Aquarium in Stone

Visiting Fossil Butte National Monument
Double rainbow over the grasslands and rocky butte of Fossil Butte National Monument
– National Park Service

During the Eocene Epoch, 56 to 33.9 million years ago, a series of inland lakes extended through Wyoming, Utah, and Colorado. Over the course of more than two million years, sediments accumulated while plants and animals died and settled to the bottom of the lake. Bones, shells, and fiberous plant material mixed with limey ooze—calcium carbonate deposits carried into the lake by rivers—on the lakebed.

Fossil of now-extinct Diplomystus dentatus fish eating another fish Knightia

This fossil depicts a fish (Diplomystus dentatus) that most likely died from starvation or suffocation because it could not spit the smaller fish out (Knightia).

National Park Service

Salty and with little to no oxygen, the lake bottom was a hostile environment that lacked decomposing bacteria or scavengers, giving recently deceased organisms a chance to lay undisturbed until they were encased in lays of sediment. By the time the lake completely dried up, sediment had piled hundreds of feet thick, compressing the very bottom layers into limestone, emtombing fish, alligators, turtles, horses, and other plants and animals. These fascinating ancient fossils can be seen today at Fossil Butte National Monument.

Fossil of leaf at Fossil Butte National Monument

Fossilized unidentified leaf

National Park Service

Fossil Butte National Monument protects a part of what was Fossil Lake, the deepest of three great lakes during that period. Because the environment created some of the best conditions for fossilization, the amount and quality of preservation is one of the highest in the world, giving scientists some of the finest records of the earth’s ancient history.

Fairly complete fossil of softshell turtle (Trionychidae) that was 5’6” from Fossil Lake

This giant softshell turtle from Fossil Lake was 5’6” long

National Park Service

Often, researchers have only fragmented and incomplete fossil records to work with. However, at Fossil Butte, fragile organisms like fish with skin and eyes, plants, and birds, have been almost perfectly preserved.

Over 300 fossils on display at the park, including birds, dragonflies, soft-shell turtles, and even a 13-foot crocodile! There are also numberous hiking trails, a historic fossil quarry, and views of modern-day wildlife and plants of the high desert, including wildflowers, sagebrush, grove of aspens, elk, jackrabbits, and more.

Fairly complete fossil of dog-sized horse (Protorohippus sp.) found in the Green River Formation

Fairly complete fossil of dog-sized horse (Protorohippus sp.)

National Park Service

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