Oregon Oligo-Miocene herbivore community structure: Insights from morphology and stable isotope analysis

Extant ungulates exhibit a wide range of niche-partitioning strategies to reduce competition. Niche partitioning is thought to facilitate species coexistence in highly diverse communities. However, how ungulates partitioned plant-food resources in the past and how those relationships changed over time in response to vegetation dynamics remains unclear. We present a synthesis of stable carbon and oxygen isotope compositions from newly analyzed (n = 134) and previously published (n = 77) fossil tooth enamel specimens from three Oregon Oligo-Miocene fossil assemblages (John Day, Mascall, and Rattlesnake Formations). We pair these data with body mass and tooth-crown height estimates to investigate ecological changes in Oregon's ungulate communities during the Oligo-Miocene (∼32–5 Ma), a time when grassland expansion coincided with significant changes in North American ungulate diversity. We find isotopic evidence that ungulates partitioned food resources in C₃-dominated ecosystems, particularly before and during the Mid-Miocene Climatic Optimum (MMCO). In contrast, the post-MMCO Rattlesnake Formation ungulates consumed isotopically similar C₃ plants. Morphological evidence supports these shifts, with a transition from low-crowned to mesodont and hypsodont teeth and a decline in small-bodied taxa. This work underscores the value of integrating isotopic and morphological data to better understand the ecological evolution of extinct communities.