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Green River Formation Fossils

One of the most important fossil sites for understanding the Eocene is found at Green River, located in western Colorado, eastern Utah and southwestern Wyoming in the United States. During the Eocene, this region was located at much the same latitude it is today, though global climate was more equable. Therefore, the climate in which the organisms lived differs somewhat from that of the present-day western United States. The fossils, especially plants, found at this site indicate that the climate was moist temperate or sub-tropical, with temperatures ranging from 15 to 20 degrees Celsius. In addition to the plants, another piece of evidence suggesting that the climate was sub-tropical was the presence of fossilized crocodiles. Crocodiles can only survive in areas with a constant, warm temperature.

From the National Park Service comes this bit of scientific wisdom:

This 50-million year old lake bed is one of the richest fossil localities in the world. Recorded in limestone are dynamic and complete paleoecosystems that spanned two million years. Preservation is so complete that it allows for detailed study of climate change and its effects on biological communities.

Well, it ain't limestone. So much for the government geniuses. Here's the real scoop from Fossil Museum:

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 them. The formation is one of the largest lacustrine (i.e., lake) sedimentary accumulations in the world, averages some 2,000 feet thick, and spans the period 40 to 50 million years ago during the Eocene Epoch of the Cenozoic Era.

Green River Formation Fossils

Lepisosteus Simplex
(gar fish - 29 inches) Very rare

The Green River Formation is actually a heterogeneous complex of lakes differential in ecological, geological characteristics, timeframe and hence fauna and flora. The complex comprises three primary lakes formed as a consequence of drainage from tectonic highlands evolved 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.

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, as well as abundant invertebrates and plants. 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.

Green River Formation Fossils

Knighta Mortality Plate
Length: Up to 10 Inches

Green River Formation Fossils

Diplomystus Dentatus
Length: Up to 26 inches

The majority of fish fossils are taken from the Fossil L area from two layers: 1) the so-called 18-inch layer; and 2) the spilt fish layer (aka the F1 layer). The best preserved fish come from the 18-inch layer. Because the sediment is highly laminated, the fish can often be removed nearly whole. This layer, in the area near Fossil Butte, does indeed average about 18 inches in thickness, and represents some 4,000 (years) of deposition. The composition of the limestone (actually oil shale - where do they keep getting limestone from?) indicates that the layer was formed in deep water far from shore. By contrast, the so-called split-fish layer is unlaminated making extraction and preparation of the best fossil fish far more difficult. The layer is about six feet thick, and the fauna indicates water that was better circulated than that associated with the 18-inch layer. Some 19 genera of Eocene fish come from the Green River formation.

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