60-57 Faunal and Ecological Turnover Across the Eocene–Oligocene Transition in the U.S. Western Interior

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

Presenting Author:

Authors:

Abstract:

The Eocene–Oligocene transition marks a critical global climate shift from a warm, ice-free greenhouse world to a cooler, glaciated icehouse regime. This change is well recorded in the vertebrate fossil assemblages of the White River Formation/Group across the intermontane basins and Great Plains of the U.S. Western Interior. While previous studies have explored this stratigraphic interval’s faunal composition, paleoecology, sedimentology, and stratigraphy, quantitative analyses of faunal turnover across this boundary remain limited. 

To better characterize faunal change across this interval, we quantitatively assessed taxonomic diversity and functional ecological shifts using fossil vertebrate occurrence data—including mammals, reptiles, and birds. These data were compiled not only from the Paleobiology Database but also from museum collections at the University of Wyoming, Tate Geological Museum, University of Colorado Boulder, and South Dakota School of Mines. Importantly, museum collections represent an untapped resource for obtaining detailed vertebrate occurrence data that can greatly enhance our understanding of faunal dynamics in deep time. 

Our analyses reveal notable changes in vertebrate diversity and ecology across the Eocene–Oligocene boundary. Rarefaction analyses show both assemblages are well sampled, with curves nearing their asymptotes. Despite a smaller sample size and lower generic richness, the Chadronian curve rises higher than the Orellan curve. This suggests that Eocene diversity may be underestimated and indicates a decline in vertebrate diversity across the boundary. Functional ecological data reveal a shift in life habits: both faunas were dominated by browsers and mixed herbivores (frugivores, granivores), consistent with forested habitats. However, the Orellan assemblage features grazers and lacks omnivores, suggesting a transition to more open, heterogeneous landscapes. Together, these results indicate significant taxonomic and ecological restructuring of vertebrate communities in the U.S. Western Interior during the Eocene–Oligocene transition, likely driven by environmental changes linked to the onset of Antarctic glaciation during the OI-1 event. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9209

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