COMPARISON OF LOSTCABINIAN (LATE EARLY EOCENE) MAMMALIAN ASSEMBLAGES FROM THE GREEN RIVER AND WIND RIVER BASINS, WYOMING

Abundant fossil vertebrates from the Eocene Lostcabinian Subage (latest Wasatchian, Wa7, Lambdotherium Zone) are known from the type area in the Wind River Basin and from new localities at The Pinnacles in the Green River Basin. New collections allow for comparisons between similar aged faunas in adjacent basins derived from different depositional environments and produced by different field methods.

The Pinnacles are an area of badlands representing the top 40 meters of the Main Body of the Wasatch Formation exposed beneath an escarpment formed by the overlying Green River Formation at the northern fringe of the Red Desert. The deposits consist of crevasse splay sandstones and flood basin mudstones overprinted by mature paleosols. Isolated ribbon sand bodies representing meandering stream channel deposits contain a rich and diverse vertebrate assemblage preserved as a lag of small teeth and bones along cross-bed laminae. Most specimens are collected by prospecting and screening of anthills. The Lost Cabin area of the Wind River Basin exposes the upper Wind River Formation which is lithologically similar to the Wasatch, but most fossils are recovered by surface prospecting and quarrying in overbank mudstone and mixed lime-mud deposits.

The Pinnacles have yielded over 800 mammalian specimens making it one of the world’s richest Wa7 sites. The assemblage includes ~53 species of mammals representing 15 orders and 30 families. The type Lostcabin has produced over 500 specimens with ~65 species, 17 orders, and 31 families represented. Of these species, only ~25% overlap between the two assemblages.

Important differences among small mammalian taxa include: insectivorous forms being considerably more diverse in the type area while omomyids are more diverse at The Pinnacles; marsupials, microsyopids and palaeanodonts represented by predominantly different taxa in each area; and multituberculates being absent from The Pinnacles. In terms of abundance, the type area is dominated by insectivores while The Pinnacles are dominated by rodents, Microsyops and Hyopsodus.

Many dissimilarities in the assemblages can be attributed to differences in taphonomic and collecting biases in the samples, but some must reflect original faunal discordance due to the riparian vs. paludal/flooplain habitats where the fossils originated.