Articulated skeletons of the aetosaur Typothorax coccinarum Cope (Archosauria: Stagonolepididae) from the Upper Triassic Bull Canyon Formation (Revueltian: early-mid Norian), eastern New Mexico, USA

ABSTRACT

We report two nearly complete, articulated skeletons of the crurotarsan archosaur Typothorax coccinarum from the Upper Triassic Bull Canyon Formation of east-central New Mexico. These are the most complete, articulated aetosaurs from North America and provide a wealth of new anatomical and paleobiological data, including articulated presacral armor that confirms the distinctiveness of T. coccinarum from the closely related T. antiquum and from Redondasuchus. Cervical vertebrae are small, but the corresponding reduction in armor is accomplished by a reduced number of cervical osteoderms. The third row of osteoderms includes a thin, elongate, lateral spike. The ventral armor consists of 10 thoracic columns and four caudal columns of osteoderms. Spiked osteoderms near the cloacal vent are the first spikes reported in aetosaurian ventral osteoderms. The forelimb of T. coccinarum was very short, only ∼0.65 the length of the hind limb, possesses some adaptations found in digging taxa, and was held in a sprawling or ‘semi-erect’ position. In contrast the hind limb is much more robust, ‘pillar erect,’ and functionally mesotarsal. The articulated pes, including unguals, has, minimally, the phalangeal formula 2-3-3?-4?-3? with relative digit lengths III > II > IV > I > V, digits I–IV equally as wide as long and other characteristics of the footprint ichnogenus Brachychirotherium, often attributed to an aetosaurian trackmaker. Both specimens are ∼2.5 m long and the preserved armor and limb bones are as large or larger than known Typothorax fossils, suggesting that this approximates the upper size limit of T. coccinarum, and we calculate body mass estimates of ∼100–104 kg for both specimens.

ACKNOWLEDGMENTS

Volunteer P. Sealey found NMMNH P-12964 and S. Sucher discovered NMMNH P-56299. Mr. Cresap allowed the NMMNH to collect P-12964 from his land and Mr. Box facilitated NMMNH access to Badlands Ranch on multiple occasions, including when P-56299 was collected. Numerous volunteers with the New Mexico Friends of Paleontology (NMFOP) assisted with the excavation of these fossils. Initial preparation of P-12964 was conducted by T. Benson, J. Harris, and K. Zeigler, with oversight of the molding process by P. Reser. The mold was cast by J. Smith with help from a team of NMFOP volunteers. Bill Ortman prepared P-56299. Preparation of fossils was supported by the Sandoval Fossil Preparation fund and the NMFOP. The Appalachian State University ‘Triassic trip’ that helped collect P-56299 included the senior author as well as chairman Dr. J. Waters and students A. Abernethy, J. Camp, J. Richards, B. Snow, and W. Waters, and was supported by the Department of Geology's field trip fund. A University Research Council Grant from Appalachian State University supported the senior author's work on this paper. M. Sundstrom prepared the initial line art of the Revuelto Creek specimen. The NMMNH Foundation and Appalachian's College of Arts and Sciences supported some of the publication costs associated with this paper. We thank editor C. Sidor and reviewers M. Benton and N. Fraser for constructive suggestions that improved the manuscript.