FAUNAL ANALYSIS OF AN EARLY PALEOCENE (TORREJONIAN) FAUNA FROM THE CHINA BUTTE MEMBER, FORT UNION FORMATION, GREAT DIVIDE BASIN, WYOMING

Decades of paleontological fieldwork in the Fort Union Formation in the Great Divide Basin (GDB) of southern Wyoming have recovered dozens of fossil vertebrate localities ranging in age from the earliest Paleocene (early Puercan) through late Paleocene (Tiffanian) time. Here, we report a new early Paleocene (Torrejonian) mammalian faunal assemblage from UCM locality 2011059 in the upper China Butte Member of the Fort Union Formation that builds upon the work of others and refines the age of these strata. We identified 14 mammalian genera from six orders from UCM locality 2011059, based upon 48 isolated teeth. Ellipsodon inaequidens, as an index taxon of the To2, indicates that this fauna is Torrejonian in age. First occurrences in the GDB include the neoplagiaulacid multituberculate Neoplagiaulax jepi, the hyopsodontid ‘condylarth’ Ellipsodon inaequidens, and the periptychids Anisonchus sectorius and Periptychus carinidens which also suggest a Torrejonian age to the fauna. The first appearances of Ptilodus montanus, Swaindelphys sp., Acmeodon hyoni, Arctocyon corrugatus, Ellipsodon inaequidens, and Paramomys maturus suggest that the minimum age for UCM locality 2011059 is in the To2 interval zone (or middle Torrejonian time). Mesodma thompsoni, Oxyclaenus cuspidatus, and Ellipsodon inaequidens suggest the fauna is no younger than To2, as these taxa make their last appearance elsewhere in this interval zone. Although N. jepi was previously known to occur in To3 in the Hannah Basin, Wyoming, the other mammalian taxa from UCM locality 2011059 support a middle Torrejonian (To2) age for the locality. Therefore, we interpret the occurrence of N. jepi in the GDB as a temporal range extension. Additionally, the occurrence of E. inaequidens in the GDB represents a considerable geographical range extension northward from the San Juan Basin, New Mexico. Along with prior faunal analyses, our study illuminates the Torrejonian mammalian diversity in the GDB. Future research will focus on recovering more fossil mammals to better correlate the GDB assemblages to other localities in the Western Interior and to better understand the Torrejonian mammalian fauna.