This is a fossil of an extinct fat-tailed stingray from the 52 million year old Fossil Lake deposits of Wyoming. Most living stingrays live exclusively in saltwater environments. Asterotrygon were one of two types of stingrays that became adapted to the freshwater environment in the now-vanished Fossil Lake which once spread over 60 miles of southwestern Wyoming during the warm, wet Eocene Epoch. The “fat-tailed” appearance is created by a layer of denticles (small, bony structures) surrounding the tail base. The stinger, a spine-like defensive structure that delivers venom, can be seen crossing the tail in this specimen.