
Arnaud Brayard (University of Lyon) for his helpful taxonomic advice
and patience shown while answering questions and reviewing the final
version of this paper and offering valuable advice. The writer is also
deeply grateful to Yasunari Shigeta of the National Museum of Nature
and Science, Tokyo for providing morphological information relating to
the two Russian species of Churkites and for reviewing the final version
of this paper and providing helpful guidance. The author is also especially grateful to Spencer Lucas (NMMNHS, Albuquerque) for reviewing the final version of the paper and for providing valuable suggestions
for improvement. The writer is appreciative of Royal Mapes (Ohio
University), Wolfgang Weitschat (University of Hamburg, retired), Hans
Rieber (University of Zurich, retired), Jean Guex (University of Lausanne)
and Hugo Bucher (University of Zurich) for the many helpful and stimulating discussions regarding the paleontology, paleobiology and biostratigraphy of Triassic ammonoids. In one of these fruitful discussions,
Wolfgang Weitschat affirmed the validity of the writer’s evidence and
assertion that the beds at Crittenden Springs are likely overturned. The
writer is also grateful to Norman Silberling for encouraging him to describe these specimens and for his generous help and support over the
years.
REFERENCES
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