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Green River Formation

Green River Formation Fossils:

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Location: Sweetwater County, Wyoming

Time: 50 to 45 million years ago, during the Tertiary period

About this Famous Find: The Green River Formation, exposed in northwest Colorado, southwest Wyoming, and northeast Utah, is what remains of a large lake that existed in the area ~ 60 to 38 million years ago. The lake system formed as water from the retreating Cretaceous Western Interior Seaway was trapped in basins in the foothills of the newly forming Rocky Mountains.

During the Early Tertiary (Eocene), this region was located at much the same latitude as it is today, though the fossils, especially the plants, indicate that the climate was more sub-tropical. If you were able to visit the Green River at that time, you would have seen palms, cat-tails, sycamores, and other familiar plants from North America, but you also would have seen others more common to eastern Asia, such as Acrostichum, a large, coarse fern indigenous to Guam.

The formation contains one of the most complete fossil assemblages of plants, insects, reptiles, fish, and mammals in the world. Approximately 60 vertebrate taxa have been found and although the majority of fossil material is fragmentary, some complete skeletons exist. The bat Icaronycteris index was extremely well preserved. Not only are its skeleton, membranes, and cartilage intact, but food remains are fossilized within its body cavity. This rare preservation makes the bat fossil very useful for paleontologists, providing evidence of the flora, algae, pollen, and arthropods living at the time. Invertebrate fossils are also abundant, and remains of snails and insects are fairly common.

Dr. John Evans collected the first fossil fish from the Green River Formation in 1856, but work on the fossils from this site continues to this day, with specimens housed at various institutions, including the Museum of Western Colorado, the United States National Museum, the University of Colorado Museum, and the University of California Museum of Paleontology.


Parks (1 listing)

Fossil Butte National Park: Designated a national park in 1972, Fossil Butte has preserved some of the most fossil-rich exposures of the Green River Formation for all to see. At this website, you can find information on the fossils and rocks in the park, as well as how to get there.

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Education and Exhibits

Virtual Exhibits (1 listing)

Localities of the Eocene: The Green River Formation: Visit this exhibit for more information on the rocks, fossils, and history of research of the Green River Formation, including references. Explore other exhibits on the UC Museum of Paleontology site to find out about the Tertiary period and the animals that lived during that time.

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Field Guides (1 listing)

Green River Paleobotany Project: This site is geared towards amateurs and professionals who collect fossil plants from the Parachute Creek Member of the Green River Formation in Colorado and Utah. The site provides an identification guide for more than 250 species of fossil plants and encourages collectors to submit images of their own fossils if they think they have found a new species.

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