PALEOFAUNA FROM THE LATEST EOCENE CHADRON FORMATION, HIGHWAY 44 LOCALITY, IN SOUTHWESTERN SOUTH DAKOTA, AND ITS IMPLICATIONS CONCERNING THE EOCENE/OLIGOCENE TRANSITION

The Chadron Formation has been extensively studied for 150 years, but the assemblage from the Peanut Peak Member (late Chadronian (Ch3)) remains relatively poorly known. The Highway 44 local fauna from the upper Chadron Formation is important for understanding the climatic shift that occurred at the Eocene/Oligocene boundary. This assemblage aids in the determination of the proposed faunal turnover that has been given as evidence for climatic change.

Within the Highway 44 assemblage are the first documented appearances in the South Dakota Chadron Formation of Serpentes and Cricetidae. The range of Metanoiamys, an eomyid known from the Uintan and Duchesnean of southern California and Saskatchewan, is extended upward to the late Chadronian. First appearances of the florentiamyid rodent, Ecclessimus, and the anguid lizard, Peltosaurus are extended from the Orellan to the late Chadronian.

This assemblage when compared to other Chadronian and Orellan paleofaunas was found to be most similar to other Ch3 assemblages. The Highway 44 assemblage was found to contain approximately equal faual content from the Chadronian and Orellan paleofaunas suggesting this assemblage is near the Chadronian/Orellan transition.