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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LATE TRIASSIC TETRAPOD TRACKS OF WESTERN NORTH AMERICA
ADRIAN P. HUNT AND SPENCER G. LUCAS
New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 87104-1375
Abstract—Significant tetrapod footprint assemblages from the Late Triassic of western North America include the
following ichnotaxa: Gwyneddichnium majore Bock, 1952, Rhynchosauroides ispp. Maidwell, 1911,
Brachychirotherium parvum (Hitchcock, 1889) Brachychirotherium thuringiacum Rühle von Lilienstern, 1938,
Apatopus lineatus Baird, 1957, Grallator cursorius Hitchcock, 1858, Anchisauripus sillimani Hitchcock, 1843,
Eubrontes isp. Hitchcock, 1845, Eosauropus cimarronensis Lockley et al., 2006a, Evazoum sirigui Nicosia and
Loi, 2003, Barrancapus cresapi Hunt et al., 1993a, Atreipus milfordensis (Bock, 1952), Pentasauropus isp.
Ellenberger, 1970, Brasilichnium elusivum Leonardi, 1981 and Ameghinichnus isp. Casamiquela, 1964. Most
tracksites are from the youngest (Apachean = late Norian-Rhaetian) portion of the Chinle and Glen Canyon
groups. Track sites occur in Wyoming, Utah, Colorado, Arizona, New Mexico and Oklahoma. There are four
named ichnocoenoses present in the Late Triassic of western North America: Evazoum, Brachychirotherium
(Brontopodus ichnofacies), Grallator (Grallator ichnofacies), and Brasilichnium (Chelichnus ichnofacies). In
addition, the Characichnos ichnofacies is present. There are two superposed ichnofaunas in the Chinle/Glen
Canyon – the upper ichnofauna can be distinguished by the presence of sauropodomorph (and other) tracks.
INTRODUCTION
Upper Triassic strata of western North America have yielded an
important record of tetrapod tracks over the last one hundred years (e.g.,
Riggs, 1904; Lockley and Hunt, 1995). There is an extensive literature on
these ichnofaunas, but there has been no attempt to synthesize all the
data. The purpose of this paper is to provide the first review of all Late
Triassic tracks from western North America, including discussion of
ichnotaxonomy, tracksites, ichnofacies and biochronology.
UPPER TRIASSIC LITHOSTRATIGRAPHY
Lucas (1993) named the Chinle Group to encompass a genetically-related suite of nonmarine redbeds in western North America that
extends from Idaho in the north to Texas in the south (Fig. 1). The Chinle
Group is composed of a number of formations that, in general, include a
basal conglomeratic interval (e.g., Shinarump, Gartra, Camp Springs formations), a lower fine-grained interval (e.g., Blue Mesa Member of Petrified Forest Formation; Bluewater Creek, Tecovas, Garita Creek and
Monitor Butte formations), a medial, sandstone-dominated interval (e.g.,
Sonsela Member of Petrified Forest Formation; Moss Back, Poleo and
Trujillo formations), an upper fine-grained interval (e.g., Painted Desert
Member of Petrified Forest Formation; Bull Canyon and Owl Rock
formations) and an upper silty/fine sandstone interval (Rock Point,
Redonda, Sloan Canyon formations).
Overlying, Upper Triassic strata at the base of the Glen Canyon
Group include parts of the Wingate and Moenave formations. The Wingate
Sandstone is a prominent, cliff-forming unit in the Four Corners region
that thins and is erosionally truncated to the east and south, and interfingers
laterally to the west, with the Dinosaur Canyon Member of the Moenave
Formation (e.g., Harshbarger et al., 1957; Clemmensen et al., 1989). Over
much of its outcrop belt, the Wingate Sandstone appears to conformably
overlie the sheet sandstones and siltstones that comprise the Rock Point
Formation of the Chinle Group (sensu Lucas, 1993; Lucas et al., 1997b,
2006c; Lucas and Tanner, 2007). The Dinosaur Canyon Member, as
described at the type area east of Cameron, Arizona, comprises siltstones and sandstones, in varying proportions, deposited in streams,
lakes and eolian dunes at the margin of the Wingate erg (Harshbarger et
al., 1957; Clemmensen et al., 1989; Lucas et al., 2006b, c; Tanner and
Lucas, 2007).
Locally, portions of the upper Chinle Group in New Mexico may
be age equivalent to the lower Wingate Sandstone and Dinosaur Canyon
FIGURE 1. Distribution of Late Triassic tracksites in western North America.
1, Wind River basin – Bell Springs Formation. 2, Dinosaur National
Monument area – Rock Point Formation and “Glen Canyon Sandstone.” 3,
Northwestern Colorado – Rock Point Formation. 4, Gateway – Colorado
National Monument area – Rock Point Formation and Wingate Sandstone.
5, Moab area – Rock Point Formation and Wingate Sandstone. 6, Glen
Canyon National Recreation Area region/Shay Canyon – Rock Point
Formation and Wingate Sandstone. 7, Grand Staircase-Escalante National
Monument – Owl Rock Formation. 8, St. George area – Shinarump Formation.
9, Ward Terrace – Dinosaur Canyon Member of Moenave Formation. 10,
Petrified Forest National Park – Petrified Forest Formation. 11, Fort
Wingate – Bluewater Creek Formation. 12, East-central New Mexico – Bull
Canyon and Redonda formations. 13, Dry Cimarron Valley – Sloan Canyon
Formation and Sheep Pen Sandstone. 14, Oklahoma Panhandle – Sheep
Pen Sandstone. 15, Southeastern Colorado – Sloan Canyon Formation and
Sheep Pen Sandstone.
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Member. These units are the Sheep Pen Sandstone of northeastern New
Mexico and the Wallace Ranch Member of the Redonda Formation in
east-central New Mexico, which at times has been termed Wingate.
LATE TRIASSIC BIOCHRONOLOGY
The most complete sequence of Late Triassic nonmarine tetrapod
fossil assemblages (faunas) is in the Chinle Group of western North
America (Lucas, 1997). These faunas have formed the basis of a
biochronological scheme that has global utility (Lucas and Hunt, 1993;
Lucas, 1997, 1998, 1999). Thus, Lucas and Hunt (1993) defined four
land-vertebrate faunachrons (lvfs) to encompass Late Triassic time based
on the faunas of the Chinle Group, which are, in ascending order:
Otischalkian (late Carnian), Adamanian (late Carnian), Revueltian (earlymid Norian) and Apachean (?late Norian/Rhaetian). Subsequently, Lucas
and others (Lucas et al., 1997a; Lucas, 1998; Hunt, 2001; Hunt et al.,
2005a) further refined this biochronology and subdivided the Adamanian
into the St. Johnsian and Lamyan sub-lvfs and the Revueltian into the
Barrancan and Lucianoan sub-lvfs.
The Chinle Group is entirely of Late Triassic age (OtischalkianApachean). The Wingate Sandstone straddles the Triassic-Jurassic boundary, as does the laterally equivalent Dinosaur Canyon Member of the
Moenave Formation (Lucas et al., 2005, 2006b, c; Lucas and Tanner,
2007).
ICHNOTAXONOMY
Despite the extensive recent literature on Late Triassic tracks
from western North America, there has been relatively little discussion
of the ichnotaxonomy below the ichnogeneric level. Thus, most papers
identify ichnotaxa at only the ichnogeneric level, and we believe that
most ichnogenera are probably monospecific because: (1) ichnogenera
usually represent family-level or higher taxonomic units based on body
fossils (Olsen and Galton, 1984; Lucas, 2007); (2) the taxonomically
most thoroughly revised ichnofaunas (those of the Early Permian) indicate monospecificity of most ichnogenera (e.g., Haubold, 1996; Lucas
and Hunt, 2006); and (3) most recently named ichnogenera are monospecific.
One reason for the lack of use of ichnospecies in discussing western North American Upper Triassic footprints is related to the quality of
preservation. Upper Triassic tracks from western North America are
typically preserved as outline imprints in convex hyporelief or concave
epirelief. The overall quality of preservation is not as fine as for comparable ichnofaunas from the Newark Supergroup of eastern North America.
Furthermore, in most western ichnofaunas, there are not enough well
preserved specimens to allow ichnospecific identification. The following
is a brief overview of the principal ichnotaxa that are known from the
Upper Triassic of western North America. We apply ichnospecies-level
designations where possible, and hope that this will be a stimulus to
further ichnotaxonomic study (Fig. 2).
Gwyneddichnium majore Bock, 1952
Bock (1952) described three ichnospecies of Gwyneddichnium
from the Norian Lockatong Formation of Pennsylvania. We consider G.
minore and G. elongatum to be junior subjective synonyms of G. majore
(cf. Olsen and Flynn, 1989). Lockley (2006) and Lockley et al. (1991,
1992a-c) described walking and swimming traces of Gwyneddichnium
from northeastern Utah and northwestern Colorado. Gwyneddichnium is
a quadrupedal, digitigrade ichnite with a five-toed pes and manus.
Gwyneddichnium walking trackways have pes digits I-III that are elongate, closely spaced and decreasing in length, have digit IV subequal in
length to II and separated from III, and have a short digit V originating
posterior to the other digits. The manus has less elongate digits with
similar relative lengths and separations. The western North American
specimens are about twice the size of the Lockatong specimens.
The putative swimming traces of Gwyneddichnium preserve only
pes impressions with short, thin digit marks that are characterized by the
inner three digits (presumably I-III) connected by webbing. The digits
decrease in length from IV to I. The fourth digit (?IV) is not conjoined to
the others by webbing, although all four digits are otherwise evenly
separated.
The tanystropheid Tanytrachelos is a convincing track maker for
Gwyneddichnium (Baird, 1986; Lockley, 1986; Olsen and Flynn, 1989).
This is the only Late Triassic track that can be assigned to a particular
genus of body fossil. Gwyneddichnium is relatively uncommon in western North America, only occuring in the Rock Point Formation of northeastern Utah and northwestern Colorado (Lockley et al., 1991, 1992a-c;
Lockley, 2006) and possibly the Owl Rock Formation of Grand Staircase-Escalante National Monument in Utah (Foster et al., 2000, 2001,
2003).
Rhynchosauroides ispp. Maidwell, 1911
Rhynchosauroides is a pentadactyl and heteropod lacertoid track
(e. g., Demathieu, 1970; Melchor and De Valais, 2006). The pes is often
tridactyl and digitigrade with digits decreasing in length from IV to II.
The digits are slender and curved. The manus is smaller than the pes. The
pace angulation is 110o. The Rhynchosauroides track maker has been
considered to be a sphenodontid (e.g., Baird, 1957) or a prolacertoid
(e.g., Diedrich, 2002). Given the abundance of Rhynchosauroides, it is
most parsimonious to consider that Late Triassic representatives of this
ichnogenus represent the tracks of a sphenodontian.
There are at least 21 potentially valid ichnospecies of Triassic
Rhynchosauroides (e.g., Haubold, 1971), and the ichnogenus is in dire
need of taxonomic revision. There are at least two distinct morphologies
of Rhynchosauroides present in western North America. One, a small
form that constitutes the majority of specimens in the Chinle Group, has
a pes length of 1-2 cm (e. g., Lockley and Hunt, 1995, fig. 3.15); the
second is a form that is more than twice as large and has straighter digits
(Lucas et al., 2001, fig. 3B). Pending a revision of the ichnogenus, we do
not assign the Chinle tracks to a specific ichnospecies. Rhynchosauroides
is widely distributed in the Upper Triassic of western North America
(Table 1) and it is locally abundant (e. g., Lockley and Hunt, 1995, fig.
3.15; Gaston et al., 2003, fig. 2; Klein et al., 2006).
Brachychirotherium parvum (Hitchcock, 1889)
Brachychirotherium thuringiacum Rühle von Lilienstern, 1938
Brachychirotherium represents a quadruped in which the pes and
manus are pentadactyl and plantigrade or semiplantigrade (Karl and
Haubold, 1998). The manus is about one-fourth the size of the pes. The
pes is semiplantigrade and has digit V set apart from the other digits, all
of which face forward and are thick (spatulate) and terminate in fine,
curved claws. Digit III is the longest, and thick digital pads are often
evident. Digits I-IV are each about as broad as long. The manus is nearly
overstepped by the pes, and the manus is usually represented by a
tridactyl, digitigrade undertrack. Brachychirotherium is the most common Late Triassic chirotherian ichnogenus, and its stratigraphically highest occurrence is Late Triassic (e.g., Haubold, 1971; Silvestri and Szajna,
1993; Szajna and Silvestri, 1996; Klein and Haubold, 2003, 2004; Szajna
and Hartline, 2003; Demathieu and Demathieu, 2004; Lucas et al., 2005,
2006b-c; Lucas and Tanner, 2007; Lucas, 2007). Chirothere tracks assigned to Brachychirotherium have been assigned to derived crurotarsans
(Karl and Haubold, 1998) such as rauisuchians (e.g., Olsen et al., 2002)
or aetosaurs (Lockley and Hunt, 1995). The abundance of
Brachychirotherium in a range of sedimentary environments suggests
that it represents a herbivore. Additionally, based on their body fossil
record, rauisuchians are rare components of Late Triassic faunas (e.g.,
Long and Murry, 1995; Hunt, 2001), whereas aetosaurs are much more
abundant. Therefore, we consider that most Late Triassic
Brachychirotherium most parsimoniously represent aetosaurs.
Karl and Haubold (1998) recognized four species of
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FIGURE 2. Principal ichnotaxa from the Late Triassic of western North America. A, Gwyneddichnium majore Bock, 1952 (from Lockley, 2006). B,
Rhynchosauroides isp. Maidwell, 1911 (from Lockley and Hunt, 1995). C, Brachychirotherium parvum (Hitchcock, 1889) (from Conrad et al., 1987).
D, Brachychirotherium thuringiacum Rühle von Lilienstern, 1938 (from Lockley and Hunt, 1995). E, Apatopus lineatus Baird, 1957 (from Foster et al.,
2003). F, Grallator cursorius Hitchcock, 1858 (from Conrad et al., 1987). G, Anchisauripus sillimani Hitchcock, 1843 (from Lockley and Hunt, 1993).
H, Eubrontes isp. Hitchcock, 1845 (from Hunt and Lucas, 2006a). I, Evazoum sirigui Nicosia and Loi, 2003 (from Lockley and Hunt, 1995). J,
Eosauropus cimarronensis Lockley et al., 2006a (from Lockley and Hunt, 1995). K, Barrancapus cresapi Hunt et al., 1993a (from Hunt et al., 1993a).
L, Atreipus milfordensis (Bock, 1952) (from Lockley and Hunt, 1995). M, Pentasauropus isp. Ellenberger, 1970 (from Lockley and Hunt, 1995). N,
Brasilichnium elusivum Leonardi, 1981 (from Lockley et al., 2004). O, Ameghinichnus isp. Casamiquela, 1964 (from Lockley et al., 2004).
Brachychirotherium – B. hassfurtense, B. thuringiacum, B. parvum and
B. eyermani. No specimens from western North America have been
assigned to a particular ichnospecies. This ichnogenus is in need of revision, but we offer the following observations: (1) Brachychirotherium
specimens from the western United States tend to have a relatively
elongate digit V that is oriented at an acute angle to the direction of travel
(e.g., Conrad et al., 1987, fig. 5A; Lockley et al., 1992a, fig. 2; Klein et al.,
2006, fig. 7); (2) Brachychirotherium parvum and B. eyermani from the
Newark Supergoup of eastern North America have similarly configured
digits V and therefore (3) B. parvum and B. eyermani are probably
synonymous with the former the senior synonym of the latter; (4) the
German ichnospecies B. hassfurtense and B. thuringiacum differ from
the Chinle forms in the more lateral orientation of digit V (Karl and
Haubold, 1998, figs. 3-9), so therefore (5) we believe that B. hassfurtense
and B. thuringiacum may be synonymous; and (6) the German ichnotaxa
are older than most of the diagnostic Apachean Chinle specimens. In
conclusion, we tentatively assign the majority of Chinle specimens to
Brachychirotherium parvum.
Specimens of Brachychirotherium from Shay Canyon, Utah, appear to differ from the other Chinle tracks and to be more similar to the
German specimens in the lateral placement of digit V (e. g., Lockley and
Hunt, 1995, figs. 3.8B, 3.9), so we tentatively assign these specimens to
Brachychirotherium thuringiacum. Brachychirotherium is a relatively
common track in Late Triassic ichnofaunas of western North America.
Its oldest record is two poorly-preserved specimens present in the upper Carnian portion of the Chinle Group (Hunt and Lucas, 2001, 2006a).
Apatopus lineatus Baird, 1957
Apatopus is a large heteropod, quadrupedal track (Baird, 1957).
Both the manus and pes are pentadactyl, with straight and relatively
broad digits. The pes is plantigrade, with a long, sub-triangular heel
imprint. Digit V is short, and digits IV-I decrease in size. The manus is
much smaller than the pes and wider than long, with digits short relative
to the pes. Digit III is longest, and the other digits decrease in length
laterally and medially. There is webbing between all the digits of both the
manus and pes.
Apatopus has been long considered the track of a phytosaur (e.g.,
Baird, 1957; Foster et al., 2000), although Parrish (1986) disputed this
interpretation. Nevertheless, Parrish’s (1986, fig. 4.9) illustrations of a
hypothetical phytosaur track and Apatopus are strikingly similar. Fur-
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TABLE 1. Late Triassic tetrapod track localities in western North America.
219
220
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thermore, Apatopus retains the “primitive” diapsid digital proportions
(Olsen and Huber, 1998); the only other logical Late Triassic candidates
for a track maker are rhynchosaurs (Parrish, 1986). However,
rhynchosaurs are rare and small in the Upper Triassic of North America,
and they become extinct by the early Norian (Hunt and Lucas, 1991;
Hunt, 2001; Lucas et al., 2002), whereas Apatopus tracks persist later
into the Norian and into the Rhaetian. Therefore, we consider it most
parsimonious to conclude that the Apatopus track maker was a phytosaur.
Well-preserved specimens of Apatopus are restricted to the Owl Rock
Formation of Grand Staircase-Escalante National Monument in Utah
(Foster et al., 2000, 2001, 2003). Conrad et al. (1987) referred several
trackways from the Sloan Canyon Formation to Apatopus.
of Eubrontes in Europe, Australia and southern Africa (Lucas et al.,
2005, 2006d). Furthermore, the claim of Olsen et al. (2002) is based on
the occurrence of Eubrontes in strata of the Newark Supergroup that
they interpreted as Hettangian in age, an interpretation that is now seriously in doubt (Kozur and Weems, 2005; Lucas et al., 2006d). The
ichnogenus thus has a stratigraphic range that crosses the Triassic-Jurassic boundary. There are many potential ichnospecies within this
ichnogenus (e.g., Olsen et al., 1998; Rainforth, 2005). Pending a revision,
we assign all diagnostic specimens to the type species, Eubrontes
giganteus. The stratigraphically lowest record in the Chinle Group is
Eubrontes isp. at Petrified Forest National Park, in the Newspaper Rock
Bed (Petrified Forest Formation, Blue Mesa Member) at the Teepees
(Hunt and Lucas, 2006a).
Grallator cursorius Hitchcock, 1858
Eosauropus cimarronensis Lockley et al., 2006a
Grallator is the ichnogenus name applied to relatively small (generally <15 cm long pes) tridactyl tracks of functional bipeds (Olsen et al.,
1998). The pes is narrow, and digit III projects relatively far anterior to
digits II and IV. The track maker is a small theropod dinosaur. Grallator
tracks are very common in, and well-described from, the Upper Triassic
Chinle Group and Triassic portion of the Wingate Sandstone, as well as
in Lower Jurassic strata of the Moenave and Kayenta formations, as
they are in Triassic-Jurassic strata of the Newark Supergroup (e.g.,
Lockley and Hunt, 1995; Olsen et al., 1998; Gaston et al., 2003; Lucas et
al., 2006b-c). Many ichnospecies have been assigned to Grallator (e.g.,
Olsen et al., 1998; Rainforth, 2005). Pending a revision of the ichogenus,
we assign all diagnostic specimens to Grallator cursorius.
Eosauropus (previously referred to Tetrasauropus [e.g., Lockley
and Hunt, 1995; Lockley et al., 2001] or Chirotherium [e.g., Conrad et
al., 1987]) refers to tracks of a large quadruped with strong heteropody
(pes larger than manus) and a short step and stride. The pes track is
elongate and oval, tetradactyl to pentadactyl, with the long axis and
prominent distal claw impressions that are outwardly rotated. The manus
is transverse and tetradactyl to pentadactyl, with outwardly-rotated
digit impressions and a concave posterior margin (Lockley et al., 2006a).
Eosauropus is the track of a sauropodomorph dinosaur, most likely a
sauropod (Lockley et al., 2001).
In the American Southwest, Eosauropus is widespread and occurs in the Rock Point Formation of northeastern Utah, western Colorado, the Wingate Sandstone of western Colorado and central Utah, the
“Glen Canyon Sandstone” of northeastern Utah, Moenave Formation of
northeastern Arizona, the Sloan Canyon Formation of northeastern New
Mexico and the Redonda Formation of eastern New Mexico (Table 1).
However, Eosauropus is only truly abundant in the Sloan Canyon Formation of Furnish Canyon in southeastern Colorado (Lockley et al.,
2001, fig. 4).
Anchisauripus sillimani Hitchcock, 1843
Anchisauripus is the name applied to medium-sized (pes length
15-25 cm) tridactyl tracks of bipeds (Olsen et al., 1998). Digit III is
usually relatively shorter than in Grallator and longer than in Eubrontes,
but other than size, the three track morphologies are essentially identical.
There is no current consensus on whether Anchisauripus is a valid
ichnogenus (e. g., Olsen et al., 1998) or a junior subjective synonym of
Grallator or Eubrontes (e. g., Lucas et al., 2006d). Although, Anchisauripus
is essentially only diagnosable by its size we believe that it represents a
distinct morphology that has utility in biostratigraphy. The Anchisauripus
track maker is a medium-sized theropod dinosaur. There are many potential ichnospecies within this ichnogenus (e.g., Olsen et al., 1998;
Rainforth, 2005). Pending a revision of the genus, we assign all diagnostic
specimens to the type species, Anchisauripus sillimani. Anchisauripus
is relatively uncommon in the Upper Triassic of western North America;
its principal records are in the Rock Point Formation of Shay Canyon,
Utah and the Sloan Canyon Formation in Sloan Canyon, New Mexico
(Lockley and Hunt, 1993, fig. 3, 1995, fig. 3.10).
Eubrontes giganteus Hitchcock, 1845
Eubrontes is the ichnogeneric name applied to relatively large (pes
length greater than 25 cm) tridactyl tracks of bipeds (Olsen et al., 1998).
Digit III is usually relatively shorter than in Grallator and Anchisauripus,
but other than size, the track morphologies are essentially identical. A
large theropod dinosaur is the Eubrontes track maker. Olsen and collaborators (e.g., Olsen et al., 2002) have advocated the idea that the
stratigraphically lowest occurrence of Eubrontes is equivalent to the
base of the Jurassic, but there are well-established Late Triassic records
Evazoum sirigui Nicosia and Loi, 2003
Evazoum tracks from the Chinle Group have previously been
referred to Pseudotetrasauropus (e.g., Lockley and Hunt, 1993, 1995;
Lucas et al., 2001; Lockley et al., 2001, 2006b). Evazoum encompasses
bipedal tetradactyl tracks that are ectaxonic to mesaxonic and often functionally tridactyl (digit I often absent or poorly impressed) (Nicosia and
Loi, 2003). The track is about as wide as long (if tetradactyl) with
rounded metapodial pads posterior to digits II and IV. Digit III is longest,
with II and IV being subequal in length. The digits have well-developed
pads and long, triangular claw marks. Pes angulations range from 140o to
170o. We do not believe that there is any convincing evidence that all
specimens previously assigned to Evazoum are extramorphological variants of Brachychirotherium (contra Klein et al., 2006).
Evazoum (“Pseudotetrasauropus”) has long been considered to
be the track of a prosauropod (e. g., Lockley and Hunt, 1995; Lockley et
al., 2001). It is widespread (occurring in the Owl Rock Formation of
Grand Staircase-Escalante National Monument, the Rock Point Formation of Dinosaur National Monument area, Utah and in western Colorado, and the Sheep Pen Sandstone of northeastern New Mexico) and
locally abundant (in the Redonda Formation of eastern New Mexico).
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Barrancapus cresapi Hunt et al., 1993a
The pes impression is mesaxonic, relatively short and broad with
relatively wide, blunt digit impressions (Hunt et al., 1993a). The pes is
pentadactyl. The smaller manus has a medially extended digit I and
relatively short digits II-IV. Manus digit V is elongate. Barrancapus may
represent a prosauropod trackway (Hunt et al., 2001). It is only known
from the early Revueltian of the Petrified Forest National Park in Arizona and the Bull Canyon Formation of eastern New Mexico (Table 1).
Atreipus milfordensis (Bock, 1952)
Atreipus is a quadrupedal track with a tridactyl pes and a tetradactyl
manus (Olsen and Baird, 1986). The pes impression is 9-14 cm long with
relative digit proportions similar to Grallator. The metatarsal-phalangeal
pads of digits II and IV are oval to circular and frequently impressed. The
distal phalangeal pads are often confluent with the more proximal pads.
The hallux is not impressed, even in deep tracks. The manus is slightly
wider than long, with digit III being the longest, followed by digits II, IV
and I. The manus is functionally tridactyl, and digit I is only impressed
in A. acadianus.
Olsen and Baird (1986) recognized three ichnospecies other than
the type ichnospecies, A. milfordensis. We consider A. sulcatus, A.
acadianus and A. metzeri to be junior subjective synonyms of A.
milfordensis. Olsen and Baird (1986) argued that Atreipus is likely the
track of an ornithischian. However, Olsen (in Lucas and Sullivan, 2006)
posited that this track might represent a non-dinosaurian dinosauromorph,
but Lucas and Sullivan (2006) argued that this was unlikely for temporal
reasons. Atreipus is currently only known in western North America
from the Rock Point Formation of Glen Canyon National Recreation
Area in southeastern Utah (Lockley et al., 1992d, 1998; Hunt and Lucas,
2006a).
which tracks are of cynodonts and which are of mammals. There are
probable records of Ameghinichnus from the Rock Point Formation of
Gateway and the Redonda Formation of eastern New Mexico (Lockley
et al., 2004, figs. 5A, D, 7; Klein et al., 2006, figs. 11C, D).
Pentasauropus maphutsengi Ellenberger, 1970
Pentasauropus is a large track with a very short stride and subequal
manus and pes impressions. Both the manus and pes are pentadactyl,
and each usually consists of an arc of rounded, equant digit impressions.
As Olsen and Galton (1984) noted, the only obvious track maker with
large enough, short-toed feet is a dicynodont. However, dicynodonts are
both rare in the Upper Triassic, and the bone record indicates that they
become extinct in North America at the end of the late Carnian, whereas
the supposed Chinle examples are all Norian or ?Rhaetian in age. Either
the Chinle tracks were not made by dicynodonts or (less likely) they
represent a range extension of this group that has no body fossil record.
Tracks from the Rock Point Formation of Shay Canyon, Utah and the
Rock Point Formation of the Gateway area, Colorado may represent
Pentasauropus (Lockley and Hunt, 1995, fig. 3.10; Gaston et al., 2003,
fig. 12).
TETRAPOD TRACK LOCALITIES
Otischalkian
St. George, Utah
The only possible Otischalkian tetrapod track localities in western North America are from the Shinarump Formation of southwestern
Utah (Lockley and Milner, 2006). Two localities yield specimens of
swimming traces and the tracks of a ?small theropod (Table 1).
Adamanian
Brasilichnium elusivum Leonardi, 1981
Brasilichnium is the track of a small pentadactyl quadruped that
is principally, or solely, restricted to dune faces in eolian strata (Hunt and
Lucas, 2006b-c, 2007). This ichnogenus can be diagnosed from the similar Chelichnus by: (1) track width (and subsequently pace angulation –
Brasilichium has a narrower trackway); and (2) relative size and preservation of the manus print – Brasilichium has a relatively smaller manus
that is often not preserved – hence the nomina dubia and likely junior
synonyms “Bipedopus” and “Semibipedopus” (e.g., compare Lockley
and Hunt, 1995, fig. 2.8 and 4.23; Hunt and Lucas, 2006b).
Schultz-Pittman et al. (1996) and Lockley et al. (2004) documented tracks of synapsids (including possible mammal tracks) from the
Wingate Sandstone. Most of these are small (pes length <5 cm) tracks of
quadrupeds that have pentadactyl manus and pes prints with straight
digits and in which digits I and V are shorter than digits II-IV. Some
Wingate tracks from the Gateway area of western Colorado have been
referred to Brasilichnium (Schultz-Pittman et al., 1996). We also believe
that the tracks described by Lockley et al. (2004) from the same area as
synapsid tracks (Lockley et al., 2004, figs. 13-14) or small mammal/
mammal-like, non-mammalian synapsid (Lockley et al., 2004, figs 1516) pertain to Brasilichnium.
Ameghinichnus patagonicus Casamiquela, 1964
Ameghinichnus is a small quadrupedal track with subequal-sized
manus and pes. The digits in both the manus and pes are also subequal in
length. Either the metacarpus and metatarsus were short or shorter than
the phalanges or the track maker was digitigrade (Olsen and Galton,
1984). Ameghinichnus specimens, depending on their size, probably
represent mammals or tritylodonts (Olsen and Galton, 1984).
Lockley et al. (2004) discussed the global record of small synapsid
and mammal tracks of Late Triassic-Early Jurassic age and indicated a
need to resolve the ichnotaxonomy of these small tracks, and to establish
Petrified Forest National Park, Arizona
Tetrapod tracks have been reported at four localities at Petrified
Forest National Park (Santucci and Hunt, 1993; Santucci et al., 1995,
1998, 2006: Martin and Hasiotis, 1998; Hunt et al., 2005b; Hunt and
Lucas, 2006a), but only one, in sandstone in the Teepees area, is
Adamanian in age. This locality is in the Newspaper Rock Bed of the
upper Carnian Blue Mesa Member (Adamanian lvf; St. Johnsian sublvf), not the Monitor Butte Member as reported by Martin and Hasiotis
(1998). Specimens from this locality have been described or mentioned
by several authors (Caster, 1944; Santucci and Hunt, 1993; Santucci et
al., 1995; Martin and Hasiotis, 1998; Hunt et al., 2005b; Santucci et al.,
2006). This unit has produced the majority of walking traces from the
park, included numerous specimens of the limulid trace Kouphichnium
(Caster, 1944; Hunt et al., 1993b). The tetrapod tracks from this locality
are: Rhynchosauroides isp., cf. Grallator isp., Eubrontes isp., and indeterminate large tracks and swimming traces. This is the oldest occurrence
of Eubrontes in western North America (Hunt et al., 2006a).
Gateway, Colorado
Tracks have been known for over 20 years from the Alabaster Box
Mine in the Gateway area (Parrish and Lockley, 1984; Lockley and
Jennings, 1987; Lockley and Hunt, 1995; Gaston et al., 2003). These
tracks are from stratigraphically low in the Chinle Group, just above the
Shinarump Formation, probably from the ?Monitor Butte Formation
(Adamanian; St. Johnsian). Specimens from this locality pertain to cf.
Grallator isp. and cf. Brachychirotherium isp.
Fort Wingate, New Mexico
There is a small number of specimens from the Bluewater Creek
Formation (Adamanian; St. Johnsian) near Fort Wingate in west-central
New Mexico (Hasiotis et al., 1994; Heckert et al., 2000; Lucas and
Heckert, 2002). These specimens represent Grallator isp. and aff.
Brachychirotherium isp. (Lucas and Heckert, 2002; Hunt and Lucas,
2006a).
Conchas Dam, New Mexico
Hunt and Lucas (2001) described a tetrapod track specimen from
the Garita Creek Formation (Adamanian; Lamyan) near Conchas Dam in
east-central New Mexico. This poorly preserved specimen represents
Brachychirotherium isp.
Revueltian
The Revueltian lvf can be subdivided into an older Barrancan and
a younger Lucianoan (Hunt, 2001; Hunt et al., 2005a). The majority of
Revueltian tetrapod tracks in the Chinle Group are of Barrancan age.
Petrified Forest National Park, Arizona
There are two Revueltian (Barrancan) track localities at Petrified
Forest National Park (Hunt et al., 2006a). Both are in the Agate Bridge
Bed of the Sonsela Member of the Petrified Forest Formation: (1) near
the Rainbow Forest (Rhynchosauroides isp. and cf. Barrancapus isp.);
and (2) Flattops area (Rhynchosauroides isp.).
Barranca Creek, New Mexico
The Barranca Creek badlands in east-central New Mexico yield a
large body fossil fauna and a small tetrapod ichnofauna (Hunt et al.,
1993a; Hunt, 2001). This ichnofauna from the Bull Canyon Formation
(Revueltian; Barrancan) includes the holotype of Barrancapus cresapi
and swimming traces (Hunt et al., 1993a; Hunt et al., 2001).
Grand Staircase-Escalante National Monument, Utah
The only Lucianoan tracksites in the Chinle are at Grand Staircase-Escalante National Monument in Utah. The Brinkerhof locality is
in the upper portion of the Owl Rock Formation in the northeastern
Circle Cliffs (Foster et al., 2000). The most unusual specimen from this
locality is a trackway of Apatopus lineatus, a taxon that is otherwise
almost exclusively known from the Newark Supergroup of eastern North
America (Baird, 1957; Foster et al., 2000, 2003). Other specimens represent Rhynchosauroides isp. and ?Gwyneddichnium isp. A second locality at Long Canyon Pass yields tracks of Grallator isp. and Evazoum
isp. (Foster et al., 2001).
Apachean
Hunt and Lucas (1992) first noted that the majority of tracks from
the Chinle Group are from the upper part of the lithosome, the Apachean
interval. Recently, Lucas and co-workers (e.g., Lucas et al., 2006b-c)
have demonstrated that the lower track-bearing portions of the Wingate
Sandstone and Dinosaur Canyon Member of the Moenave Formation
are also of latest Triassic (Apachean) age (Lucas and Tanner, 2007).
Southern Wyoming
Branson and Mehl (1932) described Agialopous wyomingensis
from the Bell Springs Formation of the Wind River basin (Lucas, 1994).
This ichnotaxon is a subjective junior synonym of Grallator cursorius.
Glen Canyon National Recreation Area and Vicinity, Utah
The spectacular canyons of the Glen Canyon area expose large
outcrops of the Chinle Group and Wingate Sandstone. The only track
record from the Rock Point Formation (Chinle Group) within Glen Canyon National Recreation Area is Atreipus milfordensis, the only western
occurrence of this ichnogenus (Lockley et al., 1992d, 1998; Lockley and
Hunt, 1995). The cliff-forming Wingate Sandstone in this area yields
many more tracks, but all are assignable to Grallator cursorius (Riggs,
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1904; Lockley et al., 1992d, 1998; Lockley and Hunt, 1995). North of
the recreation area, the Rock Point Formation yields numerous specimens of Grallator cursorius in the Dirty Devil River valley (Lockley
and Eisenberg, 2006).
Shay Canyon, Utah
Shay Canyon, east of Canyonlands National Park, preserves a
large tracksite in the Rock Point Formation. This tracksite includes multiple specimens and long trackways (Lockley and Hunt, 1995, figs. 3.83.10). The ichnofauna includes Brachychirotherium thuringiacum,
Anchisauripus sillimani and Pentasauropus isp. (Lockley, 1986; Lockley
and Hunt, 1995).
Dinosaur National Monument Area, Utah
Strata of the Rock Point Formation in and around Dinosaur National Monument in northeastern Utah have yielded a diverse ichnofauna,
including: Gwyneddichnium majore, Brachychirotherium parvum,
Grallator cursorius, Evazoum sirigui, Eosauropus isp. and a synapsid
track (Lockley et al., 1991, 1992a-c; Lockley, 2006).
In northeastern Utah, the basal unit of the Glen Canyon Group,
the “Glen Canyon Sandstone,” is equivalent to the Wingate Sandstone
(Poole and Stewart, 1964). About 7 m above its base are tracks of
Brachychirotherium reported by Lockley et al. (1992a-c), who also mentioned probable Grallator and Eosauropus tracks from the Wingate Sandstone equivalent in this area (Lucas et al., 2006b-c).
Moab, Utah
There are a few tracks preserved in the Rock Point Formation in
the Moab area that pertain to Grallator isp. Tracks are more common in
the overlying Wingate Sandstone. There are two in situ tracksites in the
lower part of the Wingate in Kane Springs Canyon, south of Moab,
about 15 m above the base of the Wingate (Lucas et al., 2006b). These
sites yield numerous Grallator cursorius and Brachychirotherium isp.
and less common Eosauropus isp. Two sites north of Moab, near Corral
Canyon, are about 2 m above the Wingate base and yield numerous
Grallator cursorius and Brachychirotherium isp. (Lucas et al., 2006b).
The total thickness of the Wingate Sandstone in the Moab area is about
100 m, so these tracksites are stratigraphically low in the Wingate (Lucas
et al., 2006b).
Northwestern Colorado
The Rock Point Formation in northwestern Colorado yields a
significant ichnofauna that includes Grallator cursorius,
Rhynchosauroides isp. and Gwyneddichnium majore (Lockley and
Gillette, 1989; Lockley and Hunt, 1995; Lockley, 2006).
Colorado National Monument, Colorado
In Colorado National Monument near Grand Junction, there are
six tracksites in the Wingate Sandstone. These sites yield tracks assignable to Grallator cursorius, Eosauropus isp., ?Brachychirotherium isp.
and ?Brasilichnium isp. The tetrapod tracks in the Wingate Sandstone at
Colorado National Monument are in the lower one-fourth of the unit
(Lucas et al., 2006b).
Gateway, Colorado
The deep canyons associated with the Dolores River Valley south
of Gateway yield abundant tracks in the uppermost Rock Point Formation. This ichnofauna includes Rhynchosauroides isp., Brachychirotherium
isp., Grallator cursorius, Evazoum sirigui, Eosauropus cimarronensis
and Pentasauropus isp. (Gaston et al., 2003; Lockley et al., 1996, 2004).
Lockley et al. (2004) reviewed the numerous tracksites in the
Wingate Sandstone (also see Lockley, 1991; Lockley and Hunt, 1995;
Schultz-Pittman et al., 1996; and Lockley and Peterson, 2002). Most of
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the Wingate tracks in this area are on fallen blocks (though a few sites are
in situ) that can be confidently assigned to the lower third of the Wingate
Sandstone cliff (Lucas et al., 2006b). The tracks are mostly Grallator
cursorius and Brasilichnium elusivum, but also include records of
Brachychirotherium isp. and Eosauropus isp. (Lockley, 1991; Lockley
and Hunt, 1995; Schultz-Pittman et al., 1996; Lockley and Peterson,
2002; Lockley et al., 2004; Lucas et al., 2006b; Lucas and Tanner, 2007).
Southeastern Colorado
In southeasternmost Colorado, there are limited exposures of trackbearing Upper Triassic strata. The largest tracksite is in Furnish Canyon,
where multiple trackways of Eosauropus cimarronensis are preserved
in the Sloan Canyon Formation (Lockley et al., 2001). Closer to the
Three Corners area, the Sheep Pen Sandstone yields a small ichnofauna
that includes Grallator cursorius and ?Brachychirotherium (Conrad et
al., 1987).
Ward Terrace, Arizona
On Ward Terrace, in the Navajo Nation of northeastern Arizona,
the Moenave Formation is about 100 m thick and is mostly fine-grained
sandstone, siltstone and minor shale (Lucas et al., 2006b). In the lower
part of the Moenave Formation, eolian sandstones represent an
interfingering tongue of the Wingate Sandstone that is well exposed from
Tohachi Wash to Dinosaur Canyon (Lucas et al., 2005, 2006b; Lucas and
Tanner, 2007). In this Wingate tongue, there is a tracksite that covers an
area of approximately 930 m2. Morales (1986) first reported this tracksite,
at which the majority of tracks pertain to Grallator cursorius, but there
are also small (up to 6 cm long), poorly preserved tracks that may be
synapsid, and large tracks of Eosauropus isp. (Lucas et al., 2006b).
Dry Cimarron Valley, New Mexico
The deep canyon of the Dry Cimarron Valley parallels the New
Mexico/Colorado border and exposes a Triassic-Cretaceous sequence of
strata. The largest tracksite in this area is preserved in the Sloan Canyon
Formation at Peacock Canyon (Conrad et al., 1987). Extensive exposures of several bedding planes expose multiple trackways (Conrad et
al., 1987, fig. 3). The ichnofauna includes Rhynchosauroides isp., Apatopus
lineatus, Brachychirotherium eyermani, Grallator cursorius, Eosauropus
cimarronensis and a possible therapsid.
Farther east down the valley are track-bearing exposures in the
Sloan Canyon area. Here, a tracksite in the Sloan Canyon Formation
yields Brachychirotherium isp., ?Eosauropus isp. and Anchisauropus
sillimani (Lockley and Hunt, 1993). In the same drainages, an exposure
of the overlying Sheep Pen Sandstone contains a tracksite comprising
Grallator cursorius and Evazoum sirigui (Lockley et al., 1993).
East-central New Mexico
The mesalands of Quay County contain extensive exposures of
the upper Chinle Group. The Redonda Formation (San Jon Creek Member: Lucas et al., 2006a) yields extensive ichnofaunas in two areas. Mesa
Redonda yields an extensive ichnofauna that has been collected for threequarters of a century and includes Rhynchosauroides isp.,
Brachychirotherium eyermani, Grallator cursorius, Evazoum sirigui and
Ameghinichnus isp. (Hunt and Lucas, 1989; Hunt et al., 1989; Lockley et
al., 2000; Lucas et al., 2001; Klein et al., 2006). Farther east, the Redonda
is exposed along the margin of the Llano Estacado in Apache Canyon and
at Red Peak where it yields Rhynchosauroides ispp., Brachychirotherium
spp., Grallator cursorius and Evazoum sirigui (Hunt and Lucas, 1989;
Hunt et al., 2000; Lucas et al., 2001). Note that the larger form of
Rhynchosauroides discussed above only occurs at Apache Canyon.
Oklahoma Panhandle
Cimarron County, in the easternmost Panhandle of Oklahoma,
has limited outcrops of Upper Triassic strata. The Sheep Pen Sandstone
in this area contains well-preserved specimens of Grallator cursorius
and ?Brachychirotherium isp. (Conrad et al., 1987).
ICHNOFACIES
Introduction
Hunt and Lucas (2007) provided a discussion of terminology
relevant to the study of tetrapod ichnofacies. An ichnocoenosis can be
defined as a trace fossil assemblage produced by a biological community
that can be characterized by morphological criteria (independent of depositional environment or biological affinities) (e.g., Bromley, 1996; McIlroy,
2004; Hunt and Lucas, 2007).
Seilacher (1964, p. 303) introduced the term ichnofacies for “general trace associations, or types of ichnocoenoses, representing certain
facies with a long geologic range.” Subsequently, Hunt and Lucas (2007)
defined five archetypal tetrapod ichnofacies for nonmarine environments:
Chelichnus, Grallator, Brontopodus, Batrachichnus and Characichnos
ichnofacies (Table 2).
Batrachichnus Ichnofacies
Hunt and Lucas (2007) proposed the Batrachichnus ichnofacies
for ichnofaunas in which the majority of tracks are of quadrupedal carnivores with a moderate-high diversity (four to eight ichnogenera). This
ichnofacies represents tidal flat-fluvial plain environments from the Devonian to the Cretaceous. Hunt and Lucas (2006b, 2007) recognized two
ichnocoenoses of this ichnofacies in the Triassic (Table 2).
Hunt and Lucas (2006c) identified the distinctive and pervasive
Apatopus ichnocoenosis throughout much of the Upper Triassic portion
of the Newark Supergroup in eastern North America. Ichnofaunas of this
ichnocoenosis lack Evazoum and Eosauropus, contain less than 50%
Brachychirotherium and Grallator, and are characterized by ichnotaxa
that are rare or absent elsewhere, including Apatopus and Gwyneddichnium.
Brontopodus Ichnofacies
Hunt and Lucas (2007) proposed the Brontopodus ichnofacies for
medium diversity ichnofaunas in which the majority of tracks are of
terrestrial herbivores with a small quantity (generally ~10%) of terrestrial carnivore tracks. This ichnofacies includes coastal plain-marine shoreline environments and some lacustrine shorelines, and ranges from Late
Permian to Recent in age. Hunt and Lucas (2006c) recognized four Triassic ichnocoenoses within this ichnofacies (Table 1).
Hunt and Lucas (2007) redefined Lockley’s (2007) Late Triassic
Grallator-Brachychirotherium-Rhynchosauroides ichnofacies as the
Grallator ichnocoenosis of the Grallator ichnofacies. Further study of
Late Triassic ichnofaunas by Hunt and Lucas (2006c) indicated that four
individual ichnocoenoses can be discriminated within this plexus, two of
which can be assigned to the Brontopodus ichnofacies, and one each to
the Grallator and Batrachichnus ichnofacies.
Hunt and Lucas (2006c) recognized the Evazoum ichnocoenosis
for Late Triassic ichnofaunas that are numerically dominated by tracks
of quadrupedal herbivores (e.g., Lockley and Hunt, 1995, fig. 3.25). The
Redonda Formation of east-central New Mexico is typical of these
ichnofaunas (Klein et al., 2006). The ichnogenus Evazoum also occurs in
Wales and Lesotho and often co-occurs with Eosauropus (Lockley et al.,
2006b), for example in western North America, Lesotho and Wales
(Lockley and Hunt, 1995; Lockley and Meyer, 2000). In Switzerland
and Greenland, Eosauropus occurs without Evazoum (Lockley and
Meyer, 2000). Hunt and Lucas (2006c) diagnosed the Evazoum
ichnocoenosis as consisting of ichnofaunas in which more than 40% of
the specimens represent Evazoum and/or Eosauropus.
Other Late Triassic ichnofaunas, dominated by the tracks of quadrupedal herbivores, are composed of >50% Brachychirotherium tracks
(e.g., sites in the Chinle Group of western North America, including Shay
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TABLE 2. Late Triassic ichnofacies and ichnocoenoses of western North America.
Canyon in Colorado and sites in Sloan Canyon in New Mexico). These
tracksites typically have 0-10% Evazoum or Eosauropus. Hunt and
Lucas (2006c) assigned these ichnofaunas to the Brachychirotherium
ichnocoenosis.
stone in western North America (Utah, California, Colorado). The
Brasilichnium ichnocoenosis is also locally present in the lower Wingate
Sandstone in western Colorado (Schultz-Pittman et al., 1996; Hunt and
Lucas, 2006c).
Grallator Ichnofacies
Characichnos Ichnofacies
Hunt and Lucas (2007) proposed the Grallator ichnofacies for
medium to high diversity ichnofaunas (five to eight ichnogenera) dominated by tracks of tridactyl avian and non-avian theropods (usually
dominant) or of other habitual bipeds. Tracks of bipedal and quadrupedal ornithischians, sauropods and herbivorous mammals are also locally
common in this ichnofacies. This ichnofacies extends from the Late Triassic to the Recent and often characterizes lacustrine margin environments. Hunt and Lucas (2006c) recognized four ichnocoenoses of the
Grallator ichnofacies during the Triassic (Table 2).
As noted above, Hunt and Lucas (2007) recognized a Grallator
ichnocoenosis in the Late Triassic. There are many Late Triassic
ichnofaunas in which the most abundant ichnogenus is Grallator (>50%).
Notable ichnofaunas occur at the very top of the Chinle Group or in the
overlying Wingate Sandstone in Colorado (Gaston et al., 2003; Lucas et
al., 2006b-c). Other prominent examples are in Wales, France, Germany,
Italy, Switzerland and Greenland (Lockley and Meyer, 2000, figs. 4.4,
4.10, 4.14).
Hunt and Lucas (2006c) noted that there is potential to subdivide
the Grallator ichnocoenosis, and it clearly includes several subichnocoenoses: for example, on the Colorado Plateau, the upper and
lower Wingate Sandstone have different sub-ichnocoenoses: a lower
Eosauropus sub-ichnocoenosis that includes Brasilichnium,
Brachychirotherium, Eosauropus, and an upper Otozoum subichnocoenosis that includes Eubrontes, Batrachopus and Otozoum (Hunt
and Lucas, 2006c; Lucas et al., 2006b).
Hunt and Lucas (2007) proposed the Characichnos ichnofacies
for medium diversity ichnofaunas in which the majority of tracks are
swimming traces (parallel scratch marks) and fish swimming trails
(Undichna). This ichnofacies represents shallow lacustrine (and tidal)
environments. Swimming traces are relatively common in the Chinle
Group (Upper Triassic) in Arizona and New Mexico (e.g., Hunt et al.,
1993a; Hunt and Lucas, 2006a). There is no named ichnocoenosis of this
ichnofacies in the Triassic.
Chelichnus Ichnofacies
Hunt and Lucas (2007) proposed the Chelichnus ichnofacies for
ichnofaunas that have a low diversity (fewer than five ichnogenera) of
tetrapod tracks in which manus and pes tracks are equant in shape, of
subequal size and have short digit impressions. This ichnofacies is recurrent in dune faces of eolian environments, and it extends from the Early
Permian to the Early Jurassic.
Hunt and Lucas (2007) redefined the Brasilichnium ichnofacies of
Lockley et al. (1994) as an ichnocoenosis (Table 2). Brasilichnium is
abundant in the Early Jurassic Navajo Sandstone and coeval Aztec Sand-
Discussion
There are thus four named ichnocoenoses present in the Upper
Triassic of western North America: Evavoum, Brachychirotherium
(Brontopodus ichnofacies), Grallator (Grallator ichnofacies), and
Brasilichnium (Chelichnus ichnofacies). The Evazoum ichnocoenosis is
principally restricted to western North America, where it is replaced in
the uppermost Chinle, Wingate and Sheep Pen formations (latest Triassic) by the Grallator ichnocoenosis. The Shay Canyon tracksite in Utah
pertains to the Brachychirotherium ichnocoenosis and is stratigraphically
low in the upper Chinle. It may represent a lateral equivalent of the
Evazoum ichnocoeonosis or it may be stratigraphically lower, which
suggests a temporal progression of ichnocoenoses from
Brachychirotherium to Evazoum to Grallator (Hunt and Lucas, 2006c).
The ichnofaunas of the lower Chinle are poorly known, but they
include Brachychirotherium, lack Evazoum and several include
Barrancapus (Hunt and Lucas, 2006c). These ichnofaunas may represent the Brachychirotherium ichnocoenosis (possibly a Barrancapus
sub-ichnocoenosis).
BIOSTRATIGRAPHY AND BIOCHRONOLOGY
Introduction
Lucas (2007) reviewed the Phanerozoic record of tetrapod tracks
(Devonian-Neogene) and noted that three principal factors limit their use
in biostratigraphy and biochronology (palichnostratigraphy): (1) invalid
ichnotaxa based on extramorphological variants; (2) slow apparent evolutionary turnover rates; and (3) facies restrictions. Tetrapod footprints
226
have generally been oversplit ichnotaxonomically, largely due to a failure
to appreciate extramorphological variation. Thus, many tetrapod footprint ichnogenera and most ichnospecies are useless “phantom” taxa that
confound biostratigraphic correlation and biochronological subdivision.
Tracks rarely permit identification of a genus or species known from the
body fossil record. Indeed, almost all tetrapod ichnogenera are equivalent
to a family or a higher taxon (order, superorder, etc.) based on body
fossils. This means that ichnogenera necessarily have much longer temporal ranges and therefore slower apparent evolutionary turnover rates
than do actual genera. Because of this, footprints cannot provide as
refined a subdivision of geological time as do body fossils. The tetrapod
footprint record is also much more facies controlled than the tetrapod
body fossil record. The relatively narrow facies window for track preservation, and the fact that tracks are almost never transported, redeposited or reworked, limits the facies that can be correlated with any trackbased biostratigraphy (Lucas, 2007).
Triassic Biostratigraphy and Biochronology
There is much literature on Triassic tetrapod footprint biostratigraphy, especially based on the European and North American records.
The most comprehensive are the publications of Demathieu (e.g., 1977,
1982, 1984, 1994; Demathieu and Haubold, 1972, 1974), who established the presence of three different Triassic footprint assemblages in
Europe that Lucas (2007) validated. These are the chirothere assemblage
of Olenekian-early Anisian age (early-Middle Triassic), the
dinosauromorph assemblage of late Anisian-Ladinian age (late Middle
Triassic) and the dinosaur assemblage of Carnian-Rhaetian age (Late
Triassic). Lucas (2007) suggested that a fourth footprint assemblage,
based on earliest Triassic dicynodont footprints from Gondwana, may
also be discernable.
Late Triassic Biostratigraphy – Previous Work
There have been attempts to identify two temporally successive
Late Triassic footprint assemblages. Thus, Olsen (1980) identified three
footprint assemblages in the Newark Supergroup of eastern North
America, two of Late Triassic age and one of Early Jurassic age. More
detailed stratigraphic data have shown that the two Late Triassic assemblages should be combined into one characterized primarily by
Brachychirotherium, Gwyneddichnium, Grallator, Atreipus and
Rhynchosauroides (Silvestri and Szajna, 1993; Szajna and Silvestri, 1996;
Lucas and Huber, 2003; Lucas, 2007). Olsen and Huber (1998) raised the
possibility that an older, distinctive footprint assemblage may be present
near the base of the Newark Supergroup, but when extramorphological
variation is considered, this assemblage consists of characteristic Late
Triassic ichnotaxa, including Apatopus, Grallator and Brachychirotherium.
Haubold (1986) followed Olsen’s (1980) zonation, applying it to the
European and South African records. However, current ichnotaxonomy
and understanding of stratigraphic distribution make it clear that only
one Late Triassic footprint assemblage can be identified in these regions
(Lucas and Hancox, 2001; Lucas and Huber, 2003).
Lockley (1993) and Lockley and Hunt (1994, 1995) also presented a similar zonation for the upper Chinle Group and the Glen
Canyon Group in the western United States. They identified four successive zones, two of which are Late Triassic in age: (1)
Brachychirotherium and small grallatorid zone of the upper Chinle Group
(Rock Point sequence of Lucas, 1993); and (2) medium-size grallatorid
assemblage of the Wingate Sandstone. However, subsequent collecting
and stratigraphic data demonstrate that assemblages 1 and 2 are a single
assemblage dominated by Grallator and Brachychirotherium that Lucas
et al. (2006b) termed the Brachychirotherium assemblage zone.
Upper Triassic Track Biostratigraphy in Western North America
We can potentially distinguish five distinct track-bearing intervals
within the Upper Triassic strata of western North America (Fig. 3):
FIGURE 3. Temporal distribution of Late Triassic tetrapod tracks in western
North America.
1. The Otischalkian-Adamanian portion of the Chinle Group has
a poorly preserved ichnofauna that is characterized by the presence of
Rhynchosauroides isp., Brachychirotherium isp., Grallator isp. and
Eubrontes isp.
2. The early Revueltian (Barrancan) fraction of the Chinle Group
has few tracksites, and they are characterized by Rhynchosauroides isp.
and Barrancapus cresapi.
3. Late Revueltian (Lucianoan) tracks are only known from two
localities, which include ?Gwyneddichnium isp., Rhynchosauroides isp.,
Apatopus lineatus, Grallator isp. and Evazoum isp.
4. The majority of tracksites occur in the Apachean formations of
the uppermost Chinle Group (Rock Point Formation and equivalents),
and they preserve a diverse ichnofauna that includes Gwyneddichnium
majore, Rhynchosauroides isp., Brachychirotherium eyermani,
Brachychirotherium hassfurtense, Apatopus lineatus, Grallator cursorius,
Anchisauripus sillimani, Evazoum sirigui, Eosauropus cimarronensis,
Atreipus milfordensis, Pentasauropus isp., and Ameghinichnus isp.
5. The tracks of the lower Wingate Sandstone (superposed above
the Rock Point Formation) include Rhynchosauroides isp.,
Brachychirotherium isp., Grallator cursorius, Evazoum isp., Eosauropus
cimarronensis, Brasilichnium elusivum and Ameghinichnus isp.
There are three issues to consider in evaluating this biostratigraphic distribution – sample size, preservation, and facies. The
Otischalkian-Adamanian ichnofaunas are both small in number and poor
in preservation, so it is impossible to evaluate whether they constitute a
distinct biostratigraphic interval. The early Revueltian is potentially
characterized by the presence of Barrancapus, but sample sizes here
also are small.
Tracks of the late Revueltian are only known from a small number
of localities, but this interval includes distinct ichnotaxa that also characterize the Apachean (?Gwyneddichnium isp., Apatopus lineatus, and
Evazoum isp.). The Apachean clearly has the largest sample size and
good preservation. The Wingate ichnofauna potentially represents a
younger Apachean interval characterized by the distinctive ichnotaxon
Brasilichnium. However, Brasilichnium is restricted to dune faces, so it
227
is most parsimoniously considered to be a facies-dependant fossil whose
appearance relates to an onset of eolian conditions.
In conclusion, it appears that there are probably only two readily
distinguishable biostratigraphic intervals in the Upper Triassic strata of
western North America: (1) an upper interval that includes
Gwyneddichnium majore, Apatopus lineatus, Anchisauripus sillimani,
Evazoum sirigui, Eosauropus cimarronensis, Atreipus milfordensis,
Pentasauropus isp. and Ameghinichnus isp.; and (2) a lower interval that
does not include any of these ichnotaxa. Although it is not good practice
to establish a biostratigraphy based on absences, the large size and distinctive morphology of the majority of many of these tracks makes it
unlikely that they would not be preserved or recognized in older
ichnofaunas (assuming there are no facies dependency issues).
Late Triassic Track Biochronology in Western North America
It appears that the distinctive nature of the Apachean ichnofauna
may have biochronological significance. The sauropodomorph traces
Evazoum and Eosauropus first appear in the later Norian or Rhaetian
worldwide, specifically in western North America, Greenland, Switzerland and Lesotho (e.g., Lockley and Hunt, 1995; Lockley and Meyer,
2000; Lockley et al., 2006a-b; Lucas et al., 2006c). Older Late Triassic
ichnofaunas lack these ichnotaxa, and their appearance corresponds with
the evolution of larger sauropodomorphs in the Norian.
Barrancapus appears to be a potential index ichnotaxon of the
Barrancan sub-lvf of the Revueltian lvf. Eosauropus and Evazoum are
index ichnotaxa of the Apachean lvf (Hunt and Lucas, 2006c).
ACKNOWLEDGMENTS
We thank Jerry Harris for a thoughtful and helpful review.
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