MICROWEAR ANALYSIS OF UNGULATES FROM THE ASHFALL FOSSIL BEDS: EFFECTS OF VOLCANIC ASH ON MICROWEAR

The Ashfall Fossil Beds of Nebraska (Late Miocene, ~12 Ma) contain a taxonomically diverse mass death assemblage (including the rhinocerotid Teleoceras and camelid Procamelus), which formed from with a large volcanic eruption. Bone pathologies indicate animals persisted for days to weeks after the eruption, with larger mammals lasting longer and found stratigraphically higher. Evidence of consumption of vegetation covered in silica-rich volcanic ash (mean grain size = 64.3 µm) by these animals could come from microscopic tooth wear analysis. Microwear analysis provides paleodietary interpretations (i.e., grazer vs. browser) by comparing tooth wear patterns (e.g., scratches and pits) of extinct mammalian herbivores to those of their extant relatives. Controlled feeding trials show abiotic silica (i.e., grit ) can increase relative abundances of microwear features, but it remains unclear whether these trials are relevant to natural conditions (e.g., ash, dust, sand). Mammals feeding on ash-covered vegetation in this unique environment are ideal for testing the hypothesis that consumption of silica-rich volcanic ash obfuscates differences in microwear patterns between browsers and grazers.

To assess ‘grit effect’ in a natural environment, we analyzed microwear on fossil teeth of Ashfall ungulates, including 21 specimens of 3 grazing taxa (families Equidae and Rhinocerotidae) and 13 specimens of 3 browsing taxa (families Camelidae, Moschidae, and Merycoidodontidae). Dietary category was based on phylogeny and hypsodonty. Since microwear is overwritten on short timescales (i.e., the “Last Supper Effect”), the tooth wear of the Ashfall mammals reflects their final ash-covered meals.

We found scratches to be the predominant wear feature on the occlusal surfaces of all individuals, including browsers. The scratch-dominated wear patterns of Ashfall browsers offer striking contrast to the less worn patterns of related taxa from non-volcanic localities, such as Coffee Ranch (Texas) and Cambridge (Nebraska). The mean scratch:pit ratios for Ashfall browsers and grazers are statistically indistinct (Students t-test, p=0.6113), which suggests ingested ash at Ashfall contributed toprevalence of microwear scratches among local ungulates and shows that exogenous grit contributute to microwear.