THE IMPACTS OF VOLCANISM ON UNGULATE TOOTH WEAR AND ECOLOGY AT JOHN DAY FOSSIL BEDS NATIONAL MONUMENT

The John Day Fossil Beds National Monument (JODA) maintains and displays an impressive collection of fossils that includes floral and faunal specimens spanning nearly 50 million years of Cenozoic history (~53 Ma to ~5 Ma). The strata and fossils of the John Day Basin document changes in climate and landscape, including major climatic transitions (e.g., Eocene-Oligocene transition) and frequent volcanic events. Volcaniclastic strata are punctuated by numerous ignimbrites, flood basalts, and tuffs that not only allow high-resolution dating, but also signify major landscape-modifying events that affected floral and faunal communities. The relationship between volcanism and JODA biota is best represented by the Turtle Cove Member (~32 Ma to 25.9 Ma) of the John Day Formation, a series of 14 fossiliferous units with 6 dated tuff and ignimbrite layers.

Over the last few years, the Geoscientists-in-the-Park (GIP) program has provided opportunities to elucidate the relationships between the fauna, landscape, volcanism, and climate of JODA. Previous GIP studies on the Turtle Cove fauna focused on faunal turnover of mammals following volcanic eruptions and corresponding changes in the tooth wear gradients of hypertragulids (mouse-deer), which represent the most abundant taxa in the fauna. We have expanded the research on tooth wear by analyzing cheek teeth of numerous hoofed mammals (ungulates), including equids (horses), merycoidodontids (oreodonts), camelids (camels), and agriochoerids (clawed oreodonts). Combined with the hypertragulids, this assemblage provides contrasts in body size (used as a proxy for feeding height) and dentition (e.g., selenodont versus lophodont) that represent the diverse ecological roles of ungulate taxa in the early Oligocene. We assessed the mesowear (i.e., cusp shape and relief) and microwear (i.e., relative abundance of microscopic scratches and pits) of nearly 500 ungulate specimens and made inter- and intraspecifc comparisons to quantify changes in dietary abrasiveness over time and examine differences related to body size and relative species abundance. We document increased variance in tooth wear values following a supervolcanic eruption (the Picture Gorge Ignimbrite, 28.7 Ma), reflective of the increased openness of habitats following volcanic events.