Fossil Butte National Monument in southwestern Wyoming preserves part of the fifty-million-year-old Green River Formation, including deposits from ancient Fossil Lake, an Eocene subtropical freshwater basin. Established on October 23, 1972, the monument protects what is considered the finest paleontological record of Cenozoic aquatic communities in North America. Its exceptionally preserved fossils include fish, turtles, crocodiles, bats, insects, birds, palm trees, and an early dog-sized horse, revealing an ecosystem unlike modern Wyoming. The area’s fossil history began with coal mining and commercial collecting, but it is now protected as both a research site and a public window into ancient life.