Description: 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.

This specimen comes from the so-called 18 inch layer noted for the fish coming out complete on one side of the matirx and exquisite soft-tissue preservation. The preservation here is particularly superb.

Genus Priscacara is perhaps the most popular of the Green River fish fossils. A member of the Family Priscacaridae, the name Priscacara means "primitive head". Shaped rather like a sunfish, the genus sports sturdy, protective dorsal and anal spines. Among the two

species, liops and serrata, serrata is uncommon. The species name comes from the diagnostic serrated preopercle, seen here in closeup. Liops is the smaller species, never exceeding 150 mm, whereas serrata have been found up to 375 mm. The genus went extinct at the end of the Miocene, and is thought by some to be related to the modern-day Cichildae.

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.