Recent and Current Research Projects


   



Since the early 1990's when Emmett Evanoff, Peter Robinson and Paul Murphey of the University of Colorado Museum initiated a focused effort to collect fossils and study the stratigraphy and other aspects of the Bridger Formation and its paleontology, thousands of fossils from over 500 precisely documented fossil localities have been collected, and a detailed stratigraphic framework has been established. Participants have included colleagues from other academic institutions and the USDI Bureau of Land Management, students from several field schools, numerous field assistants, and many good friends. In 1991, Dr. Emmett Evanoff, then a Research Associate and Instructor at the University of Colorado, was the first UCM worker to undertake geologic field work in the Bridger under a greant from British Petroleum to study channel sandstone geometries. The Bridger Basin Project was first named by then UCM Curator Professor Peter Robinson in the third field season, after his discovery of the now well-known Omomys Quarry (UCM Loc. 93026). The project subsequently included Paul Murphey's Masters Thesis and Doctoral Dissertation, as well as a number of publications by Murphey and other project participants, including a series of geologic maps. All of the fossils collected during the 1990's are housed at the UCM. Those collected by Paul Murphey, Steve Walsh and their crews since 2005 are housed at the San Diego Natural History Museum.

Some of the findings of our research on the upper Bridger Formation (Twin Buttes and Turtle Bluff members) and its fossils is summarized below and on the "Products" page of this website. As discussed below, the primary focus of our current work is the faunal transition from the Bridgerian to the Uintan North American Land Mammal Age (NALMA). Part of this work is ongoing in the Turtle Bluff Member (Bridger subdivision E) of the Bridger Formation, where efforts to supplement this critical but faunally and temporally poorly understood interval by intensive screenwashing are underway.


Portion of a mural depicting middle Eocene Wyoming painted by J. Matternes for the National Museum of Natural History. Mural includes reconstructions of the crocodile Crocodilus (lower right), the condylarth Hyopsodus (upper right), the horse Orohippus (upper left), the saber-toothed creodont Machaeroides preying on the monitor-like lizard Saniwa (lower left), and the turtle Echmatemys (lower middle).


History of stratigraphic nomenclature of the Bridger Formation.


As reported by Evanoff et al. (1998), Murphey (2001), and Murphey and Evanoff (2006), three of W.D. Matthew’s original subunits, the Bridger B, C and D, have now been further subdivided into lower, middle and upper subunits using additional marker limestones, ash fall tuffs, and a widespread sheet sandstone. The names of the marker units we use to subdivide the Twin Buttes Member are, from lowest to highest, the Sage Creek limestone, Soap Holes limestone, Henrys Fork tuff, Lonetree limestone, Basal Blue Sheet sandstone, and Upper White limestone. The Turtle Bluff Member is bounded by the Basal E limestone below and the Behunin Reservoir Gypsum Bed near the top, and unconformably overlain by the Oligocene Bishop Conglomerate.


The Bridger marker units have now been mapped over approximately 1,000 square kilometers of the southern Green River Basin. The classic upper Bridger area in the vicinity of Sage Creek Mountain, Hickey Mountain, the southwest side of Cedar Mountain, and the Henrys Fork/Lonetree Divide, has been correlated across the central part of the basin east to Twin Buttes and Black Mountain using the previously unrecognized Henrys Fork tuff as a datum. Detailed stratigraphic sections have been measured across the basin, and, as described below, sedimentologic, paleontologic and taphonomic studies have been undertaken to interpret Bridger paleoenvironments and microfossil accumulations.

Three ash fall tuffs have been dated using 40Ar/39Ar techniques, and several more ash samples are currently undergoing 40Ar/39Ar analysis. The Church Buttes tuff (base of middle Bridger B) has a date of 47.96 +/- 0.13 Ma. The Henrys Fork tuff (base of upper Bridger C) has a date of 46.92 +/- 0.17 Ma. The Basal Bridger E tuff (8 m below base of Turtle Bluff Member) has a date of 46.16 +/- 0.44 Ma. These dates indicate relatively rapid and consistent average rock accumulation rates of 24.0 cm/1000 years for the middle and upper Bridger B (within Blacks Fork Member), and the Bridger C and D (Twin Buttes Member). Other tuffs, including two samples from the Turtle Bluff Member, are currently undergoing isotopic analysis.


Will Clyde (center) and student (right) from the University of New Hampshire collecting a sample of the Henrys Fork tuff for a paleomagnetism pilot study while Steve Walsh of the San Diego Natural History Museum (left) looks on (August, 2005).


                          Composite bedrock geologic map of the southern Green River Basin, Wyoming (from Murphey, 2001).


Steve Walsh on the west slope of "Peratherium Pinnacle" sampling potentially fossiliferous rock matrix for later screenwashing (SDSNH Loc. 5842). This locality is stratigraphically 87 m above the base of the Turtle Bluff Member (subdivision E) of the Bridger Formation on the southwest flank of Cedar Mountain, Uinta County, Wyoming (August, 2006).

Depositional environments that are recognized by Murphey (2001) and Murphey and Evanoff (2006) in the Twin Buttes and Turtle Bluff members of the Bridger Formation include fluvial (stream channel and floodplain), lacustrine, playa lacustrine, paludal, marginal mudflat, basin margin, and volcanic.

The Bridger Formation has produced the holotypes of most presently recognized Bridgerian mammalian taxa. Despite the fact that terrestrial vertebrate fossils are numerically most abundant and taxonomically most diverse in lake-margin deposits (deposited on the landward but regularly-flooded side of lake shorelines), most Bridger sediments were deposited on floodplains. These volcaniclastic floodplains appear to have been eposidically flooded, perhaps annually. They persisted for up to 85% of depositional time, and contain few fossils.

Studies of the fossil fauna, taphonomy and sedimentology of two highly fossiliferous units, the Henrys Fork limestone and Hickey Mountain limestone, have been completed. Other units, including the Whisky Reservoir limestone, Butcher Knife limestone, Horse Ranch red bed, Burnt Fork limestone, and Hickey Reservoir limestone, have been stratigraphically documented, described and at least partially mapped.

Sedimentologic data indicate that the basin floor was topographically flat, even near the Uinta Mountain front, although alluvial fan (basin margin) deposits derived from the Uinta Mountains are present along its foothills. However, sediment input from the adjacent Uinta Mountains was minimal. The basin axis was an elongate east to west trough that formed in a structural downwarp along the north side of the Uinta Mountains, and this is where lakes persisted for the longest period of time. Based on fossil mollusks, most lakes and ponds were shallow (less than 5 m deep), mostly groundwater-fed, well oxygenated, perennial bodies of water with freshwater salinities. Based on rock accumulation rates, they typically persisted for less than ten thousand years before being overwhelmed by influxes of volcaniclastic mud washed into the basin from the north. The data suggest that as a result of increased volcanism in the Absaroka Volcanic Field in northwestern Wyoming, the mode of deposition of the Bridger was fundamentally different than that of the intertonguing and underlying (slightly older) Green River Formation, in which lake expansion was largely associated with cooler and wetter climatic conditions. During deposition of the upper Bridger Formation, wetter climatic conditions corresponded with times of increased volcaniclastic input to the basin and floodplain deposition, not lake expansion. Groundwater-fed lakes and ponds developed during times of low sediment transportinput to the basin. Lake and pond sediments are characterized by extremely fine grain-sizes, a lack of associated deltaic facies, and abundant aquatic but uncommon terrestrial fossils. Terrestrial fossils, including vertebrates but also trees and plants, are associated primarily with lake margin facies. As reflected in the geometries of lacustrine deposits, lake margin facies advanced and retreated as lake waters expanded and contracted through time.


 





Visual analog for a Bridgerian lake approximately 47 Ma during deposition of the Twin Buttes Member, with the foothills of the Uinta Mountains in the background (Lake Magadi, Kenya).




Drying fossiliferous matrix from Red Lenses (SDSNH Loc. 5843) prior to screenwashing at the 2006 field camp. Red Lenses is stratigraphically 106 m above the base of the Turtle Bluff Member (subdivision E) and 414 m above the base of the Twin Buttes Member of the Bridger Formation on the southwest flank of Cedar Mountain, Uinta County, Wyoming.

 

Based on our stratigraphically well documented fossil collections, it appears as if the major faunal break within the Bridger Formation (Bridger A-D inclusive) does not occur at the Bridger B-C boundary, as previously reported, but is characterized by much more gradual faunal change.

The uppermost member of the Bridger, the sparsely fossiliferous Turtle Bluff Member (subdivision E), once widely regarded as belonging to the Bridgerian North American Land Mammal Age, has a "transitional Uintan fauna." This is based largely on fossils collected from UCM Loc. 92189 (discovered in 1992 by Donna Engard), which occurs stratigraphically at the base of the Turtle Bluff Member on Cedar Mountain, Uinta County, Wyoming.

The age of the Basal Bridger E tuff indicates that the Bridgerian-Uintan faunal transition was underway at approximately 46 Ma. As stated above, efforts are now underway to better document the fauna of the Turtle Bluff Member and the Bridgerian-Uintan faunal transition by intensive screenwashing of all levels of this steep and remote unit, as well as isotopic dating of newly sampled ash-fall tuffs. The preliminary results of the screenwashing approach have been far better than hoped for in terms of fossil yield, although we do not expect to publicize these results until SVP in 2007.


Ecological Diversity Analysis (EDA) is a method in which particular mammalian adaptations (diet, locomotor behavior, and body mass) are correlated with certain vegetation types. EDA has been used in paleobiology to predict vegetation, or habitats, for mammalian fossil assemblages. It has been very successful in predicting habitats for Neogene mammalian assemblages, but has only rarely been used for interpreting Paleogene assemblages. Using a discriminant model developed by evaluating the ecological diversity of 20 Neotropical mammalian faunas representing four macrohabitats in Central and South America, 9 Bridger Formation facies faunas represented by over 3,000 specimens from over 500 localities were analyzed (Murphey and Townsend, 2005; Townsend and Murphey, in preparation). The purpose was to infer probable habitat types for each depositional facies independent of existing taphonomic and sedimentologic information. The facies faunas include fossil samples from 7 stratigraphically distinct lake margin deposits, 1 combined sample representing all fossils collected from stream channel deposits, and 1 combined sample representing all fossils collected from floodplain deposits. The lake-margin and stream channel samples were all classified as forest habitats, and the floodplain sample was the only sample that classified as an open country habitat with few to no trees.


 


Screenwashing fossiliferous matrix from "Roll the Bones" (SDSNH Loc. 5844) at the 2006 field camp. Roll the Bones is stratigraphically 105 m above the base of the Turtle Bluff Member (subdivision E) and 413 m above the base of the Twin Buttes Member of the Bridger Formation on the southwest flank of Cedar Mountain, Uinta County, Wyoming.

 


Stratigraphic distribution of UCM mammalian fossils in the Twin Buttes Member and basal part of the Turtle Bluff Member. Blue lines indicate numbers of fossils that were preserved in lake and lake margin facies, and brown lines indicate numbers of fossils found in fluvial (stream channel and floodplain) facies.

 
In summary, the EDA results suggest that the stratigraphic and geographic pattern of fossil distribution in the Twin Buttes Member reflects paleohabitat types rather than differential fossil preservation attributable to taphonomic factors and/or rates of sedimentation. For example, although rates of lacustrine and lake margin sedimentation were apparently at least four times less than those of floodplains, the difference is insufficient to account for the much higher concentrations of fossils in the lake and lake margin sediments. Furthermore, the results of taphonomic and sedimentologic studies indicate that floodplain sediments are geochemically suitable for preserving fossils. They contain nothing that would result in diagenetic dissolution of bone. Most floodplain fossils consist of highly fragmentary and transported bones of larger mammals (mostly perissodactyls) that exhibit high degrees of primary weathering. In combination with the EDA results, the taphonomic and sedimentologic data support the hypothesis that the volcaniclastic Bridger (Twin Buttes Member) floodplains were less favorable habitats for mammals than forested lakes margins and riparian habitats. Furthermore, the data support the hypothesis that floodplains resembled episodically--flooded volcaniclastic mudflats, with sparse vegetation and high sedimentation rates. These persisted for up to 85% of upper Bridger time. This is a significant contrast to the traditional image of the Bridger Formation as a sub-tropical forest environment throughout its deposition.
Another current long-term research project seeks to assess the rates of exposure of Bridger fossils observed at numerous localities with different rock types. The results of this effort will provide insights into biases present in surface fossil samples that will be important to future paleoecological analyses of the Bridger fauna, and will also be useful to land managers in their stewardship of paleontological resources. Preliminary results after 11 years of observations indicate that the average rate of erosional exposure of fossil bone is 6.5 mm/year. Thus, while a tooth or small jaw may be totally exposed in less than ten years, it typically takes a minimum of several decades for larger elements to be fully exposed. The remains of a very large animal could take several hundred years to become fully exposed. Simultaneously, degradation of exposed fossil bone due to secondary weathering is a relatively rapid process, and may lead to total destruction of small vertebrate fossils in several years time.
 


A sublime sunset as viewed from the 2005 field camp looking south towards the uppermost rim of Cedar Mountain, Sweetwater County, Wyoming.

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