ECOLOGICAL PREFERENCES FOR LARGE DINOCERATANS FROM WASHAKIE BASIN, WYOMING BASED ON ENAMEL STABLE ISOTOPE VALUES

The Dinocerata were large, herbivorous mammals that were a minor, but important part of western North America faunas from the Late Paleocene to Middle Eocene. As one of the first groups of mammals to achieve large body masses (> 1000 kg) in the Eocene, dinoceratans represent an early stage in the evolution of the North American megafauna following the end Cretaceous mass extinction. The recent discovery and collection of multiple specimens of the large dinoceratan Uintatherium from Middle Eocene sediments in the Washakie Basin of southern Wyoming provides an opportunity to assess the diet and habitat preferences of a derived member of this group through carbon and oxygen isotope analysis of tooth enamel. The stable isotope composition of an animal’s tooth enamel can serve as a tracer for it’s diet (carbon isotopes, δ¹³C) and drinking water (oxygen isotopes, δ¹⁸O), which in turn can provide insight into the environmental conditions during the time interval in which animal lived. When combined with paleobotanical and sedimentological records from the basin, enamel stable isotope results can provide new insight into the climate and vegetation conditions for this region during the Middle Eocene.

Multiple samples of enamel (n = 11), dentin (n = 8), and bone (n = 5) from Uintatherium as well as enamel and bone samples from other faunal members (e.g., brontotheriids and hyracodontids) from the Adobe Town member of the Washakie Formation were selected for δ¹³C and δ¹⁸O analysis. Enamel δ¹³C values for Uintatherium (-9.9 ± 0.6‰) were consistent with a diet of C3 vegetation from an open canopy woodland habitat (estimated δ¹³C_diet = -23.7 ± 0.6‰). Enamel δ¹⁸O values (22.7 ± 0.5‰, VSMOW) were low and exhibited little variation among individuals. Estimated drinking water values for Uintatherium (-10.3 ± 0.6‰) were significantly higher than estimates of current δ¹⁸O values for precipitation in the basin (-13.8 ± 1.0‰). Higher δ¹⁸O values for precipitation during this time interval is suggestive of significantly warmer temperatures than today in the Middle Eocene, a conclusion that is supported by records developed from other paleoclimate proxies from the Washakie Basin (e.g., leaf margin analysis, pedogenic carbonates).