Fossil Fish 54 - 48 million years ago, the Wyoming landscape was lush and sub-tropical. Dead fish sank to the bottom of large deep lakes due to low temperatures, were buried, and fossilized. Discovered in the 1840s, the limestone matrix in the ancient Green River Formation is so fine-grained that fossils include spectacular detail. The most common fish species found is Knightia eocaena, a 4-8 inch herring-type fish. The carnivorous Diplomystus dentatus reached up to 2 feet in length and has, on occasion, been preserved with smaller fish inside its stomach. Other notable species include spiny Priscacara and trout-like Amphliplaga.