New Mexico Geological Society Annual Spring Meeting — Abstracts

Barrancapus cresapi was named by Hunt et al. (1993, NMMNH Bulletin 3) for a small, quadrupedal trackway from the lower Bull Canyon Formation (Upper Triassic) of Quay County, New Mexico. The holotype slab (NMMNH P-4782 is a plastotype) from NMMNH locality 55 preserves two parallel-trackways in convex hyporelief preserved in a lenticular, conglomeratic sandstone that represents a low-sinuosity channel with fluctuating discharge.

The trackways display a range of extramorphological variation. The best-preserved pedal impression has length and widths Of 60 mm. It is pentadactyl and mesaxonic with broad, tapering digit impressions. The associated manus imprint is 40 mm long and 50 mm wide. This manus impression, as well as at least three others, are markedly entaxonic. The manus impression preserves ?five digit impressions with digit impressions I and V being the longest.

Barrancapus cresapi is the only named footprint ichnotaxon from the Revueltian lvf (Norian) strata of western North America. Other tetrapod ichnotaxa from this time interval are restricted to swimming traces of phytosaurs(?) from near the type locality of Barrancapus and enigmatic tracks described from the Painted Desert Member of the Petrified Forest Formation, Petrified Forest National Park, Arizona. The Painted Desert tracks have mean track (pedal track?) lengths and widths of 63 mm and 58 mm, respectively, which are very similar to values of Barrancapus cresapi.

The most notable feature of the morphology of Barrancapus cresapi is the large inferred size of digit I and its medial orientation. Among Triassic tetrapods, this condition is most closely matched in prosauropod dinosaurs. Two factors argue against Barrancapus cresapi representing a prosauropod: (1) the apparently large, inferred size of digit I; and (2) the hypothesis that the clawed pollex was held off the ground (Galton, 1971). In conclusion, it is most parsimonious to assign Barrancapus cresapi to the Prosauropoda, but we adopt a conservative approach and consider it as ?Prosauropoda. Barrancapus cresapi is the oldest track attributed to a prosauropod dinosaur. Other Revueltian prosauropod occurrences from western North America are limited to isolated specimens from the Bull Canyon Formation of Quay County, New Mexico and Garza County, Texas.