Evaluating the Relationship between Pedofacies and Faunal Composition: Implications for Faunal Turnover at the Paleocene-Eocene Boundary

Abstract

Fossil vertebrates commonly are preserved in paleosols that originally formed on the floodplains of ancient rivers. Laterally adjacent paleosols that differ in terms of their chemistry and morphology due to geological and environmental gradients on the ancient floodplain are called pedofacies. Vertebrate fossil assemblages from the Willwood Formation (Bighorn Basin, Wyoming) are investigated to determine if there is a relationship between faunal composition and pedofacies. This question is particularly important for the Willwood Formation because it records dramatic temporal changes in both faunal composition and pedofacies across the Paleocene–Eocene boundary—an interval marked by pronounced global warming. To understand the underlying causes of these temporal patterns, it is important to know the degree to which pedofacies and faunal composition are linked. Two separate analyses are conducted to investigate the relationship between pedofacies and faunal composition. Analysis 1 evaluates faunal compositions of established localities that are stratigraphically adjacent but characterized by different mean pedofacies. Results indicate that faunal compositions of these localities are not related to pedofacies but instead correspond to differences in the estimated body masses of the taxa. Results of Analysis 2, which evaluates new paleosol-specific collections from the same stratigraphic interval, indicate that different pedofacies are characterized by nearly identical faunas. The only exception is lizards, which are significantly more abundant in mature paleosols. Environmental gradients on the ancient floodplain seem to have exerted very little control on faunal composition at this spatial scale, but size sorting may have been an important taphonomic process in these Willwood collections.