Paper No. 196-6
Presentation Time: 9:00 AM-6:30 PM
CHANGES IN MAMMALIAN ABUNDANCE THROUGH THE EOCENE-OLIGOCENE CLIMATE TRANSITION IN THE WHITE RIVER GROUP OF NEBRASKA, USA
High-resolution marine records show major cooling at high latitudes during the Eocene-Oligocene Climate Transition (EOCT). Studies in the White River Group of Nebraska suggest both drying and disputed levels of cooling appear to have affected terrestrial faunas, but the lesser resolution of the EOCT in continental strata has hindered a direct comparison of climate change from the marine record and faunal changes on land. The present study compiles and analyzes data from 4,875 fossil specimens in the University of Nebraska State Museum (UNSM) collection to test the hypothesis that climate change across the Eocene-Oligocene (E-O) boundary resulted in significant abundance changes in mammals. Most specimens are associated with information that allows them to be assigned to specific stratigraphic levels relative to established marker beds. I examined taxonomic abundance changes using six binning schemes. One scheme followed the lithological zones of C. B. Schultz and T. M. Stout of the UNSM, two schemes were based on sediment accumulation rates, and three more were created by applying a cubic spline curve to published 206Pb/238U zircon ash dates and high-resolution (±0.5 m) carbon and oxygen isotope stratigraphies from enamel of the oreodont Merycoidodon. The greatest pulse of abundance changes corresponds to EOCT, but it is unclear if this is driving the change or not. Chord distance analyses, or measures of overall similarity, reveal the faunal composition of Orellan zones are more similar to each other than they are to those of the Chadron zones. This similarity is likely caused by the extinction, or near extinction, of Chadron taxa like Megacerops around the EOCT. Despite this, diversity indices of the fauna do not significantly change between any of the bins or zones. However, numerous taxa also undergo extended increases or decreases in relative abundance through time. Archaeotherium, a water-dependent taxon, continually decreases in relative abundance through time just as Poebrotherium, a water-independent taxon, steadily increases in abundance through time. These changes in abundances are consistent with a drying environment. The level of water-dependence in other taxa is less clear, and their changes in relative abundance cannot be confidently explained through diet, dentition, body mass, or locomotion.