Paleoenvironmental Implications of Small-Animal Fossils from the Black Mountain Turtle Layer and Associated Layers, Eocene of Wyoming
Authors:   ,   Andrews University, McLarty Jeremy 
Published: June 2023

The Bridger Formation is an Early Middle Eocene deposit in southwestern Wyoming that preserves a rich record of life from North America. Some horizons within the Bridger Formation contain abundant fossil turtle shells, but turtle skulls are rarely found. Previous research focused on one of these fossil-rich horizons, the Black Mountain turtle layer, to develop a model for the abundance and taphonomic condition of the fossil turtles. The proposed model begins with a limestone layer deposited in a lake. Then, an influx of fine-grained volcanic ash (forming the Black Mountain turtle layer) was rapidly deposited into this lake likely causing the mass death of turtles in the lake and rapid (but not instantaneous) burial of the turtles. Subsequent units of the lithofacies association were deposited as the lake was filled with sediment, eventually transforming into a floodplain environment. Although the depositional model developed to explain the concentration of fossil turtles in the Black Mountain turtle layer envisions accumulation within a broad, shallow lake, subsequent investigation of the taxonomic makeup and taphonomic condition of small-animal fossils associated with the fossil turtles showed that terrestrial animals, especially lizards, were the most common taxa present and that these fossils were probably not transported into the system. These findings seemed puzzling in light of the lacustrine model because large numbers of terrestrial fossils would be unexpected in a lake deposit unless transported into the lake. In the current study, I analyzed small-animal fossils from nine sites along exposures of the Black Mountain turtle layer to test the proposed model that the limestone and overlying mudstone were deposited in a broad, shallow lake. Small-animal fossils were identified as specifically as possible and assigned to ecological groups that were identified as either aquatic or terrestrial. The geographic and stratigraphic patterns of the fossils were compared to the predictions of the proposed lake environment. My findings confirmed that the presence of terrestrial fossils, especially lizards, was much higher than would be expected for a lacustrine deposition of the limestone layer and Black Mountain turtle layer. Sites towards the west had the highest proportion of terrestrial fossils, while sites to the east had the highest proportion of aquatic fossils. However, there was no clear gradient in the geographically intermediate sites. The majority of sites had a high proportion of terrestrial fossils in the basal limestone layer with little change in this proportion over time. These findings contradict the proposed model of a lacustrine deposition for the limestone layer and Black Mountain turtle layer. An alternative depositional environment of a fluctuating paludal/lacustrine environment does account for several patterns observed in the small-animal fossils, but does not appear to account well for the fossil turtles. These findings highlight the need for further work in reconstructing the paleoenvironment of the Black Mountain turtle layer. This study also emphasizes the need for inclusion of small-animal fossil data in paleoenvironmental reconstructions.