Turtles are among the most abundant reptile fossils. They are usually found as isolated shell plates, although many complete shells are known. The turtles Trionyx ([Fig. 25]), Emys, Baena, and Notomorpha are known from the Fossil Basin.
The largest Eocene reptiles were the crocodiles ([Fig. 26]). The presence of these predaceous animals is further evidence for a prevailing warm and humid climate during the Eocene times since their tolerance ranges include only comparable conditions at the present time. A few incomplete specimens have been recovered from near the monument.
BIRDS.
Birds, which first appeared in Jurassic time, are rare as fossils in rocks of any age because their skeletal remains are exceedingly fragile. Their fossilization requires special conditions because many of their bones are hollow as a weight-reducing adaptation for flight. These hollow bones are especially susceptible to breakage and poor preservation. Scattered remains of birds, mostly limb bones, are found in the Green River sediments. Feather imprints have also been recovered. A complete skeleton of Gallinuloides, a bird related to the South American fowl-like chachalaca, is one of the most significant finds from the Green River Formation. Recently, Brodkorb (1970) described the wings of puff birds similar to those now living in South America from near Fossil Butte.
Fig. 25. An aquatic turtle, Trionyx, from the shore-line deposits of Lake Gosiute. Several turtles have been found in the fish beds near Fossil Butte.
Fig. 26. The skull of a large crocodile from near the shoreline of Lake Gosiute. Several crocodile fragments have been found at Fossil Butte beds. Length of original, 75 cm. Collection of University of Wyoming.
In the adjacent Green River Basin to the east, paleontologists from the University of Wyoming have found nesting grounds of Eocene flamingos in slightly younger sediments of the Green River Formation. These “rookeries” have yielded many hundreds of fossil bones of this curious bird. Since these bird fossils were found proximal to Fossil Butte, they demonstrate the near subtropical nature of the environment in the Fossil Butte area in Green River times.