Knightia Eocaena is the most common fish in Fossil Lake and may be the most common articulated vertebrate fossil in the world. It is Wyoming's State Fossil. Length: up to 25 cm (10 in).
Fifty million years ago ancient Fossil Lake existed in what is now southwest Wyoming. Of its estimated maximum extent of 930 square miles, approximately 500 square miles of sediment remains. The 230 square miles across the center of the ancient lake-bed contain exceptionally fossiliferous sediments and associated geologic features including deltas, beaches, springs, and rocks from the center and nearshore environments.
The unusual chemistry of fossil lake prevented decay and scavenging of dead organisms while millimeter-thick layers of alternating limestone matter slowly accumulated. The result is laminated limestones that contained the highest concentration of fossil fish in the world. These fish, other aquatic organisms, and associated geologic features make Fossil Lake the world's best Paleogene record of the freshwater lake ecosystem.
Since the discovery in the 1870's, many perfectly preserved fossil fish have been recovered. Preserved with the fish in the laminated limestone is a complete aquatic ecosystem: cyanobacteria, plants, insects, crustaceans (shrimp, crawfish, 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, apatemyid (an arboreal insectivore), miacad (primitive carnivore), insects, and more than 325 types of leaves, seeds, and flowers.