Dubbed “America’s Aquarium in Stone” by the National Park Service, Fossil Butte is exceptional because it has many well-preserved fossils, especially those of aquatic animals. It is considered by researchers to be one of the richest paleontology sites on earth. Paleontologists have recovered fossils of crustaceans, gars, stingrays, plankton, crocodiles, turtles, bivalves, and birds, among many others, preserved for 56 to 34 million years. Fossil Butte used to be the smallest of three prehistoric lakes from the Eocene Epoch in what is now southwest Wyoming, northeast Utah, and northwest Colorado, collectively known as the Green River Lake System. The bottom of the lake is thought to have contained hydrogen sulfide, which is poisonous, which prevented fish from scavenging off of the lake bottom. When an animal in the lake died, it sank to the bottom of the lakebed and remained undisturbed, eventually to be covered and preserved beneath the mud. Fossil Butte is also famous for fossils of mass mortalities, where many fish all died at the same time and became fossilized next to each other. This could be caused by a variety of factors like seasonal changes in water salinity or PH. It is likely as well that the hydrogen sulfide became disturbed from time to time and caused poisonings.
Though some explorers like Ferdinand V. Hayden and John C Frémont already knew about the fossil beds in this area, the full extent of Fossil Butte’s paleontological significance wasn’t understood until the late 1860s. The Union Pacific Railroad’s construction passed through the formation, and railroad workers showed fossils to paleontologists. One collector, Robert Craig, began quarrying fossils there in 1897. The site gained interest from museums around the world for its richness in high-quality fossils. As a result of this fame, Congress created the Fossil Butte National Monument in 1972 to preserve some of it for public viewing.
The visitor center of the Fossil Butte National Monument Visitor Center has over eighty fossils on display of various types of prehistoric aquatic creatures, as well as a variety of prehistoric bats, birds, plants, and insects. The center shows a thirteen-minute video to inform visitors about the discoveries paleontologists have made within the site. Visitors are allowed to make their own “fossil imprints” as souvenirs and access an educational computer program about fossils and the natural history of the monument.