12 - Lucas et al (Tr Eubrontes).p65

Harris et al., eds., 2006, The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History and Science Bulletin 37.
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TRIASSIC-JURASSIC STRATIGRAPHIC DISTRIBUTION OF THE
THEROPOD FOOTPRINT ICHNOGENUS EUBRONTES
SPENCER G. LUCAS1, HENDRIK KLEIN2, MARTIN G. LOCKLEY3, JUSTIN A. SPIELMANN1,
GERARD D. GIERLINSKI4, ADRIAN P. HUNT1 AND LAWRENCE H. TANNER5
1
New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, NM 87104;
2
Rübezahlstrasse 1, D-92318 Neumarkt, Germany;
3
Dinosaur Tracks Museum, University of Colorado at Denver, PO Box 173364, Denver, CO 80217;
4
Polish Geological Institute, Rakowiecka 4, PL00-975, Warsaw, Poland;
5
Department of Biology, Le Moyne College, 1419 Salt Springs Road, Syracuse, NY 13214
Abstract—Eubrontes is an ichnogeneric name applied to relatively large (pes length > 25 cm) tridactyl tracks of a
bipedal theropod dinosaur. Eubrontes tracks are well known from Lower Jurassic strata, especially in southern
Africa, western Europe, eastern North America and the American Southwest, and some have advocated that the
lowest occurrence (LO) of Eubrontes corresponds to the Triassic-Jurassic boundary. However, there are well
documented Late Triassic records of Eubrontes in Australia, southern Africa, western Europe, Greenland and
North America. Indeed, the LO of Eubrontes in the Newark Supergroup of eastern North America, long considered
to be equivalent to the base of the Jurassic, is demonstrably of Late Triassic age based on radioisotopic ages and
microfossil biostratigraphy. Furthermore, body fossils of Late Triassic theropods large enough to make Eubrontes
tracks are known from Europe and North America. The theropod footprint record across the Triassic-Jurassic
boundary indicates a gradual increase in theropod maximum body size across the system boundary. The idea of a
sudden appearance of Eubrontes trackmakers at the beginning of the Jurassic, the result of a rapid (thousands of
years) evolutionary response by the theropod survivors of a mass extinction (“ecological release”), can be rejected.
INTRODUCTION
Eubrontes is an ichnogeneric name long applied to some relatively
large, bipedal and functionally tridactyl tracks of early Mesozoic age,
widely considered to have been made by a theropod dinosaur (Fig. 1).
Since the 1980s, some workers have argued that the lowest occurrence
(LO) of Eubrontes coincides with the Triassic-Jurassic boundary (TJB).
This idea has been important to placement and correlation of the TJB in
nonmarine strata. However, the LO of Eubrontes does not coincide with
the base of the Jurassic, as there are various well-documented Triassic
Eubrontes records (Lucas, 2003; Lucas et al., 2005b, c). Here, we review
these records to establish the biostratigraphic distribution of Eubrontes
and to discuss its implications for theropod dinosaur evolution across
the Triassic-Jurassic boundary.
ICHNOTAXONOMY AND TRACKMAKER
We use the ichnogeneric name Eubrontes E. Hitchcock, 1845, as
redefined by Olsen et al. (1998). Their diagnosis of Eubrontes reads:
Large (>25 cm long) bipedal, functionally tridactyl ichnite
with a relatively short digit III, a broad pes, and a hallux
which is rarely, if ever, impressed. Divarication of outer
digits averaging 25o-40o (Olsen et al., 1998, p. 590).
Several authors have argued (most recently Rainforth, 2005) that
Eubrontes and the smaller Grallator should be the same ichnogenus, as
they are only reliably distinguished on the basis of size (Fig. 1). While we
agree with this in general, we still use Eubrontes here because of the
biostratigraphic significance that has been attached to this ichnogenus,
understood as a Grallator-like pes imprint larger than 25 cm long. As
noted by Olsen (1980), Lockley (1999) and Milner et al. (this volume)
Grallator tracks are more elongate, with a greater anterior projection of
digit III, than Eubrontes. This is what Weems (1992) refers to as “toe
extension.” Placing Grallator in the same ichnogenus as Eubrontes, as
suggested by Rainforth (2005), requires an allometric argument that implies “lumping” or synonymy. Such an approach explicitly allows the
synonymy of two morphologies that represent end members of a
FIGURE 1. Eubrontes (left) and Grallator (right) showing possible
configurations of phalanges based on congruence (A) or incongruence (B)
of pads and phalanges. Not to scale; after Thulborn (1990).
Grallator-Anchisauripus-Eubrontes plexus, originally proposed by Olsen
(1980) under the double barreled “sub-ichnogenus” labels Grallator
(Grallator), Grallator (Anchisauripus) and Grallator (Eubrontes). This
cumbersome and ichnologically-unprecedented scheme also has some
merit for fans of allometry, and has been accepted by some authors and
discussed by others (Gierlinski, 1991; Weems, 1992; Gierlinski and
Ahlberg, 1994; Lockley, 2000). It provides ample food for ongoing debate on the perennial problems of theropod track ichnotaxonomy between the lumper versus splitter factions. Although it is outside the
scope of this paper to undertake a taxonomic revision of the plexus, we
note that Olsen et al. (1998), who undertook the most recent and thorough investigation of the problem, still maintain the Grallator-Eubrontes
distinction, which moves away from the implied synonymy of the original allometric plexus argument of Olsen (1980). We follow this more
87
recent position (Olsen et al., 1998) in maintaining the Grallator-Eubrontes
distinction.
Eubrontes, as used here, also encompasses other large grallatorid
ichnotaxa from the Triassic-Lower Jurassic, such as Kayentapus,
Dilophosauripus and Gigandipus, considered by some authors as distinct from Eubrontes, as well as several forms described under separate
names from the Elliot Formation of South Africa (Ellenberger, 1970,
1972, 1974). Full agreement on the synonymy of these ichnogenera has
not been reached, and Kayentapus is still used by at least one of us (GG).
We consider Anchisauripus, an ichnotaxon that was originally described
by Lull (1904) and redefined by Olsen et al. (1998) as grallatorids of
intermediate size (15-25 cm length) and morphology between Grallator
and Eubrontes, to be a junior synonym of Eubrontes (also see Rainforth,
2005). The name is not used here, but we adhere to the minimum 25-cmlong pes to define Eubrontes because this is the definition of Eubrontes
that is of supposed biostratigraphic significance. Olsen et al. (1998) and
Rainforth (2005) illustrate a range of extramorphological variation in
Eubrontes tracks that subsumes all tracks that are here referred to
Eubrontes.
There is virtually universal agreement that the Eubrontes
trackmaker was a relatively large, early Mesozoic theropod dinosaur,
such as the ceratosaur Dilophosaurus. Weems (2003) argued that a
Plateosaurus-like prosauropod was the Eubrontes trackmaker, but the
disparity between prosauropod foot structure and Eubrontes tracks is
so great that we dismiss Weem’s contention, as have others.
TRIASSIC RECORDS OF EUBRONTES
Here, we review Triassic records of Eubrontes tracks (Fig. 2).
Australia
Staines and Woods (1964) reported a trackway found in roof
shales of the Striped Bacon Coal Seam at Rhonda Colliery in the Sydney
basin of eastern Australia (Fig. 3). The best-preserved track is 43 cm long
(31 cm long for the digitigrade portion) and 38 cm wide, and the stride of
the trackway is 2 m (Hill et al., 1965; Bartholomai, 1966; Molnar, 1991;
Thulborn, 1998; Lucas and Tanner, 2004). The tracks closely resemble
tracks of Eubrontes giganteus from the Newark Supergroup as redescribed
by Olsen et al. (1998). The Australian tracks are from the Blackstone
Formation of the Ipswich Coal Measures near Dinmore in southeastern
Queensland, a unit of well-established Triassic age (probably late Carnian:
Balme and Foster, 1996).
Thulborn (2003) argued that the Australian Triassic record of
Eubrontes refutes the notion that its LO is at the base of the Jurassic.
Olsen et al. (2003), nevertheless, claimed that the Australian Eubrontes
tracks are actually tridactyl underprints of a pentadactyl chirothere track.
FIGURE 2. Map of Late Triassic Pangea showing Eubrontes locations
discussed in the text. Localities are: 1, Sydney basin, Australia; 2, southern
Africa; 3, Great Britain; 4, France, Germany and Poland-Slovakia; 5, Scania,
Sweden; 6, eastern Greenland; 7, eastern North America; 8, American
Southwest.
However, the footprint of Eubrontes is mesaxonic (symmetrical around
its long axis), as are the Australian Eubrontes tracks (Fig. 3). Tridactyl
underprints of chirotheres are paraxonic (asymmetrical around their long
axis). Therefore, the Eubrontes tracks from the Upper Triassic of Australia are correctly identified.
Southern Africa
Relatively large tetrapod tracks from the Triassic of southern
Africa were documented in detail by Ellenberger (1970, 1972). They
come from footprint-rich red beds of the Lower Elliot Formation, which
is Norian in age (Lucas and Hancox, 2001). Ellenberger, following his
own ichnological concept, assigned new ichnogeneric names such as
Quemetrisauropus, Prototrisauropus or Deuterotrisauropus to the pes
imprints of large bipedal dinosaurs from these strata. The morphology
and size (pes length up to 35 cm) of these tracks support assignment to
Eubrontes (Fig. 4). Moreover, the grallatorid footprint pattern and digit
proportions of these lower Elliott theropod tracks preclude their identification as incomplete chirothere tracks, which are also present on the
same surfaces. Similar theropod footprints are found in the upper Elliot
Formation, which is Lower Jurassic (Ellenberger, 1970, 1972, 1974).
Thus, the southern African records of Eubrontes encompass Upper Triassic and Lower Jurassic strata.
Great Britain
An archosaur track assemblage composed of grallatorid and
chirothere imprint forms is known from 88 trackways and numerous
isolated tracks from different coastal exposures of Wales (Thomas, 1879;
Sollas, 1879; Bassett and Owens, 1974; Tucker and Burchette, 1977;
Lockley et al., 1996). The tracks come from a marginal facies of the
Mercia Mudstone Group (Norian-Rhaetian), which is overlain by the
marine Penarth Group. On the track surfaces, tridactyl grallatorid footprints of varied sizes, showing the characteristic pattern of digit proportions and preserved pads, are common. Although few measurements
have been published, in some cases large tracks are as much as 26 cm
long, reaching the lower size range of Eubrontes.
Switzerland
Thirteen trackways of large theropods with Eubrontes-like imprints have been observed on a steeply-inclined surface at the Piz dal
Diavel in the Swiss Alps (Furrer, 1993; Lockley and Meyer, 2000).
They are preserved in a limestone bed that belongs to the Diavel Formation of the Hauptdolomit Group (Norian). The trackway pattern shows
a narrow trackway width and stride lengths up to 2.2 m. The size of the
FIGURE 3. Australian Triassic Eubrontes trackway (after Staines and Woods,
1964) and photograph of cast of track (after Bartholomai, 1966).
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FIGURE 4. Eubrontes tracks from southern Africa (Lesotho) in the Ellenberger collection, lower Elliott Formation (“Molteno” zones A2 [B-C] and A4
[A]). A, LES 44.2. B, LES 6.1. C, LES 6.2.
footprints (pes is 25-30 cm long: Furrer, 1993) as well as their overall
morphology support assignment to Eubrontes.
France
From the Keuper strata of d’Anduze (Norian) of southern France,
Ellenberger (1965) and Ellenberger et al. (1970) described large tridactyl
footprints that are associated with chirothere tracks. Several trackways
appear on in situ track surfaces in a riverbed near Anduze (département
Gard). Unfortunately, most of the material was destroyed by a flood a
few years ago (P. Ellenberger, personal commun., 2005). However,
Ellenberger’s figures clearly show the grallatorid morphology (digit three
longest) of the imprints, which reach a length of 50 cm, and a narrow
trackway pattern with stride lengths up to 3 meters. Hence the determination and identification as Eubrontes or Eubrontes-like theropod footprints is justified. Indeed, Haubold (1971) referred these tracks to
Eubrontes. The track layer is about 25 m below the base of the Rhaetian,
in strata that yield the conchostracan Euestheria minuta, and are considered to be of Norian age (Ellenberger, 1965).
Another possible Triassic record of Eubrontes from France at
Vendée was documented by Lapparent and Montenat (1967). These are
numerous tridactyl tracks, some more than 40 cm long, from strata of
likely Rhaetian age (Demathieu et al., 2002). If this age is correct, then
this is one of the best documented and most extensive Triassic records of
Eubrontes.
Germany
From the TJB (“Rhätolias”) strata of northern Bavaria, Kuhn
(1958) described and illustrated a large pes imprint under the name
Coelurosaurichnus sassendorfensis (also see Haubold, 1971, fig. 42.4)
The specimen was not collected, but the sketch and published measurements indicate an obvious similarity to Eubrontes. The tridactyl,
grallatorid pes imprint (25.6 cm long) is digitigrade with digit three longest, and has large, tapering claws. However, the exact stratigraphic position of the track-bearing sandstone block within a siliciclastic succession
comprising the Triassic-Jurassic transition is not certain, so its age is not
clear. According to Kuhn (1958), this could be uppermost Rhaetian or
lowermost Liassic.
Poland-Slovakia
The Tomanová Formation in the Tatra Mountains of Poland and
Slovakia yields large tridactyl theropod tracks assignable to Eubrontes
(Fig. 5A), as revised by Michalik and Kundrat (1998), and by Gierliñski
and Sabath (2005). These tracks are associated with characteristic Triassic ichnogenera listed by Gierlinski and Sabath (2005) as “Tetrasauropus”
(= Eosauropus: Lockley et al., this volume) and “Pseudotetrasauropus”
(=Evazoum Nicosia and Loi, 2003; = Brachychirotherium: Klein et al.,
this volume). The age of the Tomanová Formation is latest Triassic
(Rhaetian) based on macrofloral and palynological analyses (Michalik et
al., 1976; Fijalkowska and Uchman, 1993).
Sweden
Bölau (1952) first documented large (up to 38 cm long pes) tridactyl dinosaur tracks from the Höganäs Formation in northwestern Scania
(Fig. 5B). Bölau (1952) made no ichnotaxonomic assignment, but Haubold
(1971, 1986) assigned the Swedish tracks to Eubrontes. Gierlinski and
Ahlberg (1994) redescribed this record, which is from the roof beds of
coal mines in the Bjuv Member of the Hogänäs Formation. This interval
is assigned a latest Triassic (Rhaetian) age based on sequence stratigraphy, plant biostratigraphy and the presence of a Triassic amphibian
body fossil (e.g., Nilsson, 1934; Pienkowski, 1991). The best-preserved
tracks are tridactyl and up to 32 cm long, and certainly referable to
Eubrontes (Fig. 5B). A stratigraphically higher level in the Hettangian
portion of the Höganäs Formation also yields large theropod tracks
assigned to Grallator (Eubrontes) soltykovensis by Gierlinski and Ahlberg
(1994), later revised as Kayentapus soltykovensis (Gierlinski, 1996), so
in Scania the stratigraphic range of large theropod tracks encompasses
the TJB.
Greenland
Theropod trackways from the Ørsted Dal Member of the Fleming
Fjord Formation (Norian) of Jameson Land in east Greenland were described by Jenkins et al. (1994) and Gatesy et al. (1999). Jenkins et al.
(1994) report that the grallatorid imprints reach a maximum length of 28
cm, which is within the size range of Eubrontes.
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FIGURE 5. Triassic Eubrontes from Slovakia and Sweden. A, Eubrontes from Tomanová Formation (Rhaetian), Ticha Dolina (Slovakia). B, Eubrontes cf.
E. giganteus, specimen LO 5462t from Höganäs Fm. (Middle Rhaetian), “Gustaf Adolf” coal mine in Scania (Skane), Sweden. The scale bar above the
hypex beween digits II and III equals 5 cm.
Eastern North America
The idea that the LO of Eubrontes corresponds to the base of the
Jurassic has almost become doctrine for those who work on the tetrapod
track record of the Newark Supergroup in eastern North America (e.g.,
Olsen, 1983; Olsen and Galton, 1984; Silvestri and Szajna, 1993; Szajna
and Silvestri, 1996; Olsen et al., 1998, 2002a, b, 2003; Szajna and Hartline,
2003). Nevertheless, the LO of Eubrontes in the Newark basin, which is
just below the lowest basalt sheet of the Newark extrusive zone, does
not correspond to the Triassic-Jurassic boundary (Fig. 6). This is because the palynological criteria that supposedly placed the TJB below
the lowest basalt sheet do not identify the TJB, and radioisotopic dating
of the TJB in marine strata indicate it is no older than 200 Ma, which is
younger than the 201 Ma age of the lowest basalt sheet (Lucas and
Tanner, 2004; Kozur and Weems, 2005). Recent conchostracan biostratigraphy of Newark Supergroup strata that encompass the TJB also
indicates the system boundary is above the lowest basalt sheet and thus
above the LO of Eubrontes (Kozur and Weems, 2005).
Weems (1987, p. 16-17, fig. 4, pl. 1B) documented relatively large
(~27 cm long pes) tridactyl tracks of a biped (a 14 footfall trackway) that
he assigned to Eubrontes from the Balls Bluff Siltstone (Norian) at the
Culpeper Crushed Stone Quarry in the Culpeper Basin of Virginia. Weems
(1992) reported measurements of all the tridactyl tracks at the Culpeper
Crushed Stone Quarry, indicating that there are many such tracks with
pes lengths greater than 25 cm. Smoot and Olsen (1989, fig. 4A) illustrated one of these tracks as a “possible Grallator,” despite its size, and
further argued that this and the other tracks Weems (1987) described
from the Culpeper Crushed Stone Quarry are indeterminate. The tracks
Weems (1987) assigned to Eubrontes are quite similar to Eubrontes tracks
illustrated by Olsen et al. (1998, fig. 4F-G), so we assign them to the
ichnogenus.
American Southwest
There is one possible record of Eubrontes from Upper Triassic
strata of the Chinle Group in the American Southwest (e.g., Lockley and
Hunt, 1995). Virtually all Chinle Group theropod tracks have been assigned to Grallator (Lucas, 1997), but a possible exception is a report of
a tridactyl theropod track that is ~ 26 cm long by Martin and Hasiotis
(1998, fig. 5). This track is from the Blue Mesa Member of the Petrified
Forest Formation (upper Carnian) in the Petrified Forest National Park,
Arizona, but is a single, incompletely preserved track, so its
ichnotaxonomic identity is uncertain. Likewise, as noted below, unnamed
tracks reaching 25 cm in length (the minimum for Eubrontes) have been
reported by Lockley and Hunt (1993) from the Sloan Canyon Formation
of northeastern New Mexico.
In the American Southwest, the LO of abundant Eubrontes is in
the Dinosaur Canyon Member of the Moenave Formation on Ward’s
Terrace in northeastern Arizona, approximately 35 m above a tracksite in
a tongue of the Wingate Sandstone that yields Triassic tracks (Lucas et
al., 2005a, fig. 4). Eubrontes also is present in the top of the Wingate
Sandstone, and in the Whitmore Point Member of the Moenave Formation, strata of well-established Early Jurassic age (Milner and Lockley,
2006; Milner et al., this volume) (Fig. 7). Eubrontes tracks are very
abundant in the overlying Lower Jurassic Kayenta Formation and Navajo Sandstone (e.g., Rainforth and Lockley, 1996; Lockley et al., this
volume).
In the American Southwest, Eubrontes tracks do not co-occur
90
FIGURE 6. Different definitions of the TJB in the Newark Supergroup of eastern North America, with respect to the LO of Eubrontes. The definition on
the right is consistent with radioisotopic ages and microfossil biostratigraphy.
with any unambiguous Triassic age indicators. Thus, it is possible that
the LO of Eubrontes in the American Southwest is an earliest Jurassic
event. However, until a more precise placement of the base of the Jurassic can be achieved, the most that can be said is that the LO of abundant
Eubrontes appears to be a possible Jurassic biostratigraphic datum in the
American Southwest (Fig. 7).
TRIASSIC THEROPODS AS EUBRONTES TRACKMAKERS
FIGURE 7. Lithostratigraphy of the TJB interval on the Colorado Plateau
with respect to the LO of Eubrontes. See Lucas et al. (2005a) for discussion
of position of TJB.
Most Late Triassic theropods were of small to moderate size and
thus potential trackmakers of Grallator (pes length generally < 15 cm).
However, a handful were significantly larger, and thus are potential
trackmakers of Eubrontes. Based principally on published reports and
illustrations we reconstruct the pes size of selected relatively large Late
Triassic theropods (Fig. 8). These reconstructions are based on the length
from the tip of the central digit to the distal articular ends of metatarsals
2 and 4. The inclusion of these articular ends allows us to estimate the
length of the “heel” of the footprint (Fig. 1), which would be generated
either by the ends of these metatarsals or a fleshy pad, the plantar
aponeurosis, directly beneath these metatarsals.
The pedes of both Coelophysis bauri and Megapnosaurus
rhodesiensis are well known, especially the pes of C. bauri, which is
known from dozens of specimens. However, the pedes of both taxa are
significantly smaller (14 and 15 cm) than would be needed to make
Eubrontes tracks. Thus, their pes sizes would allow them to make
Grallator tracks.
Liliensternus liliensterni is known from an incomplete skeleton
that includes a complete pes. This pes, at 22 cm, is almost the size
needed to make a Eubrontes track. Because L. liliensterni is known from
only one, moderately complete skeleton, it is probable that larger indi-
91
FIGURE 8. Pes skeletons and lengths of selected Late Triassic and Early Jurassic theropods. Line drawings of Coelophysis bauri and Megapnosaurus
rhodesiensis from Colbert (1989), drawing of Liliensternus liliensterni from Rowe and Gauthier (1990), and drawing of Dilophosaurus wetherilli from
Welles (1984).
viduals of this taxon could make Eubrontes tracks.
Liliensternus airelensis is known from an isolated tooth, various
vertebrae, a sacrum, and an incomplete pelvic girdle described by Cuny
and Galton (1993). Based on illustrations in Cuny and Galton (1993, fig.
14), L. airelensis has a pelvic girdle that is approximately 1.5 times the
size of L. liliensterni based on the length of the acetabular opening (11 cm
in L. airelensis versus 7 cm in L. liliensterni). Applying this same ratio to
the pes length of L. liliensterni results in an approximate pes size of 33
cm for L. airelensis, which is clearly large enough to make Eubrontes
tracks. However, the holotype material of L. airelensis was collected
from strata that do not yield a definitive assemblage of Triassic or Jurassic pollen, and thus was assigned to the uppermost Rhaetian transitional
zone, but as noted by Cuny and Galton (1993, p. 263), “it is difficult to
say if it is in the Triassic or the Jurassic.”
The early Norian ceratosaurian dinosaur Gojirasaurus quayi is
known from an isolated tooth, various axial skeletal elements, portions
of its pectoral and pelvic girdle, a tibia and a metatarsal (Carpenter,
1997). However, based on the size of the tibia, Carpenter (1997) estimated that G. quayi was 5.5 m long. This is considerably larger than
Liliensternus liliensterni, which Carpenter (1997) estimated at 3.8 m
long. Thus, based on its size, Gojirasaurus quayi could easily have made
large Eubrontes tracks.
In summary, there were theropods large enough to produce
Eubrontes-size tracks in the Late Triassic. A large Liliensternus liliensterni
could produce a very small Eubrontes track, while L. airelensis, if it is
indeed from the Late Triassic, could produce a moderately-sized Eubrontes
track. Gojirasaurus quayi was large enough to produce a very large
Eubrontes track.
THEROPOD EVOLUTION ACROSS THE TRIASSICJURASSIC BOUNDARY
The idea that the theropod dinosaur ichnogenus Eubrontes has its
LO at the base of the Jurassic began with Olsen (1983) and Olsen and
Galton (1984), who advocated this datum because of the close correspondence between the LO of Eubrontes in the Newark Supergroup and
the “palynostratigraphically-defined” TJB. Thus, use of Eubrontes to
indicate the base of Jurassic depends entirely on palynostratigraphy, not
on the stratigraphic distribution of the tracks themselves. Indeed, previously, in his global review of footprint ichnotaxa, Haubold (1971) considered Eubrontes to be both Late Triassic and Early Jurassic in age.
Subsequent to the correlations advocated by Olsen and Galton
(1984), most ichnologists considered Eubrontes tracks to be exclusively
of Jurassic age but they did not equate the LO of Eubrontes with the TJB
(e.g., Haubold, 1986; Lockley and Hunt, 1994, 1995). These authors
knew that Eubrontes occurrences are in rocks generally considered Jurassic in age, though there was no way to correlate most of the occurrences
to marine strata or, more importantly, they could not demonstrate a close
coincidence of the LO of Eubrontes and the TJB as it is defined in marine
strata.
Olsen et al. (2002a, b) argued that the sudden appearance of
Eubrontes, made by a Dilophosaurus-like theropod, in the “earliest Jurassic” strata of the Newark Supergroup indicates a dramatic size increase in theropod dinosaurs at the TJB. They interpreted this as the
result of a rapid (thousands of years) evolutionary response by the
theropod survivors of a mass extinction and referred to it as “ecological
release” (Olsen et al., 2002a, p. 1307). They admitted, however, that this
hypothesis could be invalidated by the description of Dilophosaurussized theropods or diagnostic Eubrontes giganteus tracks in verifiably
Triassic-age strata. Indeed, their hypothesis is invalidated because
theropods large enough to have made Eubrontes tracks have been known
from the Late Triassic body-fossil record for decades, and there are
Triassic records of Eubrontes, as discussed above.
The idea of a sudden size increase of theropod dinosaurs across
the TJB also runs counter to data, especially from the American Southwest, that indicate a relatively gradual increase in theropod track size
across the TJB (e.g., Lockley, 1991, fig. 9.2; Lockley and Hunt, 1995, fig.
3.28). Thus, most Late Triassic grallatorid tracks are smaller than 20 cm,
though a few are as long as 25 cm and thus reach the minimum for
Eubrontes size (Lockley and Hunt, 1993, p. 282, fig. 3; 1995, p. 80, fig.
3.10 bottom, p. 105. fig. 3.28). The oldest abundant Eubrontes tracks in
the American Southwest (from the Moenave-Wingate interval) are 25-35
cm long, whereas younger Early Jurassic Eubrontes tracks (KayentaNavajo interval) are up to 50 cm long. Thus, in the American Southwest,
there appears to be a relatively gradual increase in maximum theropod
pes (and body) size across the TJB, though it should be noted that many
small theropod tracks (Grallator) are also known in Lower Jurassic
strata. The theropod track record thus does not support the concept of
a substantial extinction or evolutionary turnover of these dinosaurs at
the TJB.
Eubrontes thus has its LO in Upper Triassic strata. The oldest
occurrences of the ichnogenus apparently are in Carnian strata of the
92
Sydney basin, Australia and possibly at the Petrified Forest National
Park in the American Southwest. African, European and other North
American records begin in the Norian and continue into the Early Jurassic. We are hesitant to attribute much significance to the regional
diachroneity in the LO of Eubrontes, though Thulborn (2003) suggested
it may indicate a Gondwanan origin of the theropod that was the Eubrontes
trackmaker. Nevertheless, at this point we are content to establish that
the ichnogenus Eubrontes has numerous Triassic records, its LO does
not equate to the Triassic-Jurassic boundary, and that the distribution of
Eubrontes does not indicate a theropod extinction at the TJB.
ACKNOWLEDGMENTS
Reviews by Jerry Harris, Andrew Milner and Robert Weems improved the manuscript.
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