Turtle Decline in the Early Oligocene of Western Nebraska

Abstract

Turtles are the most common vertebrates in the upper Eocene and lower Oligocene rocks of Nebraska. In this work we characterize a striking decline in overall turtle abundance in the early Oligocene at two Nebraska localities: Toadstool Park and Scotts Bluff National Monument. In both locations, this decline occurs at a prominent change in rock composition, the contact between the Orella and Whitney members of the Brule Formation. The two study sites are 140 km apart, suggesting that this decline in turtle fossils was a regional event. Although the reasons for the abundance changes are not entirely clear, we present preservation biases or regional cooling and drying as competing hypotheses.