New Mexico Geological Society Annual Spring Meeting — Abstracts

The Bull Canyon Formation (Upper Triassic: Norian) of east-central New Mexico has yielded the most diverse fauna of Late Triassic tetrapods from the state. The aetosaur component of the fauna includes Typothorax, Paratypothorax, Desmatosuchus, ?Aetosaurus and a new taxon. A recently collected specimen of a lateral osteoderm of an aetosaur represents a new morphotype for the Bull Canyon Formation. This
specimen was found with fragments of paramedian osteoderms of Typothorax and a posterior dorsal centrum of a Chindesaurus-sized herrerasaurid in the lower Bull Canyon Formation in eastern Quay County, New Mexico.

The new specimen is a spiked lateral osteoderm 119 mm in length. The spike is recurved at an angle of about 45° and has the cross section of an acute triangle with the posterior margin (smallest side of triangle) vertical. The spike has poorly delevloped radial grooves at its base and scattered pits on the shaft. The ventromedial flange is slightly damaged but is essentially straight anteroposteriorly, whereas the dorsomedial flange is too poorly preserved to assess its shape.

This osteoderm is similar to undescribed specimens from the Bull Canyon Formation of Garza County, Texas that represent an undescribed species of Paratypothorax. Paratypothorax is poorly known from North America. Specimens without spiked lateral osteoderms, assignable to Paratypothorax andressi, are restricted to late Carnian portions of the Chinle Group of western North America. Norian specimens from the Chinle Group are fragmentary or undescribed, but are characterized by spiked lateral osteoderms like the new specimen from east-central New Mexico. There appears to be a species-level turnover within the genus Paratypothorax at about the Carnian-Norian boundary in western North America. However, in Germany P. andressi occurs in Norian strata and it is possible that specimens of this rare species were present in the Norian of western North America and their fossils have yet to be found or identified.