The Green River Formation is, quite simply, the richest deposit of fish fossils in the world. It is so named because the fossils were first discovered on the banks of the Green River in Wyoming. The Green River Formation stretches for hundreds of miles across Wyoming, Colorado, and Utah, and is over a mile thick! The abundance of fossil fish and the wide diversity of species makes this area very important in the ongoing study of ancient fish. The complex comprises three primary lakes formed as a consequence of drainage from tectonic highlands involved in the uplift of the Rocky Mountains during Tertiary time. Fossil lake, centered in Southwest Wyoming, is the smallest and appeared briefly during the early Eocene. The Lake Gosiute deposits span the period from Lower to Middle Eocene, and the largest deposit from Lake Uinta that ranges across the Utah-Colorado border, spans most of the Eocene Epoch.
 

Some 58 million years ago, for reasons which are unclear, many fish and marine animals met their death under catastrophic conditions in the wide, shallow lakes in the Green River region of Wyoming. Their remains were covered by a fine deposit of calcium carbonate, preserving every delicate outline. Year after year limestone sediments added layers to this protective cover. Through a long fossilization process, the original bones, skin, and scales were replaced with the minerals in the sediment that buried them.

 Based on the fossil record,  the Green River region was sub-tropical to temperate during the Eocene period.  Green river has been noted for its well-preserved fish since mid-way through the 19th century. 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.