SHOSHONIAN REVISITED: DOCUMENTING THE BRIDGERIAN-UINTAN FAUNAL TRANSITION IN THE TURTLE BLUFF MEMBER OF THE BRIDGER FORMATION, MIDDLE EOCENE, SOUTHWESTERN WYOMING

Informal subdivisions A-D of the Bridger Formation (Blacks Fork and Twin Buttes members) constitute the richly fossiliferous nominal stratotype of the Bridgerian North American Land Mammal Age. However, the uppermost Turtle Bluff Member (TBM) of the Bridger Formation ("Bridger E") is sparsely fossiliferous, so its precise age is uncertain.

To further investigate the Bridgerian-Uintan faunal transition and the validity of the Shoshonian (earliest Uintan) subage, a project was initiated in 2006 to prospect and screenwash all levels of the TBM. During our first field season 1,600 kg of matrix was processed from five localities at four distinct stratigraphic levels. Among these, SDSNH Loc. 5841 (Sciuravus Surprise) is 1 m above the base of the TBM, at essentially the same stratigraphic level as UCM Loc. 92189 (Donna's Locality, discovered in 1992). SDSNH Loc. 5841 has yielded a diverse "Bridgerian-aspect" micromammal assemblage as well as the basal eomyid rodent Metanoiamys sp. and a possible sespedectine insectivore. SDSNH Locs. 5843 (Red Lenses) and 5844 (Roll the Bones) are the stratigraphically highest mammal localities known from the TBM, both occurring 105 m above the base of this member. The low-diversity assemblage is dominated by the small rodents Pauromys sp. and Metanoiamys sp. The characteristic Uintan rodent Pareumys sp. is also present, as well as a derived species of Orohippus or a primitive species of Epihippus, based on a single lower premolar. Conspicuously absent from these new localities, however, are the hypertypical Uintan taxa Protoreodon, Leptoreodon, Leptotragulus, Protylopus, and Amynodon.

If the apparent absence of undoubted selenodont artiodactyls and Amynodon in the TBM should persist in spite of future collecting, then the “Shoshonian” interval might be redefined as an earliest Uintan biochron older than the first appearances of hypertypical Uintan taxa. Alternatively, if the original faunal characterization of the beginning of the “Shoshonian” is retained, then a fourth (and truly earliest) Uintan biochron could be recognized for post-Bridgerian, pre-Shoshonian time. In either case, the Bridgerian-Uintan boundary would be characterized mainly on the basis of the first appearances of smaller-bodied taxa which are represented by more numerous fossils in bulk matrix samples.