Mioplosus labracoides Fish Fossil from Green River Formation

Mioplosus labracoides

Order Perciformes, Family Percidae

Geological Time: Eocene

Size (25.4 mm = 1 inch): 235 mm in length on a 360 mm by 200 mm matrix

Fossil Site: Green River Formation, Fossil Lake, Kemmerer, Wyoming

Mioplosus labracoides Fish Fossil from Green River FormationDescription: This 50 million year old, Eocene-Era fossil fish comes from one of the world's famous Laggerstatten, the Green River Formation in Wyoming. A small portion of the fish fossils from Green River exhibits such fine preservation. The significant extent of soft-tissue preservation that makes the site famous is evident in this specimen.

Mioplosus labracoides is believed to have been a voracious predator among the Green River fish fossils. Failure to find its numbers in mass mortality leads to further conjecture that it was a solitary hunter. A member of the Family Percidae, it has numerous relatives in Northern Hemisphere fresh water as well as fossil relatives in Asia, Europe and New Zealand. It is known as a predatory species as determined from its many pointed teeth, and the fact that several have been found with fish lodged in its throat. It may be related to the modern-day pike of the genus Stezostedion.

Distinguishing features include double dorsal fins and a fan-like tail. Mioplosus is uncommon in the Green River formation, especially the fish of this size and of this exceptional quality of preservation (both bone detail and some soft-tissue preservation are evident).

About the Green River Formation: Class Actinopterygii, the ray-finned bony fishes, comprise almost half of all known species of vertebrates, some 20,000 extant species. There are numerous locations worldwide that are noted for wondrous preservation of bony fishes, and the Green River formation that covers some 25,000 square miles of SW Wyoming, west Colorado and east Utah is one of the premier examples. The formation is one of the largest lacustrine sedimentary accumulations in the world, and spans the period from 40 to 50 million years ago during the Eocene Epoch.

During the Eocene, based on the fossil record, the region was sub-tropical to temperate. Some 60 vertebrate taxa have been described from the formation, including crocodiles, boa constrictors, and birds, as well as abundant invertebrates and plants. The unusually excellent preservation of the Green River fish fossils is usually attributed to a combination of two factors: 1) a cold period during the Eocene that would have caused dead fish to sink faster due to a less inflated swim bladder; and 2) the great depth of the lakes and the consequent anoxic conditions that would have often prevented scavengers from disturbing the carcasses.

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