ECOLOGICAL RESPONSE TO ENVIRONMENTAL CHANGE: INSIGHTS FROM THE MIOCENE SMALL-MAMMAL RECORD OF SOUTHERN CALIFORNIA

Rich assemblages of small-mammal fossils from the Cenozoic record of North America allow us to investigate ecological and evolutionary responses to environmental change over geologic time. To assess how warming during the Miocene Climatic Optimum (MCO; 17–14 Ma) influenced basin-scale environmental conditions, small-mammal paleoecology, and community assembly, we evaluated fossil records from the Crowder (17–16 Ma) and Cajon Valley (16–13.7 Ma) formations in southern California. Dental traits and isotopic composition reflect aspects of an animal’s body size and diet; therefore, we utilized small-mammal cheek teeth to 1) measure hypsodonty index and occlusal area, and 2) sample the carbon and oxygen isotopic composition of tooth enamel using in situ laser-ablation mass spectrometry. In both formations, we found stable values through time for hypsodonty, occlusal area, and carbon and oxygen isotopic composition, indicating dietary stability in each basin during the MCO. In contrast, the older Crowder and younger Cajon Valley faunas differed significantly in isotopic composition. Paleoenvironmental inference using phytoliths and carbon isotopic composition of preserved soil organic matter did not show a corresponding shift in the proportion of C₄-grass composition during this time. Therefore, the shift in small-mammal diets between adjacent basins and over geologic time (warming to cooling stages of the MCO) was likely driven by changing dietary preference and increased consumption of C₄ grasses by Cajon Valley rodents, rather than by baseline shifts in available resources. The isotopic composition of rodent teeth from these formations suggests an early, patchy presence of C₄ grasses in the region prior to C₄-grassland expansion in the late Miocene. Within fossil assemblages, we found little ecological differentiation among co-occurring rodents. This pattern contrasts with modern desert-rodent assemblages, in which co-occurring species often differ significantly in isotopic composition, body size, and hypsodonty. These results imply that constraints on community assembly during the MCO differed from what we observe in western ecosystems today.