BRIEF REPORT ON THE GEOLOGY AND PALEONTOLOGY OF FOSSIL LOCALITIES WITHIN THE CHADRON FORMATION (EOCENE) IN THE NORTH UNIT OF BADLANDS NATIONAL PARK, SOUTH DAKOTA

The Chadron Formation (Eocene) in South Dakota produces numerous fossils of extinct taxa, including brontotheriids, nimravids, and alligatorines. The majority of these fossils were historically found on private lands, national grasslands, or the Pine Ridge Reservation. Despite the fact that over 250 fossil localities were documented within the North Unit of Badlands National Park between 1979, when the park was designated, and 2009, only a single fossil locality was created within the Chadron Formation (BADL-LOC-0217: discovered in 2005). Material collected from this locality consisted mainly of fragmentary remains of brontotheriids, equids, and testudines. In 2010, two new fossil localities (BADL-LOC-0260 & 0261) were discovered within the Chadron Formation. Only 5-8 meters of the Peanut Peak Member of the Chadron Formation is exposed at each of the localities, representing a wide range of depositional environments. The lithologies at BADL-LOC-0217 and 0260 are predominantly claystones and mudstones interpreted as floodplain deposits. At BADL-LOC-0261 the lithologies consist of sandstone meandering channel fills with prominent crossbedding, and claystones to mudstones interpreted as floodplain deposits and possible oxbow lake deposits. All three fossil localities preserve numerous identifiable fossil remains. In addition to more common taxa (e.g. Eumys, Palaeolagus, Merycoidodon, Archaeotherium and Mesohippus), these localities also contain material from some of the rarer taxa including Brontops, Trigonias, Colodon, cf. Elomeryx, and Alligator. The majority of recovered specimens were in situ and well preserved, ranging from complete tooth rows to a complete cranium of Brontops. Based on the data collected from these three fossil localities, the Chadron Formation within the North Unit is shown to produce well-preserved specimens of high scientific importance. These new discoveries highlight the need for a comprehensive survey of the Chadron Formation within the North Unit similar to those previously conducted for the Brule Formation (Oligocene). Additionally, further investigation of these localities could provide important information relevant to addressing questions concerning biogeography, biostratigraphy, and the terrestrial faunal Eocene/Oligocene transition.