SPHENODONTIAN ASSEMBLAGES OF THE CHINLE FORMATION: PHYLOGENETIC, BIOGEOGRAPHIC, AND MORPHOLOGICAL IMPLICATIONS

Sphenodontians are common globally in the early Mesozoic, but are only known from a few occurrences in the Triassic of the southwest USA. Recent fieldwork focused on collecting microvertebrates from the Chinle Formation of northeastern Arizona has resulted in the discovery of three diverse sphenodontian assemblages, including four new species of both basal and derived sphenodontians. Additionally, a re-assessment of the Placerias Quarry assemblage housed at the Museum of Northern Arizona has revealed a wealth of sphenodontian material, also including a new basal sphenodontian taxa. A phylogenetic analysis of the new sphenodontian taxa described in this study significantly increases the diversity of this group in the Chinle Formation. These new discoveries also add to the morphological diversity of Chinle tetrapods, exhibiting new dentition patterns and inferred feeding strategies. It is apparent that at this early stage in rhynchocephalian evolution, members of this group had diversified and specialized to occupy different feeding niches within the same ecosystem, including insectivory and herbivory. The biostratigraphy of sphenodontian taxa in the Chinle Formation, contextualized with U-Pb isotope chronostratigraphy, add to evidence of rapid faunal turnover in the Norian of Southwest North America. Additionally, the similarity of these sphenodontian assemblages to those from Upper Triassic rocks in Europe suggest biogeographic homogeneity of sphenodontian assemblages in Laurasia during the Late Triassic.