Department of Earth and Biological Sciences
Taphonomy of fossil turtles and microvertebrates, and sedimentology, of the Eocene Bridger Formation, Wyoming
Leonard Brand, PhD, Loma Linda University
In association with:H. Paul Buchheim, PhD, Loma Linda University (sedimentology)
Tom Goodwin, PhD, Andrews University (vertebrate paleontology)
Peter Ambrose, MS, US 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 under publications on my homepage (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.
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.