page_title: Green River Formation
section_title: Fossil zones
hierarchical_section_title: Green River Formation / Fossil zones
caption_attribution_description: English: Stromatolite fossil from the Fort Laclede Bed, Laney Member, Green River Formation of south-western Wyoming in USA. It is the preserved fossilised remains of a stromatolite that lived in the ancient Lake Gosuite. This is a cross-sectional slice through the fossil, showing its internal layered structure.
context_page_description: The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine layers, a dark layer during the growing season and a light-hue inorganic layer in the dry season. Each pair of layers is called a varve and represents one year. The sediments of the Green River Formation present a continuous record of six million years. The mean thickness of a varve here is 0.18 mm, with a minimum thickness of 0.014 mm and maximum of 9.8 mm.
  The sedimentary layers were formed in a large area named for the Green River, a tributary of the Colorado River. The three separate basins lie around the Uinta Mountains of northeastern Utah:
  an area in northwestern Colorado east of the Uintas
  a larger area in the southwest corner of Wyoming just north of the Uintas known as Lake Gosiute
  the largest area, in northeastern Utah and western Colorado south of the Uintas, known as Lake Uinta
context_section_description: Within the Green River Formation of southwest Wyoming in the area known as Fossil Lake, two distinct zones of very fine-grained lime muds are particularly noted for preserving a variety of complete and detailed fossils. These layers are an Eocene Lagerstätte, a rare place where conditions were right for a rich accumulation of undisturbed fossils. The most productive zone—called the split fish layer—consists of a series of laminated or varved lime muds about 6 ft (1.8 m) thick, which contains abundant fish and other fossils. These are easily split along the layers to reveal the fossils. This thin zone represents some 4000 years of deposition. The second fossil zone, the 18 inch layer, is an unlaminated layer about 18 in (46 cm) thick that also contains abundant detailed fossils, but is harder to work because it is not composed of fissile laminae.
  The limestone matrix is so fine-grained that fossils include rare soft parts of complete insects and fallen leaves in spectacular detail. More than twenty-two orders of insects are represented in the Green River collection at the Smithsonian Institution in Washington, D.C., alone.
  Fish fossils of Diplomystus and Knightia are found in Fossil Lake but not in Lake Gosiute. Only Lake Gosiute has fossils of catfish (Ictaluridae and Hypsidoridae) and suckers (Catostomidae). The catfish are found mostly in the deepest parts of the lake. 
   
  The various fossil beds of the Green River Formation span a 5 million year period, dating to between 53.5 and 48.5 million years old. This span of time includes the transition between the moist early Eocene climate and the slightly drier mid-Eocene. The climate was moist and mild enough to support crocodiles, which do not tolerate frost, and the lakes were surrounded by sycamore ( e.g. Platanus wyomingensis ) forests. As the lake configurations shifted, each Green River location is distinct in character and time. The lake system formed over underlying river deltas and shifted in the flat landscape with slight tectonic movements, receiving sediments from the Uinta highland and the Rocky Mountains to the east and north. The lagerstätten formed in anoxic conditions in the fine carbonate muds that formed in the lakebeds. Lack of oxygen slowed bacterial decomposition and kept scavengers away, so leaves of palms, ferns and sycamores, some showing the insect damage they had sustained during their growth, were covered with fine-grained sediment and preserved. Insects were preserved whole, even delicate wing membranes and spider spinnerets.
  Vertebrates were preserved too, including the scutes of Borealosuchus, the crocodile that was an early clue to the mild Eocene climate of Western North America. Fish are common. The fossils of the herring-like Knightia, sometimes in dense layers, as if a school had wandered into anoxic water levels and were overcome, are familiar to fossil-lovers and are among the most commonly available fossils on the commercial market. There were two genera of indigenous freshwater stingray, Heliobatis and Asterotrygon. Approximately sixty vertebrate taxa in all have been found at Green River. Besides fishes they include at least eleven species of reptiles, and some birds and one armadillo-like mammal, Brachianodon westorum, with some scattered vertebrae of others, like the dog-sized Meniscotherium and Notharctus, one of the first primates. The earliest known bats (Icaronycteris index
  , and Onychonycteris finneyi), already full-developed for flight, are found here. Even a snake, Boavus idelmani, found its way into a lake and was preserved in the mudstone.
caption: Stromatolite fossil from the Eocene Lake Gosuite, Laney Member, Green River Formation of south-western Wyoming
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