TWO NEW AETOSAURS (REPTILIA:ARCHOSAURIA) FROM THE UPPER TRIASSIC PEKIN FORMATION (DEEP RIVER BASIN: NEWARK SUPERGROUP) OF NORTH CAROLINA AND THE PHYLOGENY AND DISTRIBUTION OF AETOSAURS

Aetosaurs were obligately quadrupedal, heavily armored crurotarsan (“crocodile-line”) archosaurs known from Upper Triassic strata on most modern continents. The first aetosaurs reported from North Carolina were fragmentary osteoderms collected from the Pomona Pipe B locality in the Pekin Formation. These fossils have been variously assigned to Desmatoscuhus, “Typothorax” (=Longosuchus) meadei, and Lucasuchus. Most are fragmentary, but the most complete do represent Lucasuchus. Recent discoveries at another Pekin Formation locality include NCSM 23618, an incomplete skull and partial skeleton of a small aetosaur and NCSM 21723, a disarticulated, but closely associated, specimen of a larger taxon. The smaller specimen closely resembles, but is slightly larger than, Coahomasuchus kahleorum Heckert and Lucas from the Colorado City Formation in West Texas. In particular, NCSM 23618 possesses wide paramedian osteoderms with a subradial pattern of faint grooves and ridges, as well as rectangular lateral osteoderms that are essentially flat and nearly square, lacking prominent knobs or spikes. The North Carolina specimen is slightly larger and has proportionately broader paramedian osteoderms, so we consider it Coahomasuchus n. sp. The larger aetosaur (NCSM 21723) consists of at least one paramedian and/or lateral osteoderm from each of the first 10 rows of the carapace. Lateral osteoderms of NCSM 21723 are referred to the Desmatosuchinae based on strongly flexed (~90˚) flanges bearing elongate spines or spikes and, in the cervical series, a thickened articulation that overlaps the anterolateral corner of the adjoining paramedian osteoderm. The key difference between this taxon and the most similar taxa (Longosuchus and Lucasuchus) are the relatively short cervical osteoderms and a distinctive, narrow, hexagonal cross-section with the spines on the lateral osteoderms projecting ventrolaterally. Based on these specimens we have updated and corrected existing phylogenetic datasets, however, the short cervical paramedians of the new taxon result in a massive polytomy near the base of Aetosauria. The presence of Lucasuchus and Coahomasuchus in the Pekin Formation supports previous biostratigraphic hypotheses indicating an Otischalkian (late Carnian) age for that unit.