ASSESSING THE RESPONSE OF MAMMALIAN FAUNAL STRUCTURE TO THE EARLY OLIGOCENE CLIMATE TRANSITION

The earliest Oligocene marks one of the most major climate transitions in the Cenozoic. High latitude marine temperatures indicate a temperature drop of 4-5°C over the course of 300,000 years, associated with the formation of a permanent Antarctic ice sheet. Climate change in the mid-latitudes, particularly in terrestrial environments is more difficult to resolve, although there are indications of both cooling and drying, and associated faunal and floral turnover. In North America, mammalian response to this climate shift, as recorded in the classic outcrops of the White River Group, is enigmatic. Extensive museum collections show apparently little, if any, evolutionary response (extinction or origination) of the White River Group fauna during this period.

In order to determine whether, instead of an evolutionary response, the White River Group fauna exhibited an ecological response to this climate transition, new, temporally constrained samples were collected spanning the interval of the climate transition in and around Badlands National Park, SD. Extensive taphonomic data were assembled for each collected specimen, and used to establish isotaphonomy among samples. Changes in the abundances of taxa among isotaphonomic samples indicate true ecological changes, rather than potential artefactual shifts caused by varying taphonomic bias. Analysis of the faunal structure of isotaphonomic samples across the climate transition shows a directional shift in the abundances of several taxa with time (including Palaeolagus and Merycoidodon). This is interpreted to represent an ecological response to the climate shift, as paleoenvironments become drier and more open. This is the first time such a change in faunal structure has been demonstrated using vertebrate taxa in a quantifiably isotaphonomic framework.