The Bridger Formation is a geologic formation in southwestern Wyoming. It preserves fossils dating back to the Bridgerian and Uintan stages of the Paleogene Period. The formation was named by American geologist Ferdinand Vandeveer Hayden for Fort Bridger, which had itself been named for mountain man Jim Bridger. 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, 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 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 and Wind River Indian Reservation.
The first documented fossils to be recovered from the Bridger Formation were discovered on 11 August 1849 by Captain Howard Stansbury, who documented the discovery of fossilized shells and wood in his expedition report while scouting out the region for the United States Army Corps of Topographical Engineers. 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 at the University of Pennsylvania 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, William Berryman Scott, and F. Speir Jr. The Bridger Formation also became a battleground in the Bone Wars between Edward Drinker Cope and Othniel Charles Marsh.
The Bridger Formation did not see a proper scientific mission until 1903, when Walter W. Granger and William Diller Matthew 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 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.Smith, M. E., Singer, B., & Carroll, A. (2003). 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming. Geological Society of America Bulletin, 115(5), 549-565. Limestone deposits like the Sage Creek Formation separate the three distinct members 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 Genus Species Location Stratigraphic position Material ImagesAchaenodonA. insolensWyomingAMNH 5143, fragmentary mandibleA. robustusPU 10033, partial skull and mandibleHelohyusH. lentusSweetwater County, WyomingH. milleriBridger B & CH. plicodonsouthwestern WyomingParahyusP. vagusWyomingYPM 10972, partial mandible
Homacodontidae Genus Species Location Stratigraphic position Material ImagesAntiacodonA. pygmaeussouthwestern WyomingBridger C & DA. venustusSweetwater County, WyomingBridger DYPM 11765, a toothHomacodonH. vaganssouthwestern WyomingBridger C & DYPM 13129, skull and jawsMicrosusM. cuspidatussouthwestern WyomingBridger B - D
Carnivoramorphans Stem Carnivoramophans Genus Species Location Stratigraphic position Material ImagesOodectesO. herpestoidesUinta County, WyomingAMNH 140008, nearly-complete skull
Miacidae Genus Species Location Stratigraphic position Material ImagesHarpalodonH. sylvestrisWashakie County, WyomingMiacisM. bathygnathusHenry's Fork Hill, Sweetwater County, WyomingBridger C M. edaxWyomingM. parvivorusLincoln and Uinta Counties, WyomingBridger B & CM. voraxWyomingM. vulpinusMiocyonM. majorBlue Point Marker, Park County, WyomingUintacyonU. jugulansHenry's Fork Hill, Sweetwater County, WyomingBridger CVulpavusV. ovatusLittle Dry Creek, Uinta County, Wyoming V. profectusUinta County, Wyoming
Viverravidae Genus Species Location Stratigraphic position Material ImagesViverravusV. nitidusWyoming
Carnivorans Genus Species Location Stratigraphic position Material ImagesPalaearctonyxP. meadiTwin Buttes, Sweetwater Counties, Wyoming
Cimolestans Apatemyidae Genus Species Location Stratigraphic position Material ImagesApatemysA. belluscentral and southern WyomingBridger CYPM 13512, partial mandibleA. bellulussouthwestern and central Wyoming YPM 13513, partial mandible USNM 22386, mandible
Esthonychidae Genus Species Location Stratigraphic position Material ImagesMegalesthonyxM. hopsoniWyomingTillodonT. fodiensMillersville, Uinta County, WyomingTrogosusT. castoridenssouthwestern WyomingT. gaziniUinta County, WyomingUSNM 364762, partial skeletonT. hyracoidesGranger Station, Sweetwater County, WyomingT. latidensLincoln and Sweetwater counties, WyomingT. vetulus
Pantolestidae Genus Species Location Stratigraphic position Material ImagesPantolestesP. elegansWyomingP. intermediusP. longieundusP. natansP. phocipes
Creodontans Hyaenodontidae Genus Species Location Stratigraphic position Material ImagesDidelphodusD. altidenssouthwestern WyomingEntemodonE. comptusHoneycomb Buttes, Fremont County, WyomingIridodonI. datzaeCathedral Bluffs, Sweetwater County, WyomingUMMP 103465, mandibleLimnocyonL. cuspidensIce Castle, Uinta County, WyomingL. mediusHenry's Fork Hill, Uinta County, WyomingBridger DL. veloxsouthwestern WyomingL. verusUinta County, WyomingBridger CProlimnocyonP. antiquusSweetwater County, WyomingProviverraP. americanaWyomingSinopaS. aculeatusS. agilisUinta County, WyomingS. eximiaWyomingS. grangeriCottonwood Creek, Sublette County, WyomingS. insectivorusS. majorsouthwestern WyomingS. minorChurch Buttes, Uinta County, WyomingS. pungensCottonwood Creek, Sublette County, WyomingS. rapaxsouthwestern WyomingTritemnodonT. hiansWyomingT. strenuusDavis Ranch, Johnson County, Wyoming
Oxyaenidae Genus Species Location Stratigraphic position Material ImagesAelurotheriumA. bicuspisHenry's Fork, Uinta County, WyomingA. latidensGrizzly Butte, Uinta County, WyomingMachaeroidesM. eothenTwin Buttes, Sweetwater County, WyomingM. simpsoniDavis Ranch, Johnson County, WyomingPatriofelisP. feroxcentral and southwestern WyomingP. ultasouthwestern Wyoming
Dinoceratans Genus Species Location Stratigraphic position Material ImagesBathyopsisB. fissidenssouthwestern Wyoming Bridger B.EobasileusE. cornutusUintatheriumU. anceps Bridger C & D.
Eulipotyphlans Erinaceomorpha Genus Species Location Stratigraphic position Material ImagesTalpavusT. nitidussouthwestern WyomingBridger CYPM 13511, fragmentary jaw
Geolabididae Genus Species Location Stratigraphic position Material ImagesCentetodonC. bacchanalisHyopsodus Hill, Sublette County, WyomingC. bembicophagussouthwestern WyomingC. pulcherBridger C & DMarsholestesM. dasypelixUinta County, Wyoming
Nyctitheriidae Genus Species Location Stratigraphic position Material ImagesNyctilestesN. serotinusUinta County, WyomingNyctitheriumN. gunnelliSDSNH 110393, partial mandibleN. priscusWyomingN. serotinumcentral and southwestern WyomingN. veloxHyopsodus Hill, Sublette County, Wyoming
Leptictidans Genus Species Location Stratigraphic position Material ImagesHypictopsH. syntaphusUinta County, WyomingBridger DPalaeictopsP. bridgericentral Wyoming
Macroscelids Genus Species Location Stratigraphic position Material ImagesScenopagusS. curtidenswestern WyomingS. edenensissouthwestern WyomingS. priscuswestern Wyoming
Mesonychids Genus Species Location Stratigraphic position Material ImagesHarpagolestesH. immanissouthern WyomingAMNH 13143, a partial skull and jawsH. macrocephalusLincoln and Uinta counties, WyomingMesonyxM. laniusWyomingM. obtusidensUinta County, WyomingSynoplotheriumS. caniusBitter Creek, Sweetwater County, Wyoming
Perissodactyls Amynodontidae Genus Species Location Stratigraphic position Material ImagesOrthocynodonO. antiquusWyoming
Brontotheriidae Genus Species Location Stratigraphic position Material ImagesPalaeosyopsP. fontinaliswestern WyomingP. paludosuscentral & southwestern WyomingP. robustussouthwestern WyomingTelmatheriumT. validusDuncan Ranch, Fremont County, WyomingBridger DYPM 11120, a partial skullSweetwater County, WyomingMesatirhinusM. juniusDuncan Ranch, Fremont County, WyomingPU 10008
Equidae Genus Species Location Stratigraphic position Material ImagesOligotomusO. cinctusWyomingOrohippusO. agilisYPM 1268, partial skeletonO. ballardiO. majorMillersville, Uinta County, WyomingYPM 11270, fragmentary maxillaO. osbornianusBlacks Fork, Uinta County, WyomingO. procyoninusWyomingO. progressusAMNH 12120, partial skullO. pumilussouthwestern WyomingO. sylvaticusUinta County, WyomingO. uintanusWyoming
Hyopsodontidae Genus Species Location Stratigraphic position Material ImagesHyopsodusH. minusculussouthern and central WyomingBridger BH. lepidussouthwestern WyomingBridger CH. paulusBridger B - D H. pygmaeusWyomingH. tonskinorth and central Wyoming
Hyrachyidae Genus Species Location Stratigraphic position Material ImagesHyrachyusH. eximiussouthwestern Wyoming H. minorH. paradoxusWyoming
Hyracodontidae Genus Species Location Stratigraphic position Material ImagesTriplopusT. cubitalisWyomingT. obliquidens
Isectolophidae Genus Species Location Stratigraphic position Material ImagesIsectolophusI. latidensLone Tree, Uinta County, WyomingBridger D PU 10251, fragmentary skull YPM 12563, partial skullI. radinskyiLone Spring, Uinta County, WyomingBridger D
Tapiroidea Genus Species Location Stratigraphic position Material ImagesHelaletesH. nanuscentral & southwestern Wyoming YPM 11080, partial jaws and cheek teeth YPM 11807, partial skeleton
Palaeanodonta Genus Species Location Stratigraphic position Material ImagesBrachianodonB. westorumsouthwestern WyomingMetacheiromysM. marshiBridger C
Pholidotans Genus Species Location Stratigraphic position Material ImagesTetrapassalusT. proiussouthwestern Wyoming
Plesiadapiformes Genus Species Location Stratigraphic position Material ImagesAlveojunctusA. minutusVass Quarry, Hot Springs County, Wyoming