Dietary fidelity of Miocene ungulates in the context of environmental change in the Mojave Region, western North America

The fossil record of mammals preserves evidence for dietary adaptations that allowed lineages to persist in dynamic ecosystems for tens of millions of years. We investigated ecological attributes of fossil mammals during the middle to late Miocene (17.5–8.5 Ma) in the western Mojave region of North America to evaluate the response of herbivorous ungulates to paleoenvironmental changes. Herbivores may utilize the same food resources across generations, relying on relationships with vegetation and habitats established over multiple generations. We employed stable isotopes of carbon and oxygen from ancient soils and herbivore tooth enamel to evaluate changes in vegetation and herbivore diets. We compiled isotopic data from published studies of three sequences from the warm Miocene Climatic Optimum and added new data from the Dove Spring Formation, which formed during the cooler Middle Miocene Climatic Transition. Herbivorous ungulates exhibited dietary fidelity for nearly nine million years, selectively consuming C₃ plants even as C₄ vegetation became more prevalent on the landscape. High oxygen stable isotope ratios in comparison to equids suggest that ancestral antilocaprids and camelids were likely facultative drinkers. These findings reveal enduring dietary preferences that enabled these herbivores to persist amid the profound environmental transitions of the Miocene.