Date of this Version
The White River Group in the Chadron area includes at least 400 ft (123 m) of terrigenous sediment. divisible into the Chadron Formation and Brule Formation (Orella and Whitney members) on criteria similar to those used at Toadstool Park. Fossils were collected from several levels. and are grouped into the Brecht Ranch Local Fauna (Late Chadronian). Dead Horse Local Fauna (Early Orellan). Rabbit Graveyard Local Fauna (Early Middle Orellan). and Bartlett High Local Fauna (Late Middle Orellan). No Late Orcllan fauna was found, and Whitneyan fossils were scarce. Zonation of the Orellan appears to be possible on the basis of size of Ischyromys specimens and on development of auditory bullae of Merycoidodon. Statistical analysis of oreodonts in the Dead Horse Local Fauna indicates only two abundant genera and species at that level.