DIETARY PALEOECOLOGY OF UNGULATES IN RELATION TO ENVIRONMENTAL CHANGE IN THE MIOCENE DOVE SPRING FORMATION, CALIFORNIA

Tectonic processes during the Miocene had dramatic effects on the topography and climatic gradients of western North America. Along with global cooling, these changes contributed to the contraction of forest habitats and the expansion of novel open-canopy habitats. Mammals responded by evolving feeding ecologies that exploited new vegetation resources, forming communities without modern analogues. We analyzed the dietary ecology of ungulate mammals from the Dove Spring Formation (12.5 to 8.5 Ma), located in the Mojave region of southern California. Using a well-resolved chronology and stable isotopes of carbon and oxygen, we described paleoecological changes in three families (Antilocapridae, Camelidae, and Equidae) over half-million-year time intervals.

We sampled enamel of 158 cheek teeth from three ungulate families. Carbon isotopes of herbivore enamel track the photosynthetic pathway of plants consumed, with a range of depleted δ¹³C values suggesting consumption of C3 plants and a range of wooded to open environments throughout the basin. With high-resolution stratigraphy and geochronology, we investigated correlations to environmental change linked to local tectonic history. The δ¹³C values of the Dove Spring ungulate community range from -27.0‰ to -5.7‰, becoming more depleted over time. Significant changes in diet occurred at 10.0 Ma, 9.5 Ma, and 8.5 Ma. At 10 Ma, an interval of basin rotation and westward translation coincides with reduced variation in δ¹³C. Basin extension beginning around 9.0 Ma was coeval with an increase in δ¹³C variation near the top of the formation. Individual families show similar trends, although the timing of change in their diets differs. Antilocapridae and Camelidae exhibit dietary change at 10.5 Ma and 9.5 Ma, respectively, while Equidae exhibits significant changes at 9.5 Ma and four additional 0.5-Myr intervals.

Our ¹⁸O results show no significant changes over time, suggesting no major changes in precipitation regime. We also analyzed paleosol carbonates as an independent estimate of vegetation; δ¹³C_SOIL values range from -8.7‰ to -6.1‰ and δ¹⁸O_SOIL values range from -18.1‰ to -7.7‰. Long-term trends in enamel and soil carbonate δ¹³C values may indicate an increase in canopy cover related to drainage changes triggered by uplift in the Sierra Nevada.