ANOMALOUS PALEOECOLOGY OF A NEOGENE FOSSIL SITE, GRAY, TN, USA: UTILIZING STABLE ISOTOPE ANALYSES TO INTERPRET CANOPY DENSITY

Determining relative canopy density in C₃ ecosystems is viable because of predictable differences in δ¹³C values attributed to irradiance and/or CO₂ levels. δ¹³C values decline with increasing canopy density; therefore, relative canopy density can be determined. Extant tapirs are highly conservative in dietary and migratory behavior, and typically inhabit the densest canopy environments available; consequently, tapirs are model organisms for comparing relative canopy density between sites.

Eastern Tennessee's Gray Fossil Site, biostratigraphically dated between 4.5 and 7 million years ago, is highly fossiliferous containing a unique combination of taxa, including the highest known concentration of Neogene tapirs and several taxa with Eurasian ancestry. Due to the rarity of late Cenozoic fossil sites in eastern North America and the ideal nature of fossil tapirs for such analyses, the Gray site has the potential to clarify currently unknown paleoecological phenomenon during the Neogene. In addition, the Gray site's faunal and floral macrofossils suggest the presence of North American forest refugia; therefore, stable isotope sampling of herbivorous taxa may support this hypothesis and clarify paleoecological dynamics.

Constraining our hypothesis with knowledge of temperate fossil pollen, the Gray site represents a relatively closed canopy environment. Tapiravus polkensis is a hyperbrowser that inhabited dense, closed canopy environments. Teleoceras sp., morphologically presumed grazers, yields stable carbon isotope data suggesting their presence as forest dwelling browsers. Since palynological data demonstrates negligible amounts of grass pollen, and grass pollen is typically overrepresented when found, Teleoceras sp. are presumed to be browsers as opposed to mixed feeders/C₄ grazers. Additionally, oxygen isotopes suggest average cooler temperatures at Gray than Florida fossil sites of similar age. These data further support the presence of North American “refugia” in the southern Appalachians that may have contributed towards delaying the C₄ grassland transition that was occurring in western North America at that time.