RE-EVALUATION OF THE MAMMALIAN COMPONENT OF THE CEDAR PASS LOCAL FAUNA: ASSESSING THE WHITNEYAN-ARIKAREEAN TRANSITION WITHIN BADLANDS NATIONAL PARK, SOUTH DAKOTA

The collections at the South Dakota School of Mines and Technology and at Badlands National Park contain a substantial number of specimens collected from the Cedar Pass area of Badlands National Park (~3 miles northeast of Interior, SD). The majority of these specimens were collected from a series of stratigraphically restricted localities in the Poleslide Member of the Brule Formation that together comprise the Cedar Pass Local Fauna. This fossiliferous horizon is situated 14-20 meters below a prominent white layer that is often correlated with the Rockyford Ash. This horizon, which is historically referred to as either the “Leptauchenia beds” or “Leptauchenia clays,” contains the first abundant occurrences of the taxon Leptauchenia and traditionally has been one of the archetypical late Whitneyan faunas. Conversely, the first appearances of taxa that typify the Arikareean NALMA in the local section were previously reported above the prominent white layer. Given the biostratigraphic importance of this fauna and the abundant material available for study, a detailed investigation of this fauna focused on re-assessing prior taxonomic identificatio­ns was undertaken. While this study is still ongoing, we note the presence of several characteristic Arikareean taxa within this fauna along with a deficit of many taxa with last appearances in the Whitneyan. Four species previously proposed to characterize the beginning of the Arikareean NALMA are present (Leidymys blacki, Palaeocastor nebrascensis, Palaeocastor peninsulatus, and Palaeolagus hypsodus), as is the Arikareean taxon Nanotragulus. The canid fauna is dominated by borophagines, including the Arikareean taxa Phlaocyon and Cynarctoides, while no known specimens of Hesperocyon are noted within this interval. However, not all constituents of the White River Chronofauna are absent as numerous specimens of Merycoidodon and Hyracodon were recovered from this interval. These results suggest that the Whitneyan-Arikareean transition occurs stratigraphically lower within the local section than previously proposed. Additionally, this study further highlights the paucity of comprehensive Whitneyan faunas and the vague transition from the Whitneyan to the Arikareean in the High Plains.