Seven local faunas (Little Muddy I–VII) of early to middle Eocene age are described from the Little Muddy Creek area in the southwestern Green River Basin. These assemblages were collected from approximately 400 m of interfingering alluvial and lacustrine deposits in the Wasatch, Green River and Bridger formations. This is the first time the Wasatchian-Bridgerian boundary has been delineated within the southwestern Green River Basin. A continuous stratigraphic section from the late Wasatchian (Wa7) through the late Bridgerian (Br3) is now known in the southern Green River Basin.The Lostcabinian (Wa7) in the southwestern Green River Basin is recognized by the occurrence of Lambdotherium and the presence of Meniscotherium and Notharctus nunienus. The temporal range of Anemorhysis wortmani is extended upward to include the Lostcabinian of the southwestern Green River Basin. The late Gardnerbuttean (Bridgerian Biostratigraphic Zone Br1a) is recognized in the southwestern Green River Basin by the first appearances of Palaeosyops fontinalis and Hyrachyus. “Bridger A” (Bridgerian Biostratigraphic Zone Br1b) is recognized in the southwestern Green River Basin by the first appearances of Orohippus pumilus, Omomys carteri, Leptotomus parvus, and Washakius insignis. Other important records include the earliest occurrences of Tetrapassalus sp. and Parasauromalus. The presence of a new genus and species of crocodylian and a new genus and species of helaletid perissodactyl are documented.