The Bridger Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in southwestern

Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the s ...

. It preserves fossils dating back to the

Ypresian In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by the Eocene Lutetian Age. The Ypresian ...

Epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

of the Paleogene

Period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

. The formation was named by American geologist

Ferdinand Vandeveer Hayden Ferdinand Vandeveer Hayden (September 7, 1829 – December 22, 1887) was an American geologist noted for his pioneering surveying expeditions of the Rocky Mountains in the late 19th century. He was also a physician who served with the Union Ar ...

for

Fort Bridger Fort Bridger was originally a 19th-century fur trading outpost established in 1842, on Blacks Fork of the Green River, in what is now Uinta County, Wyoming, United States. It became a vital resupply point for wagon trains on the Oregon Trail, C ...

, which had itself been named for mountain man

Jim Bridger James Felix "Jim" Bridger (March 17, 1804 – July 17, 1881) was an American mountain man, trapper, Army scout, and wilderness guide who explored and trapped in the Western United States in the first half of the 19th century. He was known as Old ...

. The Bridger Wilderness covers much of the Bridger Formation's area.

History

Before colonization, the lands making up the Bridger Formation had been inhabited by the Apsáalooke, Bannock,

Eastern Shoshone Eastern Shoshone are Shoshone who primarily live in Wyoming and in the northeast corner of the Great Basin where Utah, Idaho and Wyoming meet and are in the Great Basin classification of Indigenous People. They lived in the Rocky Mountains d ...

, Hinono'eino, Očhéthi Šakówiŋ, Só'taeo'o, Tsétsêhéstâhese, and Ute nations. European settlers began to settle the area around the Bridger Formation in the 19th century, beginning with the establishment of the

Oregon Trail The Oregon Trail was a east–west, large-wheeled wagon route and emigrant trail in the United States that connected the Missouri River to valleys in Oregon. The eastern part of the Oregon Trail spanned part of what is now the state of Kans ...

in 1830. Fort Bridger – for which the formation would later be named – was established in 1843 by Jim Bridger and Louis Vasquez. In 1868, the remaining Indigenous communities in the area were displaced by the Treaty of Fort Bridger, removing them to the

Fort Hall Indian Reservation The Fort Hall Reservation is a Native American reservation of the federally recognized Shoshone- Bannock Tribes (Shoshoni language: Pohoko’ikkateeCrum, B., Crum, E., & Dayley, J. P. (2001). Newe Hupia: Shoshoni Poetry Songs. University Press ...

and

Wind River Indian Reservation The Wind River Indian Reservation, in the west-central portion of the U.S. state of Wyoming, is shared by two Native American tribes, the Eastern Shoshone ( shh, Gweechoon Deka, ''meaning: "buffalo eaters"'') and the Northern Arapaho ( arp, ...

. The first documented fossils to be recovered from the Bridger Formation were discovered on 11 August 1849 by Captain

Howard Stansbury Howard Stansbury (February 8, 1806 – April 17, 1863) was a major in the U.S. Army Corps of Topographical Engineers. His most notable achievement was leading a two-year expedition (1849–1851) to survey the Great Salt Lake and its surroundings ...

, who documented the discovery of fossilized shells and wood in his expedition report while scouting out the region for the

United States Army The United States Army (USA) is the land warfare, land military branch, service branch of the United States Armed Forces. It is one of the eight Uniformed services of the United States, U.S. uniformed services, and is designated as the Army o ...

Corps of Topographical Engineers The U.S. Army Corps of Topographical Engineers was a branch of the United States Army authorized on 4 July 1838. It consisted only of officers who were handpicked from West Point and was used for mapping and the design and construction of federal ...

. In the early-1860s, trapper Jack Robinson claimed to have discovered a number of sites along the base of the Uinta Mountains where grizzly bears had been turned to stone. When these claims were called into question by judge William A. Carter, Robinson brought Carter a bag filled with the fossils. One of the specimens recovered by Robinson was a well-preserved skull which resembled that of a grizzly bear. Judger Carter invited Louis Agassiz to observe the local strata, but Agassiz declined as the journey would have involved riding horseback to the site, a mode of transportation Agassiz abhorred. Carter's son-in-law, Dr. J. Van A. Carter, would go on to send a number of fossils to palaeontologist

Joseph Leidy Joseph Mellick Leidy (September 9, 1823 – April 30, 1891) was an American paleontologist, parasitologist and anatomist. Leidy was professor of anatomy at the University of Pennsylvania, later was a professor of natural history at Swarthmore ...

at the

University of Pennsylvania The University of Pennsylvania (also known as Penn or UPenn) is a private research university in Philadelphia. It is the fourth-oldest institution of higher education in the United States and is ranked among the highest-regarded universitie ...

in 1869. These fossils included the first Bridgerian fossil taxa, '' Omomys carteri''; and the skull discovered by Robinson, which was described as '' Palaeosyops paludosus''. Another researcher responsible for sending off specimens was Dr. Joseph K. Corson, a close friend of Leidy's who hosted him and his family on two three to Fort Bridger in 1872, 1873, and 1879. The Bridger Formation was described and named in 1869 by H.V. Hayden while conducting a geological survey in the region on behalf of the United States Geological and Geographical Survey of the Territories. The famously fossiliferous Bridger Formation attracted a number of famed palaeontologists including

Henry Fairfield Osborn Henry Fairfield Osborn, Sr. (August 8, 1857 – November 6, 1935) was an American paleontologist, geologist and eugenics advocate. He was the president of the American Museum of Natural History for 25 years and a cofounder of the American Euge ...

William Berryman Scott William Berryman Scott (February 12, 1858 – March 29, 1947) was an American vertebrate paleontologist, authority on mammals, and principal author of the White River Oligocene monographs. He was a professor of geology and paleontology at P ...

, and F. Speir, Jr. The Bridger Formation also became a battleground in the

Bone Wars The Bone Wars, also known as the Great Dinosaur Rush, was a period of intense and ruthlessly competitive fossil hunting and discovery during the Gilded Age of American history, marked by a heated rivalry between Edward Drinker Cope (of the Ac ...

between Edward Drinker Cope and

Othniel Charles Marsh Othniel Charles Marsh (October 29, 1831 – March 18, 1899) was an American professor of Paleontology in Yale College and President of the National Academy of Sciences. He was one of the preeminent scientists in the field of paleontology. Among ...

. The Bridger Formation did not see a proper scientific mission until 1903, when Walter W. Granger and

William Diller Matthew William Diller Matthew FRS (February 19, 1871 – September 24, 1930) was a vertebrate paleontologist who worked primarily on mammal fossils, although he also published a few early papers on mineralogy, petrological geology, one on botany, one on ...

initiated a three-year survey of the strata, during which time Matthew identified the Bridger Formation's distinct members by using local limestone layers as marker beds. Later expeditions brought other researchers to the region, including Charles Lewis Gazin.

Geology

The Bridger Formation overlies the

Green River Formation 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 ...

and underlies the Bishop Conglomerate. The boundary with the former occurs in the mid-Eocene after the region completed a transition to a drier environment from a moist climate in the early Eocene. Limestone deposits like the Sage Creek Formation separate the three distinct

members Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

which make up the Bridger Formation: Blacks Fork (Bridger B), Twin Buttes (Bridger C & D), and Turtle Bluff (Bridger E). The limestone surrounding the Bridger Formation was deposited on the beds of lakes and ponds at the site during the Eocene. William Diller Matthew used this limestone as marker beds in his initial description of the Bridger Formation in 1909. Portions formerly considered to be part of the Bridger Formation have since been reassigned to the nearby Uinta Formation.

Palaeobiology

Dozens of Early Eocene (50.3 - 46.2 Ma) mammalian and invertebrate genera are known from the Bridger Formation.

Eutherians

Artiodactyls

Achaenodontidae

Homacodontidae

Carnivoramorphans

stem Carnivoramophans

Miacidae

Viverravidae

Carnivorans

Cimolestans

Apatemyidae

Esthonychidae

Pantolestidae

Creodontans

Hyaenodontidae

Oxyaenidae

Dinoceratans

Eulipotyphlans

Erinaceomorpha

Geolabididae

Nyctitheriidae

Leptictidans

Macroscelids

Mesonychids

Perissodactyls

Amynodontidae

Brontotheriidae

Equidae

Hyopsodontidae

Hyrachyidae

Hyracodontidae

Isectolophidae

Tapiroidea

Palaeanodonta

Pholidotans

Plesiadapiformes

Primates

Microsyopidae

Notharctidae

Omomyidae

Rodentia

Cylindrodontidae

Ischyromyidae

Paramyidae

Reithroparamyidae

Sciuravidae

Metatherians

Herpetotheriidae

Peradectidae

Birds

Gruiformes

Strigiformes

Fish

Amiidae

Ariidae

Lepisosteidae

Osteoglossidae

Reptiles

Crocodilia

Squamates

Anguidae

Boidae

Chamaeleonidae

Rhineuridae

Teiidae

Varanidae

Testudines

Baenidae

Carettochelyidae

Dermatemydidae

Emydidae

Geoemydidae

Testudinidae

Trionychidae

''Incertae sedis''

References

{{Chronostratigraphy of Wyoming, Cenozoic state=expanded Paleogene geology of Wyoming Lutetian Stage