Green River Formation
Time Period: 58 to 42 million years ago.
Climate: Paratropical rain forest with a dry season. As the Paleocene Epoch ended, the global climate grew warmer. The area which presently comprises Montana, Utah, and Wyoming enjoyed a seasonal climate similar to the present day Central American rain forest. Temperatures peaked in the early part of the Eocene Epoch and then trended to a cooler and drier climate.
Geography: At the beginning of the Cenozoic Era the western continental margin was much further east than today. There also was a long standing interior seaway extending from Alaska to the Gulf of Mexico. This "Western Interior Seaway" was likely the source of the marine based animals which adapted to the fresh water streams and lakes draining the highlands. The seaway dried up during the Paleocene Epoch as a result of the rising land levels caused by the Laramide orogeny which created the Rocky Mountains.
The three Eocene epoch lakes which formed the lacustrine members of the Green River Formation filled basins created by the Laramide orogeny. These lacustrine sediments contain layers of ash that indicate active volcanism throughout the time period. The three lakes that are included in the Green River formation were Lake Uinta, Lake Gosiute, and Fossil Lake. Lake Uinta was also called Lake Flagstaff in its early stages.
The size and extent of the lakes as represented in the map at right is that which existed during the early Eocene Epoch. Lake Uinta existed for the longest time, from 58 to 42 million years ago varying greatly in geographical location and extent during the 16 million years it existed. Lake Gosiute existed from about 54 to 47 million years ago and Fossil Lake from about 53 to 49.5 million years ago.
At present, all of the Green River fossils sold by Manzanita Lab are from Fossil Lake. It should be noted here that these three lakes are not the only Eocene Epoch lakes in the geographic area. A lacustrine deposit found south of Broadus, Montana (marked on the map) contains fossil Knightia vetusta specimens.
Fauna Notes: In the Early Eocene, approximately contemporary with Fossil Lake, new types of animals appeared. These included primates, rodents, plus artiodactyla and perissodactyla ungulates. Present day pigs, peccaries, hippos, camels, deer, antelopes, sheep, goats and cattle are all extant artiodactylas. Living members of the perissodactyla include horses, rhinos and tapirs.
List of aquatic vertebrates found in the Green River Formation.
Fins 101; Priscacara Identification.
Fins 102; Knightia Identification.
List of paleontology and geology terms used.
List of reference material cited.
