TRIASSIC BABIES? A CLUSTER OF SMALL AETOSAURS FROM THE UPPER TRIASSIC DOCKUM GROUP (OTISCHALKIAN – LATEST CARNIAN?) OF TEXAS

The fossil record of archosaurs currently extends into the Early Triassic. This clade exhibits a fossiliferous record across the entire Mesozoic, however the same cannot be said about young, skeletally immature juveniles within this group. In ovo embryos, nests, neonates, and hatchlings are represented within terrestrial Jurassic and Cretaceous strata, but mostly restricted to some dinosaur groups. In comparison, the fossil record of such young, skeletally immature archosaurs is sparse in the Triassic Period. Currently, no egg-bearing archosaur nests or embryos are documented from this period. When present, neonates and hatchlings are typically documented based on limited fragmentary elements intermixed with larger specimens. Currently, the only reported exception is a cluster (n = 24) of articulated hatchlings (<1 years old) of the pseudosuchian aetosaur Aetosaurus ferratus (SMNS 5770) from the Upper Triassic (middle Norian) Lowenstein Formation in Germany. Here, we present a cluster of small, skeletally immature individuals (TMM 31100-1336) from the Otis Chalk fossil assemblage within the Upper Triassic Dockum Group of Texas. This cluster preserves at least four individuals, including a 3-dimensionally well-preserved skull. Skeletal proxies such as a small body size; a large skull that is proportionately longer than the femur; loosely articulated skulls with a large, elliptical orbit that is not enclosed by the postorbital bar and bears dorsally opened supratemporal fenestrae; un-coossified atlas-axis complex; un-coossified neurocentral sutures of the vertebrae; and porous nature of the bones suggest that these individuals are young, skeletally immature juveniles. Together, TMM 31100-1336 and SMNS 5770 indicate that aetosaurs exhibited a well ossified carapace early in their development, suggesting that their osteoderms may have been partially ossified to some degree at birth, which is unlike the condition observed in extant crocodilians. Although the specimens preserved within TMM 31100-1336 are skeletally immature, the morphology of their dentition, mandible, basicranium, dorsal and ventral osteoderms support their referral to the co-occuring pseudosuchian aetosaur Coahomoasuchus kahleorum. TMM 31100-1336 brings to light characters that are subject to intraspecific variation within the Aetosauria.