STRATIGRAPHY, VERTEBRATE PALEONTOLOGY, AND PALEOECOLOGY OF THE WASATCH FORMATION, FOSSIL BUTTE NATIONAL MONUMENT, WYOMING

Fieldwork conducted in the Wasatch Formation by a joint Albion College-University of Michigan expedition in conjunction with Fossil Butte National Monument has yielded a diverse assemblage of early Eocene vertebrates. Fossil vertebrates are recovered from three stratigraphic intervals within the Main Body of the Wasatch Formation underlying the Green River Formation. The first assemblage is derived from the lowest 20 meters of the section which is characterized by predominantly mudstone units representing lake-margin and fluvial deposits overprinted by immature paleosols. The middle assemblage comes from 40 meters of the section consisting of slightly more mature paleosol overprinted mudstones interbedded with a few channel and splay sandstones. The last assemblage comes from the uppermost 20 meters of the section which consists of an even mix of channel and splay sandstones interbedded with floodplain mudstones overprinted by mature paleosols. The upper fluvial beds rapidly grade upward to lake-margin mudstones just below the contact with the Fossil Butte Member of the Green River Formation.

The combined assemblages contain at least eight species of reptiles and as many as 34 species of mammals. Reptile species include: three glyptosaurine lizards, an emydid turtle, two trionychid turtles, a crocodylid, and an alligatorid. Mammal species include: five carnivorans, three perissodactyls, seven condylarths, four artiodactyls and two notharctine primates, among other less well represented taxa. The lower two assemblages are dominated by aquatic reptiles while the upper assemblage is dominated by mammals and lizards. Each assemblage contains characteristic Lysitean (Wa6) elements, but the probable occurrences of the condylarth Ectocion superstes in the middle horizon and the brontotheriid perissodactyl Lambdotherium in the upper horizon would indicate a Lostcabinian (Wa7) age for the upper part of the Wasatch Formation in Fossil Basin and, perhaps, the overlying Fossil Butte Member of the Green River Formation as well.