New Mexico Geological Society Annual Spring Meeting — Abstracts

Viewed from a global perspective,prosauropods are a significant component of Late Triassic archosaur-dominated faunal assemblages. However, to date they are virtually unknown from skeletal remains, from this epoch, in the western United States (Chinle Group sensu Lucas 1993). Little is known of the Late Triassic track record of prosauropods except . in southern Africa, where two ichnogenera Tetrasauropus and Pseudotetrasauropus were defined by Ellenberger (1972). Ellenberger included two ichnospecies in the former ichnogenus (both quadrupedal) and eight ichnospecies in the latter, of which only two were described as quadrupedal. Thus the ichnogenus Pseudotetrasauropus is predominantly (75%) composed of bipedal ichnospecies. Both ichnogenera mainly represent large animals (pes lengths 45-60 cms), though three ichnospecies of Pseudotetrasauropus are smaller (pes lengths 18.5-30 cms).

Recent work in the Late Triassic of the western United States has revealed that a small ichnospecies of Pseudotetrasauropus is present at at least two localities in New Mexico and Utah. This is the first record of the ichnogenus in North America (Lockley and others, 1992). A re-examination of trackways from the Sloan Canyon Formation in the Cimarron Valley also suggests that the ichnogenus Tetrasauropus (or another larger species of Pseudotetrasauropus) are also present.

Conrad and others (1987) noted. that large tracks, from the Cimmaron Valley area, assigned to Chirotherium sp. "resemble those of small sauropods" and prosauropods were suggested "as possible trackrnakers". However some differences, between these forms and Tetrasauropus from Africa, caused these authors to dismiss an affinity to that ichnogenus. Instead it was inferred that the tracks were attributable to phytosaurs, another large animal known to have been abundant at that time. It now appears that this dismissal of the prosauropod interpretation may have been premature, and that we must seriously consider that the tracks may be allied to Tetrasauropus. If this interpretation is correct then this is also the first report for this ichnogenus in North America.

It is accepted, at least by some ichnologists, that the Lower Jurassic ichnogenus Otozoum is attributable to a prosauropod, and it is similar to Pseudotetrasauropus and Tetrasauropus. Given that Otozoum and another prosauropod track Navahopus are both known in the Lower Jurassic of the western United States, and that late Triassic skeletal remains of at least one prosauropod are known, it is reasonable to infer that prosauropod tracks should occur at this time. The ichnogenera Tetrasauropus and Pseudotetrasauropus warrant further study to determine their distribution in western North America and to establish if they are of prosauropod affinity, as proposed by Ellenberger.