A TAXONOMIC AND TAPHONOMIC DESCRIPTION OF A LARGE UNGULATE FOSSIL FROM THE CHADRON FORMATION OF THE BUFFALO GAP NATIONAL GRASSLANDS, SOUTH DAKOTA

In the summer of 2012, a large fossil mandible with left and right dentary was recovered through traditional field methods from Buffalo Gap National Grasslands, South Dakota. The fossil specimen was excavated from the Crazy Johnson member of the Chadron Formation. The Eocene age Chadron Formation consists of mostly fine clay material with scattered coarse-grained channel sandstones that were deposited in a fluvial environment. Due to poor preservation, laboratory preparation of the fossil consisted of cleaning, reconstructing and strengthening before observations and measurements could be made. This data was used in both qualitative and quantitative analyses to place the specimen in the correct taxonomic family as well as describing both developmental stage and taphonomy. Taphonomically, the specimen exhibited weathering pre and post fossilization, little transportation and no scavenging. After studies of tooth wear patterns and jaw size, it was determined that the animal had reached a sub-adult developmental stage. Large ungulate families from the White River Group (Chadronian NALMA) were considered for taxonomic placement. These include Camelidae, Brontotheriidae, Amynodontidae, Hyracodontidae and Rhinocerotidae. After comparing teeth and mandible morphometrics, all families were ruled out except Rhinocerotidae. The genera of Rhinocerotidae were further examined, which included Penetrigonias, Trigonias, Amphicaenopus and Subhyracodon. Analyses of tooth morphometrics conclude that the fossil mandible should be placed in the genus of Trigonias. Species level placement was not possible since there is no post-cranial data available.