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.
[Illustration: 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.]
[Illustration: 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.