THE SKELETAL MORPHOLOGY OF A HYPOTHETICALLY JUVENILE AETOSAUR SPECIMEN FROM THE LATE TRIASSIC DOCKUM GROUP (OTISCHALKIAN) OF TEXAS

The Aetosauria is a speciose clade of archosaurs documented from continental upper Triassic strata. Most of the documented taxa are known exclusively from the Late Triassic Chinle Formation and Dockum Group in the southwestern United States. Few articulated and partially complete skeletons of aetosaurs have been collected from the Dockum Group in northwestern Texas and eastern New Mexico including those of Calyptosuchus wellesi, Typothorax coccinarum, and Coahomasuchus kahleorum. Here, we present an articulated and relatively complete skeleton of an aetosaur from the Otischalk locality in Howard County, Texas. This specimen (TMM 31100-1336) preserves both an articulated skull and precaudal region that are intermixed with other bone fragments and matrix. CT-imaging indicates that the preserved vertebrae lack co-ossification between the centra and their respective neural arches which suggests that TMM 31100-1336 represents a skeletally immature individual. Additionally, its small body size suggests that the individual is a juvenile. Elements of the cranium, mandible, carapace, vertebral column, and shoulder and pelvic girdles were segmented. Homologous elements were compared with Longosuchus meadei, Lucasuchus hunti, and Coahomoasuchus kahleorum, which are also documented within the same locality, in order to assess the taxonomic affinities of TMM 31100-1336. The recurved dentition, morphology of the maxilla, mandible, parabasisphenoid, basioccipital, and lateral and paramedian osteoderms, suggests that TMM 31100-1336 represents a juvenile individual referrable to Coahomasuchus kahleorum. Following this taxonomic referral, TMM 31100-1336 suggests that lateral osteoderms do not undergo drastic morphological change in their flexure through skeletal maturity, but instead grow proportionately larger. Additionally, CT-imaging shows that the juvenile specimen TMM 31100-1336 is intermixed with elements, particularly of the skull, from at least three other individuals that are of similar size and identical in morphology. This suggests that TMM 31100-1336 preserves several hypothetically juvenile individuals referrable to Coahomasuchus kahleorum. This is only the second report of a locality preserving several juvenile individuals of a particular aetosaur taxon.