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 fine fish is Diplomystus dentatus. Early workers proposed that this fish was closely related to Knightia, an incorrect assumption, as Diplomystus is not even placed within the same Order. Diplomystus has the body form and mouth placement of a surface feeder, and is thought to have been a predator of smaller surface-feeders such as Knightia. Indeed, specimens of Diplomystus have been found with Knightia in their stomachs or mouths, a classic demonstration of a

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.

predator caught by its prey! There are the remains of another, smaller fish in the upper left corner as well. At times, a small raised surface is all that tells of the presence of a specimen within the matrix.

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.