Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 181 NEW MATERIAL OF AMYNODONTIDAE (PERISSODACTYLA) FROM BADLANDS NATIONAL PARK AND ITS IMPLICATIONS ON THE WHITNEYAN-ARIKAREEAN TRANSITION Ed Welsh 1, *, Darrin Pagnac 2 , and Clint A. Boyd 3 1 Badlands National Park Interior, SD 57750 2 South Dakota School of Mines and Technology Rapid City, SD 57701 3 North Dakota Geological Survey Bismarck, ND 58505 *Corresponding author e-mail: edtwelsh@hotmail.com ABSTRACT An astragalus belonging to an indeterminate amynodontid, within the tribe Metamynodontini, is identifed within collections from the Cedar Pass local fauna from the North Unit of Badlands National Park, South Dakota. Tis new specimen represents the youngest occurrence of Amynodontidae in North America. Prior to this study, Metamynodon makes its highest verifed occur- rence in the underlying Scenic Member of the Brule Formation (Orellan North American Land Mammal Age). Te Cedar Pass local fauna originates within the lower Poleslide Member of the Brule Formation and is considered to contain a medial Oligocene fauna (early Whitneyan North American Land Mammal Age). However, matrix afxed to the new specimen is primarily composed of sandstone and gravel consistent with the river channel deposits within the Sharps Formation, which overlies the Poleslide Member. Despite recent stratigraphic reclassifcation of underlying sediments from the basal Sharps Formation to the upper Poleslide Member, the associated Sharps fauna is distinctly characterized as Arikareean North American Land Mammal Age in character. Taxa previously recovered within the Arikaree channel deposits include the entelodont Daeodon, the canid Sunkahetanka, and the rodents Tamias, Proheteromys, and Hitonkala. Specimens of the castorids Capacikala and Palaeocastor were recovered from the underlying Poleslide siltstones. Prior referrals of the channel deposits at the top of Cedar Pass to the classic “Protoceras channels,” along with associated faunal components, confound our understanding of the Whitneyan-Arikareean transi- tion and highlight the need for further research into the fauna of the Sharps Formation in the North Unit of Badlands National Park.