Dyplomistus fish fossil from Wyoming Eocene epoch. Fifty-million years ago, ancient Fossil Lake existed where southwest Wyoming now is. Approximately 500 square miles of sediments remain and the 230 square miles across the center of the ancient lakebed contain exceptionally fossiliferous sediments and associated geologic features.
The unusual chemistry of Fossil Lake prevented decay and scavenging of dead organisms. These fish, other fossilized aquatic organisms, and associated geologic features make Fossil Lake the world's best Paleogene record of a freshwater lake ecosystem.
Since the discovery of Fossil Lake in the 1870s, 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, crustaceans (shrimp, crayfish, and ostracods), amphibians (frog and primitive salamander), alligators, turtles, birds, and mammals, including the oldest pantolestid (otter-like animal). The subtropical terrestrial ecosystem surrounding the lake is also represented by rare fossils, including a horse, two snakes, lizards, two bat species, birds, an apatemyid (an arboreal insectivore), a miacid (primitive carnivore), insects, and more than 325 types of leaves, seeds, and flowers.