Reptile fossils are very abundant in the Green River and Wasatch
formations. Also a jaw of the three-horned _Triceratops_ is known from
the Cretaceous part of the Evanston Formation.
Small lizards are normally represented only by jaw fragments and
vertebrae. Their remains are most often found by a process of washing
and screening of the sediments containing the fossils. Snakes are also
represented in the Green River Formation. Remains of these animals are
also often restricted to jaws and vertebrae. A complete skeleton of a
fossil boa, _Boavus idelmani_, has been recovered from the Green River
Formation and is the most complete fossil snake from North America
(Schaeffer and Mangus 1965).
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.