Earth and Atmospheric Sciences, Department of


First Advisor

Dr. Ross Secord

Date of this Version

Summer 7-30-2019

Document Type



Gillham, R.B. 2019. Changes in Mammalian Abundance Through the Eocene-Oligocene Climate Transition in the White River Group of Nebraska, USA. M.S. Thesis, Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, Nebraska, USA.


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Geosciences, Under the Supervision of Professor Ross Secord. Lincoln, NE: August, 2019

Copyright 2019 Robert B. Gillham


Marine records show major cooling during the Eocene-Oligocene Climate Transition (EOCT). Most proxy studies in the White River Group suggest drying across the EOCT, and some suggest cooling. The lower resolution continental record has hindered a direct correlation of the marine climate record to Nebraska. I explore various correlation schemes and what they imply for faunal changes. This study compiles and analyzes data from 4,875 specimens in the University of Nebraska State Museum (UNSM) collection to test the hypothesis that climate change across the Eocene-Oligocene (E-O) boundary caused significant abundance changes in mammals. A series of binning schemes was created. One binning scheme followed previously established lithological zones, two schemes were based on average sediment accumulation rates, and three more were created by applying a cubic spline curve to published 206Pb/238U zircon ash dates. For the purpose of correlating the marine and Toadstool sections, I constructed a high-resolution (±0.5 m) carbon isotope stratigraphy across the E-O boundary using fossil enamel from the oreodont Merycoidodon. Results show that turnover in taxonomic abundance occurs throughout the study interval and is not concentrated across the EOCT. The largest pulse of faunal change and largest abundance changes for the most common taxa, Merycoidodon and the horse Mesohippus, slightly predate the EOCT. This raises the possibility that climate change began earlier in the continental interior than indicated by the marine benthic oxygen isotope record. Chord distance analyses reveal that the faunal composition of Orellan zones are more similar to one another than they are to the faunas of Chadron zones. This similarity is likely caused by the extinction, or near extinction, of Chadron taxa like Megacerops around the EOCT. Despite the lack of significant change in evenness, numerous taxa underwent extended changes in relative abundance through time. Archaeotherium, a water-dependent artiodactyl, decreased in relative abundance through time just as Poebrotherium, a water-independent camelid, increased in abundance through time. Changes in the relative abundances of Poebrotherium and Archaeotherium are consistent with a drier environment beginning in EOCT. The level of water-dependence in other taxa is less clear, and their changes in abundance cannot be confidently explained through diet, dentition, body mass, or locomotion.

Advisor: Ross Secord