Fifty-million years ago the ancient Fossil Lake existed in what is now southwest Wyoming. Of its estimated maximum extent of 930 square miles, approximately 500 square miles of its sediments remain. The 230 square miles across the center of the ancient lakebed contain exceptionally fossiliferous sediments and associated geologic features including deltas, beaches, springs, and rocks from center and near shore environments.
The unusual chemistry of Fossil Lake created conditions that prevented decay and scavenging, while alternating layers of limestone and organic material accumulated. The resulting laminated limestones contain the highest concentration of articulated fish fossils in the world. These fish, other fossilized aquatic organisms, and associated geologic features make Fossil Lake the world's best Paleogene record of a freshwater lake ecosystem.
Fossil Lake sediments contain the world's most diverse and densely fossiliferous Eocene aquatic fossil assemblage. Since its discovery in the 1870's more than a million perfectly preserved fossil fish have been recovered. Preserved with the fish in the laminated limestone is a complete ancient aquatic ecosystem: cyanobacteria, plants, insects, amphibians, alligators, turtles, birds, and mammals. Fossil components of the subtropical terrestrial ecosystem surrounding the lake include a horse, snakes, lizards, bats, birds, insectivores, insects, and more than 225 types of leaves, seeds, and flowers.