A NEW RECORD OF A LATE TRIASSIC ICHTHYOSAUR WITH JURASSIC-LIKE DENTITION REVEALED BY MICRO-CT SCANNING

Ichthyosaurs were marine reptiles that were ecologically dominant during much of the Mesozoic. Ichthyosaur tooth and jaw morphology provides key insights into their adaptations, lifestyle, and evolutionary history. Micro-CT is a powerful tool that allows non-destructive investigation of ichthyosaur internal skull anatomy, previously only revealed by broken specimens or time-consuming, destructive techniques such as serial sectioning. Late Triassic specimens collected from Mexico in the early 20^th Century housed within the University of California Museum of Paleontology (UCMP) shed light on an important window of ichthyosaur evolution. During this transitional interval, typical Triassic forms lived alongside individuals that more closely resembled Jurassic and later forms. During investigation of this material, an unusual partial ichthyosaur jawbone–one of several jaw pieces catalogued as UMCP 27141–was scanned using micro-CT and digitally segmented. The upper jaw exhibits well-preserved premaxillae, maxillae, and vomers, as well as taphonomically displaced nasals. The lower jaw exhibits well-preserved surangulars and relatively well-preserved dentaries, as well as highly splintered splenials. Forty-eight teeth are present in the specimen, including thirteen replacement teeth. A lack of visible connection to the jawbones and tight tooth packing within a dental groove suggests aulacodont tooth implantation, typical of Jurassic ichthyosaurs. Based on c­­­­omparative evidence, this specific piece of UCMP 27141 represents a different individual than the other jaw pieces it is currently catalogued with and likely a different taxon. This previously overlooked specimen improves our understanding of ichthyosaur diversity and distribution in the Late Triassic and clarifies the sequence of acquisition of important characters prior to the major reorganization of marine reptile faunas across the Triassic-Jurassic transition.