TURTLE GRAVEYARD: A NEW DIVERSE BIRD FAUNA FROM THE EARLY EOCENE WASATCH FORMATION, WYOMING

Early Eocene bird faunas of North America are poorly known compared to contemporaneous European and younger avifaunas. Turtle Graveyard in the Wasatch Formation, Sweetwater County, Wyoming is unusual in preserving a relatively unbiased sample of the vertebrate fauna. Fossil vertebrates range in size from very small mammals and birds to the 600-kg Coryphodon and include all major clades. Both mammals and reptiles are species-rich and document a late Graybullian (Wa-5) fauna.

The bird fauna is quite diverse taxonomically and ecologically, and Turtle Graveyard is one of the most diverse sites in the Eocene of North America. There are two species of extinct wading ducks (Presbyornithidae), including Presbyornis pervetus. Other aquatic birds include the extinct crane-like geranoidids, extinct rail-like messelornithids, other small gruiforms, and a small charadriiform shore bird. Arboreal birds also are well-represented by a large number of piciform specimens (including primoscenid-like taxa). Other, rarer taxa are present, including a roller-like coraciiform. This avifauna was ecologically diverse, minimally comprising filter-feeding presbyornithids, wading geranoidids, frugivorous messelornithids, and omnivorous shorebirds and piciforms.

Turtle Graveyard was deposited prior to the expansion of the Green River Formation’s lacustrine deposits and accumulation of its diverse bird fauna. The greater antiquity of Turtle Graveyard relative to the avifaunas of the Green River Formation and Messel means that some of the fossil birds from this site are the oldest records of those clades in North America and the world. In particular, the primoscenid-like specimens appear to be their first North American records. The presence of some bird taxa prior to their first appearance in Europe in the middle Eocene suggests that dispersal of birds between Europe and North America may have occurred during the Eocene after the Paleocene-Eocene boundary and that dispersal may have been from North America to Europe. The Turtle Graveyard avifauna is very different from contemporaneous avifaunas in the Bighorn Basin, which are dominated by the large, flightless Diatryma and ground-dwelling lithornithids. These differences are consistent with warmer, wetter conditions in southern Wyoming.