Paleoenvironmental implications of size, carapace position, and incidence of non-shell elements in White River turtles

In this study, 314 fossil turtles of Nebraskaʹs White River Group were used to gain insights into the regional paleoecology of the late Eocene and Oligocene. We collected taphonomic information about the size, the associated fossils, carapace position (up vs down), the presence of non-shell elements, and the presence of pre-burial weathering. A plot of turtle size vs strata reveals that turtle size decreased dramatically in the Whitney (ca. 31 Ma), and then increased again by the time the Arikaree was deposited. These size fluctuations appear to reflect the major cooling and warming trends at the beginning and end, respectively, of the Oligocene. Analyses of carapace position (up vs down), pre-burial weathering, and non-shell elements indicate several modes of death and preservation among the different turtle populations, including scavenging, burial by flood, and prolonged cold or drought. One 10 m interval in the Whitney Member, near the 31 Ma Lower Whitney Ash, contains a large number of small, completely articulated turtles preserved in mostly upright position. This indicates an expiration and subsequent preservation environment free of scavenging or high energy disturbances that might overturn a turtle.