10" Plate of Two Fossil Fish (Knightia) - Wyoming

Cockerellites liops is a species of extinct temperate bass found in the Eocene aged Green River Formation of Wyoming. It is characterized by a sunfish-like body and its stout dorsal and anal spines. It was originally placed in the Priscacara genus but was moved to the newly created genus Cockerellites by D. Jordan and H. Hanibal in 1923. There is still some debate among researchers about whether this new genus is valid.

Cockerellites is found in large numbers in mid-lake deposits, representing 5 to 20 percent of the fish unearthed, depending on the layer. It is considerably rarer in shoreline deposits, representing only 1 to 2 percent of the fish found. Because of this, Cockerellites is thought to have been a schooling fish. Fossils have been found at a maximum size of about six inches, but they rarely exceed five inches in length.

At first glance, Cockerellites liops has a very similar appearance to the rarer species Priscacara. Size can often be used as a differentiator, since Cockerellites did not exceed 6 inches while Priscacara serrata is typically found in excess of 6 inches. Cockerellites also has more dorsal and anal fin rays than Priscacara and a much smaller mouth.

About Knightia

Knightia is an extinct genus of schooling, ray-finned, spindle-shaped, bony fish that shares a family with herrings and sardines. They lived in the freshwater (lacustrine) environments of North America and were eaten by just about everything that was bigger. They ate insects and smaller fish, and used gill rakers to feast on plankton. Knightia eocena is the largest of the three species of Knightia, with a typical length of about 15 centimeters. It is the state fossil of Wyoming.

These fish had rows of dorsal and ventral scutes running from the back of the head to the medial fins. They had heavy scales, and small conical teeth. They are popular finds in the Wyoming lagerstätte, and were a primary food source to the large and hungry vertebrates of that once hunted the Green River Formation.

The Green River Formation is an Eocene-aged geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The Eocene spanned approximately 55.8 to 33.8 million years ago. This formation has distinct stratigraphy that displays alternating light and dark layers representing seasonal erosion and deposition.

Freshwater basins, charged by the Uinta Mountains on the Wyoming-Utah border, contained an enormous representation of taxa. The beginning of the Eocene was marked by warm upper latitudes, a greenhouse atmosphere rich in methane and carbon dioxide, and local climates stabilized by large lakes populated by such creatures as crocodiles. Fossil Lake in Wyoming, of which the Green River Formation includes, is known for its well-preserved warm, lacustrine ecology.

The end of the Eocene was dramatically different, with the onset of icehouse climate characteristics, a change in atmospheric chemistry, and possible bolide impacts. The Green River fossils date about 48 million years, but cover several million years, including the transition between the moist early Eocene climate and the slightly drier mid-Eocene.

About Fossil Lake

50 million years ago, in the Eocene epoch, these fish thrived in Fossil Lake, which was fed by the Uinta and Rocky Mountain highlands. The anoxic conditions at the bottom of Fossil Lake slowed bacterial decomposition, prevented scavengers from disturbing corpses, and, most interestingly, suffocated creatures that ventured into the oxygen-starved aquatic layer. The result is a miraculous exhibition of Eocene biota: a subtropical aquatic community within sycamore forests, teeming with creatures such as freshwater stingrays, dog-sized horses, menacing alligators, early flying bats, and one of the first primates.