LARGE MAMMAL TAXONOMIC RESPONSE TO THE EOCENE-OLIGOCENE CLIMATE TRANSITION IN NORTH-WESTERN NEBRASKA

The Eocene-Oligocene sediments of North-Western Nebraska and South Dakota provide an exceptional record of faunal change associated with the global cooling event that occurs at the end of the Eocene and beginning of the Oligocene (the Eocene-Oligocene Climate Transition, or EOCT). To investigate the impact of the EOCT on the taxonomic composition of the large mammal fauna of the White River Group, we surface collected >4000 identifiable vertebrate specimens from four sites spanning ~500 kyrs either side of the Eocene-Oligocene boundary in and around Toadstool Geologic Park, NE. >3000 specimens were also collected from the same formations in South Dakota. All collected specimens were identified, and a range of taphonomic data were collected from each in order to ensure that any patterns of taxonomic change were ecologically driven, rather than an artefact of preservation. In total, 8000 specimens were taxonomically identifiable, representing at least 23 different genera. Changes in species richness and evenness were examined through the study interval, as were changes in the proportional abundances of the common taxa to reveal any climate change related faunal response.

Multivariate analyses of the preserved patterns of taphonomic modification among the eight assemblages showed no variation likely to bias our diversity estimates, giving us confidence that any observed differences in diversity/abundance are ecologically driven. When accounting for sample size differences, no significant changes in diversity were observed throughout the study interval, but significant changes in abundance were found for Eumys, Ischyromys, Megalagus, Mesohippus, and Poebrotherium. These abundance shifts were gradual throughout the studied section and not focused at or immediately after the EOCT, so it appears that they are being driven by processes other than this climate change. This argues that the environmental shift associated with the EOCT was not large enough to produce a significant response among the large White River mammals. These results provide important context for understanding organisms’ response to climate change in the past, and hence studying responses to present day climate change.