The Diplomystus an ancestor of the herring family, a very finely boned fish which is extremely difficult to prepare from the Lagerstätte varve layer of the Eocene limestones of the Green River formation 18-inch layer. These predator fish of an ancient freshwater lagoon have been discovered with other prey fish in its stomach contents, Knightia alta, Knightia eocaena and Priscacara serrata all other inhabitants of the prehistoric fossil lakes and have occasionally found to the latter fishes stuck in its gullet. These so-called aspiration deaths are rare, difficult to interrupt as to the actual cause of the death of the predatory fish. The two main theories as to why aspiration fish deaths are found in the fossil deposits are, firstly the predator fish could not swallow the prey which became lodged in its throat and suffocated the attacker. Alternatively that the deoxygenation of the lake suffocated all fishes including the fish in the act of hunting and feeding.
The lower Eocene deposits of Green River formation was thought to be a large freshwater lake or lagoon which often suffered from falls of volcanic ash smothering the lagoon and creating an anoxic environment. This reduction of oxygen suffocated the inhabitants of the lake, this circumstance preserved exceedingly well the fishes and fauna of this region, a unique occurrence in the fossil record. The lack of oxygen gave little chance for bacteria to quickly establish and decay the fishes, flora and other fauna deposited in the muds at the bottom of the lake which would eventually be transformed into soft calcite bearing shale. The calcite would be absorbed by the remains of the fish, transforming their bones into a hard mineralised deposit in the slightly softer and lighter coloured limestone shales.
The fossil formation was described by Edward Drinker Cope in 1877 an American palaeontologist and comparative anatomist. In the last quarter of the nineteenth-century, fossil collecting was so popular a personal feud between two great palaeontologists grew in momentum, Cope and another palaeontologist Othniel Charles Marsh became intensely competitive in discovering new dinosaur fossils, this led to a race of collecting termed the bone wars!
The Paleogene system of fossil lakes between Utah, Colorado and Wyoming existed for around 15 million years. The oldest lake known to exist today is lake Tanganyika at around 10 million years! The fish deposits represent about 5 million years of deposition. From approximately 53.5 m.y.a. to 48.5 m.y.a.
The eighteen-inch layer is a lower Eocene varve layer, is not fissile laminae and therefore much more difficult to split and work successfully, older in the fossil lakes system containing fish which have been preserved which calcite mineralisation. The mineralisation is caused by absorption of localised mineral elements being absorbed by the process of osmosis. Water coursing through the mud layers at the time of deposition deposited minerals in the tissues of the fish, in this case, its skeletal elements, bones teeth and cartilaginous parts. In this 18 inch layer, this exhibits in a dark calcite skeleton encapsulated in light coloured limestone of the Lagerstätte fossil varved lime muds. The mineral preserves every fine detail of this particular fossil bed, a very rare occurrence indeed.
The fossil fish beds of the Green River Formation span a period of approximately five million years from 53.5 and 48.5 million years ago. This period and fossil deposit record the transition of a moist early Eocene climate to a slightly drier middle Eocene period.