Fossils and Preservational Environments of the Bridger Formation


Rocks of the Bridger Formation preserve a highly diverse assemblage of fossil vertebrates and invertebrates, with less abundant trace fossils and plant fossils. Plants include primarily silicified wood (tree trunks, branches, and roots), with less common fruits,seeds, and leaves. Plant fossils, although uncommon throughout the formation, are typically associated with lake margin and swamp deposits. Fossilized algae is also locally present in association with lacustrine and marginal lacustrine facies. Ichnofossils include rhizoliths (root casts), fish pellets and other coprolites, avian and reptilian eggshell, and a variety of insect traces including solitary bee cases, earthworm pellets, and caddis-fly larval cases. Invertebrate fossils include a diversity of prosobranch and pulmonate gastropods (snails) and unionid bivalves (freshwater mussels). These are usually associated with lake and lake margin deposits, although they also occur locally in stream channel deposits.

Bridger Formation microfossils collected from the Omomys Quarry (UCM L. 93026), a lake margin deposit. Included is a right dentary of the tarsioid omomyid primate Omomys carteri (center); avian eggshell fragments (upper left), distal femur (upper right), and talon (lower middle); freshwater gastropod (middle right), and Lepisosteus scale (lower right).

Tibia of Uintatherium anceps (UCM 60871) exposed on a sandstone pedestal in the Twin Buttes Member of the Bridger Formation (subdivision upper C) on the north side of Sage Creek Mountain, Uinta County, Wyoming.

Vertebrate fossils are most numerous in lake, lake margin, and stream channel deposits. Floodplain facies contain fewer fossils and are associated with lower taxonomic diversity. All vertebrate groups, including fish, amphibians, reptiles, birds, and mammals, are represented. Fish fossils are typically fragmentary, and include primarily teeth, scales, vertebrae, and spines. Common fish include Lepisosteus (gar pike), Amia (bowfin), and catfish. Amphibians are represented by salamanders and frogs. Reptiles include a diversity of turtles, crocodilians, lizards and snakes. These are represented mostly by teeth, jaws, vertebrae, osteoderms, and other fragmentary bones. Turtle shell fragments are the most abundant fossils found in the Bridger Formation, as is typical in many vertebrate fossil-bearing formations. Birds are represented by primitive flamingos, owls, hawks, rails, thick-knees, stone curlews, and other species, and are uncommon. They are typically discovered in lake margin deposits.

The mammalian fauna of the Bridger Formation is highly diverse for the middle Eocene, representing some 67 genera, 30 families, and 13 orders. As a result of this diversity and the abundance of mammalian fossils from the Bridger Formation, it was designated the stratotype for the “Bridgerian” North American Land Mammal Age (NALMA). The Eocene was an extremely important time in mammalian evolution worldwide because it saw the extinction of a number of archaic groups of mammals as well as the evolution of most of the modern orders. Bridgerian mammals include the primitive ancestors of perissodactyls (horses, rhinos, tapirs), artiodactyls, carnivores, primates, rodents, insectivores, bats, opossums, dermopterans, and pholidotes. Many more bizarre forms also occur, most belonging to long-extinct groups. These include the saber-toothed six-horned herbivore Uintatherium, the ubiquitous small weasel-shaped condylarth Hyopsodus, the stocky herbivorous brontotheres, the carnivorous wolf-like Mesonyx, the chisel-toothed Stylinodon, and a diverse variety of carnivorous creodonts. Mammalian fossils are typically preserved as isolated skeletal remains such as teeth, jaws, and a variety of other bones. However, complete skeletons are rare. As with other vertebrate groups, the greatest diversity and abundance of mammalian fossils is found in lacustrine and marginal lacustrine facies, and in fluvial (stream channel) facies.

Left and right dentaries of a juvenile brontothere (UCM 69355) collected from the Twin Buttes Member of the Bridger Formation (subdivision upper D), Uinta County, Wyoming.

Portion of a mural depicting middle Eocene Wyoming approximately 45 Ma painted by J. Matternes for the National Museum of Natural History. Mural includes reconstructions of the primitive tapir Helaletes (lower left and center), the oxyaenid creodont Patriofelis (lower right), the brontothere Palaeosyops (upper middle), the adapid primate Notharctus (upper right), and the squirrel-like rodent Sciuravus (right of center).

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