Taphonomy of turtles in the Middle Eocene Bridger formation, SW Wyoming

This study seeks to document and account for the distribution, abundance, and taphonomic condition of fossil turtles in a fossiliferous section of the Bridger Formation, Unit B (Early Middle Eocene of Wyoming). The following patterns were documented: (1) Fossils were non-randomly distributed stratigraphically and sedimentologically with most specimens concentrated in mudstones within a few meters above two of three widespread limestone beds. These concentrations were not artifacts of accumulations of eroded fossils on low angle slopes. (2) Fossil concentrations above limestones were widespread in the study area-tens of kilometers in at least one case. The well-exposed Black Mountain turtle layer shows a gradient in fossil density, highest to the south and lowest to the north. (3) Most specimens from fossil accumulations exhibited a similar taphonomic condition, with many shells mostly intact and unweathered, and with no skulls and few limb elements. Few elements bore predator tooth marks. Some bones in channel deposits were abraded, but most bones in fine-grained sediment were not. The largest concentrations of turtles were associated with specific layers of fine-grained sediment. These features suggest mass mortalities of turtles, and burial before many shells disarticulated. A model is presented to account for these data. In this model, a limestone forms in a shallow, basin-wide lacustrine environment. Then, a series of fluvial/lacustrine sedimentary units resulting from a large-scale episode of volcanism accumulated in the lake and buried the turtles. The volcanic event may have been the cause of death, from breathing ash-choked air, for large turtle populations in the lake/marsh environment, which were then buried early in the volcanic episode. Turtle populations evidently did not recover significantly until another shallow lake filled the basin. (C) 2000 Elsevier Science B.V. All rights reserved.