LATE TRIASSIC TETRAPOD TRACKS OF WESTERN NORTH AMERICA

Transcription

1 Lucas, S.G. and Spielmann, J.A., eds. 2007, Triassic of the American West. New Mexico Museum of Natural History and Science Bulletin 40. LATE TRIASSIC TETRAPOD TRACKS OF WESTERN NORTH AMERICA 215 ADRIAN P. HUNT AND SPENCER G. LUCAS New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 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, 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.

2 216 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 (Otischalkian- Apachean). 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 110 o. 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

3 217 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-

4 218 TABLE 1. Late Triassic tetrapod track localities in western North America.

5 219

6 220

7 221 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. Grallator cursorius Hitchcock, 1858 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. Anchisauripus sillimani Hitchcock, 1843 Anchisauripus is the name applied to medium-sized (pes length 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 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). Eosauropus cimarronensis Lockley et al., 2006a 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). 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 140 o to 170 o. 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).

8 222 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). 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 ) or small mammal/ mammal-like, non-mammalian synapsid (Lockley et al., 2004, figs 15-16) 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 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 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

9 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, ; 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 ). 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

10 224 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 m 2. 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

11 TABLE 2. Late Triassic ichnofacies and ichnocoenoses of western North America. 225 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. Grallator 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). 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 Sandstone 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). Characichnos Ichnofacies 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. 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