RECENT NORTH AMERICAN DISCOVERIES ILLUMINATE THE EVOLUTION OF AN ENIGMATIC CLADE OF TRIASSIC MARINE REPTILE

Thalattosaurs were a group of secondarily marine reptile that were widely distributed across the Northern Hemisphere for most of the Triassic. Despite the broad geographic distribution of the group, taxa within the clade exhibit strong endemism. Among thirteen currently recognized genera, five are known exclusively from western North America, four exclusively from China and another four exclusively from Europe. Many species, and in some cases entire genera, are known from single specimens, often incomplete or poorly preserved. Thalattosaurs exhibit distinctive cranial adaptations that likely reflect specialized foraging behaviors and more subtle but informative post-cranial morphology that set them apart from contemporaneous marine reptile groups such as ichthyosaurs and sauropterygians. Thus, despite their spotty fossil record, the group remains a fascinating but poorly understood example of a terrestrial-to-marine transition during a period of revolutionary change in marine ecosystems. Recently described thalattosaur fossils from Alaska, newly discovered material from Oregon, and recently re-discovered historic specimens from California all provide important new morphological data on the group. A new phylogenetic analysis incorporating this material, including additional fragmentary specimens excluded from prior analyses, reveals key insights into the evolution of the group. The new results indicate a complex history of dispersal across ocean basins, although coastal dispersal around ocean basins, or even brief overland or freshwater mediated dispersal, cannot be excluded in some cases. Disparate postcranial morphology may reflect distinct locomotory modes that likely influenced the ecology and biogeographic history of individual lineages. Finally, pronounced rostral deflection is common for many thalattosaur taxa but the evolutionary history of this feature is complex, with multiple lineages exaggerating or reducing deflection, potentially recording dietary specialization and rapid morphological adaptation. Ongoing study of previously described and newly discovered material, aided by emerging technology such as microCT imaging and modern phylogenetic techniques, promises to reveal new clues into the evolutionary history of this enigmatic evolutionary experiment.