Taphonomy of fossil turtles and microvertebrates, and sedimentology, of the Eocene Bridger Formation, Wyoming

Leonard Brand, Ph.D. Loma Linda University

In association with:

H. Paul Buchheim, Ph.D., Loma Linda University (sedimentology)

Tom Goodwin, Ph.D., Andrews University (vertebrate paleontology)

Peter Ambrose, M.S., U. S. Forest Service (LLU thesis on turtle taphonomy)

 

This research focused on the taphonomy of the most abundant fossils in unit B of the Bridger Formation - turtles, and the sedimentological context of the turtles. The goal of the research was to understand the paleoecology and sedimentary processes that produced the fossil assemblage. This initial work has been published in a pair of papers listed in the list of publications on my home page (Brand, Goodwin, Ambrose, and Buchheim 2000; Buchheim, Brand, and Goodwin 2000). Current work involves screenwashing of sediment from our original study sites, to compare stratigraphic and geographic distribution of microvertebrate fossils with the distribution of turtles.

 

Brand, L., P. C. Murphey, and J. E. Haessig. In press. Bedrock geologic map of the Linwood Canyon 7.5' Quadrangle, Sweetwater County, Wyoming. Wyoming State Geologic Survey Open File Map, 1 sheet (scale 1;24,000)

Brand, L., P. C. Murphey, and J. E. Haessig. In press. Bedrock geologic map of the Antelope Wash 7.5' Quadrangle, Sweetwater County, Wyoming. Wyoming State Geological Survey Open File Map, 1 sheet (scale 1:24,000).

Brand, L. R., H. T. Goodwin, P. G. Ambrose, and H. P. Buchheim.   2000.  Taphonomy of  turtles in the Middle Eocene Bridger Formation, SW Wyoming.  Palaeogeography, Palaeoclimatology, Palaeoecology, 162:171-189.

Buchheim, H. P., L. R. Brand, and H. T. Goodwin.  2000.  Lacustrine to fluvial flood-plain deposition in the Eocene Bridger Formation.  Palaeogeography, Palaeoclimatology, Palaeoecology, 162:191-209.

Brand, L. R. 1995. An improved high-precision Jacob's staff design. Jour. Sedim. Res., A65:561.

Two students, Mark Loewen and Judy Holbert, collecting a turtle from a sandstone

 

Relative abundance of turtle bones in mudstones in the upper part of Bridger B, above the Lower turtle layer limestone. Data from a 30 meter-wide transect in the Devils Playground area. Turtles are abundant at specific levels, above limestones, and taphonomic evidence indicate these are mass mortality assemblages. Ltl = Lower turtle layer (mudstone above the limestone). Gbl = Golden bench limestone. BMtl = Black Mountain turtle layer (mudstone above the limestone). Utl = Upper turtle layer (mudstone above the limestone).

 

Above: Geographic distribution of turtle bones in Bridger B. At each study site (e.g. NR-10) the number of turtle bones per hectare is given, and isopach lines are drawn through areas with the same number of bones per hectare. Turtle distribution shows a basin-wide pattern of the turtle mass mortalities, rather than local concentrations.

 

Student George Kim prospecting for vertebrate fossils in the Black Mountain turtle layer.

 

Tom Goodwin, Sun Ho Kim, and student Holly Moon collecting sediment for screenwashing, from the Black Mountain turtle layer.

 

Model for accumulation of turtles and sediments in Bridger B. Time 1: A shallow lake fills the basin. Turtles are abundant in the lake. Time 2: An episode of volcanism begins to the north, in the Absarokas. Ash or volcanic gases kill many turtles, and clay to sand sized volcanic sediments begin to accumulate over the basin, delivered by air fall and/or prograding fluvial-deltaic systems.

 

Time 3: The sedimentary sequence becomes more fluvial in character, in a well-established fluvial floodplain environment with large fluvial channels representing meandering rivers. Time 4: The volcanic episode ends, and a lacustrine system again begins to fill the basin, forming another limestone. This sequence accounts for the repeating pattern of turtles concentrated in large, mass mortality assemblages right above limestones. Turtles are much less common outside of these concentrations.

 

The Bridger team. From left, Brand, Buchheim, and Goodwin.

 

The LLU research camp on the Blacks Fork River, at screenwashing site.

 

Holly Moon (left) and Tom Goodwin washing Bridger sediment.

 

Local chipmunks at our Cedar Mountain camp site make use of the wastewater bucket at our cooking trailer.

 


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