An Early Eocene Oilbird from the Green River Formation of Wyoming (caprimulgiformes : Steatornithidae)

The first diagnostic fossils of an oilbird (Steatornithidae : Caprimulgiformes), two nearly complete skeletons, are described as a new genus and species from the early Eocene Green River Formation of Wyoming. The fossil form is smaller than the living Oilbird (Steatomis caripensis) but had the characteristically shortened tarsometatarsus and distinctive mandibular morphology. The sternum and pelvis are more primitive than in Steatomis and the less specialized structure of the humerus, toes, and tail suggests that the fossil form was not adapted for hovering flight or for life in caves. Although the family Steatornithidae is now confined to South America and has been regarded as one of the endemic elements in the avifauna of that continent, the distribution of the family is now seen to be relictual. Steatomis is the only herbivorous member of the Caprimulgiformes. The same families and even genera of plants that predominate in the diet of Steatomis also occur as fossils in the Green River Formation, suggesting that herbivory could have arisen in the Steatornithidae during or prior to the early Eocene. Because these birds would have been effective disseminators of large angiosperm seeds, rapid coevolution of both seed plants and their avian dispersal agents at the beginning of the Tertiary is likely. KEY-WORDS : Aves, Eocene, Green River Formation, Steatornithidae, Caprimtdgiformes, Coraciidae, Coraciiformes, Biogeography, Angiosperm Co-evolution.