Paper No. 8
Presentation Time: 3:35 PM
THE PHYTOSAURS FROM THE NEWARK SUPERGROUP AND EQUIVALENT STRATA: NEW INFORMATION AND INTERPRETATIONS FOR PHYTOSAUR EVOLUTIONARY MORPHOLOGY AND SYSTEMATICS
Phytosaurs are a clade of non-archosaurian archosauriforms that are well known from a nearly global distribution in Upper Triassic sediments, and have been utilized heavily for biostratigraphic and biochronologic correlations of the Late Triassic. Their uniquely derived cranial morphology includes extremely elongated premaxillae, nares that are located posteriorly near the orbits, and, in later taxa, distinctive and variable plate-like or knob-like squamosals. Several recent, explicitly-cladistic phylogenetic analyses included multiple newly collected specimens and recharacterized the cranial anatomy of known specimens in light of apormorphic character states, clarified many ingroup relationships, revealed increased taxonomic diversity, and indicated that the known early phytosaurs do not represent a diversification of a single basal clade but rather represent several cladogenetic events. Non-phytosaurid phytosaurs are known from multiple depositional basins worldwide, and are often associated with Angistorhinus-like taxa, whereas leptosuchomorph phytosaurs, though known globally as well, are best documented from western North America. However, the North American Newark Supergroup and the related sediments now on the eastern side of the Atlantic also document multiple phytosaur specimens and taxa (more than 15 nominal species including Rutiodon carolinensis, Clepsysaurus pennsylvanicus, Angistorhinus talainti, ‘Paleorhinus’ magnoculus) that play a critical role in clarifying the evolution of the clade. In light of our modern understanding of the derived morphology and tested systematic relationships of phytosaurs, those taxa from Newark-equivalent strata appear to be geographically restricted but closely related to some taxa from Texas and Wyoming. Rutiodon, a taxon that has been known for over 150 years, is intermediate between Angistorhinus-like taxa and leptosuchomorphs both in terms of their cranial morphology and systematic relationships. Our modern understanding of phytosaur systematics and their evolutionary morphology can now be combined with our knowledge of the chronologic and geographic distribution of these taxa in order to better characterize the tempo and mode of archosauriform evolution in eastern North America.