Fossil collecting in the White River Group has been done for over 150 years, with limited new information on the fossil fauna. A new find out of an old discovery, in this case, provides an opportunity to re-examine specimens that have been overlooked. The Museum of Geology, at SDSM&T, has a skull of Leptauchenia (Sespia) nitida that had been on display since the museum was founded. What is interesting about this particular specimen is the recorded stratigraphic location expands the genus Sespia below its traditional stratigraphic and biostratigraphic position. Previous work on biostratigraphy, chronostratigraphy, mangetostratigraphy, and lithostratigraphy has placed a physical boundary on the Whitneyan and Arikareean North American Land Mammal Ages at the contact of the Brule Fm. and Sharps Fm. in South Dakota as well as the Whitney mbr. and “brown siltstone” mbr. in Nebraska. That same work places Sespia well above the Whitneyan-Arikareean transitional marker.
Measurements, taken under the convention of previous researchers, and morphological characters of the skull were used to validate the integrity of the initial identification. Measurements and morphology were compared to previously identified species and type specimens, which were used as taxonomic anchors. The study shows that this specimen is identified properly as Sepsia nitida. The lithology of the remaining matrix on the specimen was utilized to verify its geologic origin based on the recorded locality. The matrix secured to the skull is clay comparable to the Whitney member of the Brule Formation, as seen in Nebraska, which is close to the approximate physical location of the site where the specimen was collected. This new information from a skull collected about eighty years ago is providing new insights on the oreodont subfamily Leptaucheniinae and biogeography of Sespia.