TOOTH WEAR ANALYSES AS PROXIES FOR HABITAT OPENNESS AND AN ASSESSMENT OF DRIVERS FOR CHANGES IN BIODIVERSITY IN THE JOHN DAY BASIN DURING THE EARLY OLIGOCENE

John Day Fossil Beds National Monument represents one of the world’s most complete and continuous records of terrestrial fossils, spanning nearly 50 million years of the Cenozoic (~53 Ma – ~5 Ma). The region is also notable for evidence of significant volcanism, with fossil bearing volcaniclastic sedimentary rocks interrupted by relatively frequent and sprawling ignimbrites, flood basalts, and tuff layers. The abundance of volcanic strata provides numerous radiometric dates for biostratigraphic calibration. Recent revisions of the biostratigraphy and biodiversity of the Turtle Cove Member of the John Day Formation (~32 Ma – 25.9 Ma) provide an opportunity to conduct a high-resolution analysis of changes in paleoecology in respect to climate change and volcanic activity. The Turtle Cove Member consists of 6 dated tuff and ignimbrite layers located amongst 14 distinct fossiliferous units that yield a variety of ungulates (hoofed mammals), including hypertragulids (mouse-deer), camelids (camels), merycoidodontids (oreodonts), agriochoerids (clawed oreodonts), equids (horses), and rhinocerotids (rhinoceroses). The relative abundances of these mammals indicate a shift from closed habitats to more open habitats following a supervolcanic eruption (the Picture Gorge Ignimbrite, 28.7 Ma). Here we present the results of an extensive survey of the tooth wear (microwear and mesowear) of over 300 specimens of Turtle Cove ungulates. We quantified mesowear (i.e., the shape and relief of tooth cusps) and microwear (i.e., the relative abundance of microscopic scratches and pits) as proxies for habitat openness, based on the previously reported correlation between exogenous silica (e.g., soil) and the abrasion of tooth enamel. Our preliminary results indicate periods of increased tooth abrasion levels across taxa, following the larger volcanic events. These proxies suggest large volcanic events periodically increased habitat openness and played prominent roles in ungulate ecology and biodiversity in the John Day Basin during the early Oligocene.