January 2025 - 37

Tsoabichi greenriverensis, a small crocodilian only 75 centimeters long, is most closely related to
what we know as caimans today. (Photo by National Park Service)
This cast of a Protorohippus venticolum, or dawn horse, is one of two ancient horses found in the
Green River Formation.
include a lemur-like creature, a digging mammal
called a tube-sheep, an otter-like carnivore and a
tree-climbing carnivore.
Fossil Butte has preserved 15 kinds of reptiles.
These include two snake species, eight species of
turtles - including three different softshell turtles
- three species of lizards and two crocodilians.
One lizard - Bahndwivici ammoskius - is skeletally
indistinguishable from the crocodile lizard
currently living in China.
Of the two crocodilians found there, the Borelosuchus
wilsoni, was part of a sister group of modern
alligators and crocodiles. The largest of this species
from Fossil Butte was 15 feet long. The other
crocodilian, Tsoabichi greenriverenis, was part of
the caiman family.
Although uncommon, two amphibians have
been found at Fossil Lake: an aquatic salamander
just under 10 inches long and a species of frog.
Birds also have been found in the formation
including more than 30 species among 20 families.
These include shoreline birds, parrots, landfowl,
This cast of Onychonycteris finneyi, the most primitive bat known, is about 14 centimeters long.
Claws on each finger of its wings indicate it was probably an agile climber and crawled along and
under tree branches searching for insects.
raptors, perching birds and others. Bird parts are
more common than complete birds since they fall
off rotting carcasses kept afloat by feathers. The
most common bird fossils are isolated feathers.
Multiple arthropods have been found at the lake.
Freshwater shrimp and crayfish are found more
commonly near shore. Three species of spiders have
been found preserved at the monument including
jumping spiders, wolf spiders and crab spiders. The
most common insect found is a March fly, but
many other insect groups are represented including
dragonflies, damselflies, millipedes, crickets, true
bugs, stoneflies, moths, butterflies and more.
Other fossils include coprolites - or fossilized
dung - as well as more than 400 species of plants.
The best place to view this ancient wildlife is in
the Fossil Butte National Monument visitor center
where more than 2,000 fossils are on display. The
monument is free and open year-round.
- Tracie Binkerd is the editor of Wyoming Wildlife.
Information within this article courtesy of the National
Park Service.
Wyoming Wildlife | 37

January 2025

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