Diplomystus Dentatus is an extinct genus of freshwater fish distantly related to modern-day herrings and sardines, and is well known from lower Eocene deposits from the Green River Formation in Wyoming. Specimens range up to 25" in size.
50,000,000 years ago ancient Fossil Lake existed in what is now southwest Wyoming. Of its estimated maximum size 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.