VERTEBRATE FAUNAL DYNAMICS ACROSS THE EOCENE/OLIGOCENE BOUNDARY IN THE U.S. WESTERN INTERIOR OF NORTH AMERICA

The Eocene/Oligocene boundary marks the first significant Cenozoic global climate shift from a warm, ice-free greenhouse world to a cooler, drier icehouse climate. Faunal changes associated with this transition are preserved in the highly fossiliferous White River Group/Formation of the intermontane basins of the Rockies and Great Plains of North America. While previous studies have used range data and raw species richness to examine faunal changes at the Eocene/Oligocene boundary in North America, none have utilized fossil abundance data to investigate the diversity dynamics across this boundary.

The goal of this study is to quantitatively analyze faunal occurrence data from the White River Group/Formation across the Eocene/Oligocene boundary in Wyoming, Nebraska, South Dakota, and Colorado to explore patterns of origination, extinction, and abundance linked to the climate transition. To do this, we analyzed over 5,000 occurrences of fossil mammals, reptiles, and birds recorded in the Paleobiology Database, as well as the vertebrate paleontology collections at the University of Wyoming and Tate Geological Museum. Rarefaction analysis and diversity indices provide insight into the faunal turnover associated with this time of significant climate change.

Our results demonstrate significant vertebrate faunal turnover and shifts in abundance across the Eocene/Oligocene boundary. Approximately 55% of the latest Eocene vertebrate fauna either went extinct or were extirpated from the Western Interior during this transition. Despite this wave of extinctions, generic richness increased markedly, rising from 47 genera in the latest Eocene to 78 genera in the early Oligocene. This increase in diversity is primarily driven by the appearance of 52 new vertebrate genera in the Oligocene. These findings suggest that the OI-1 glaciation event at the onset of the Oligocene triggered profound changes in vertebrate communities, underscoring the extensive ecological impact of the Eocene/Oligocene climate transition in the American West.