Do marine faunas track lithofacies? Faunal dynamics in the Upper Cretaceous Pierre Shale, Western Interior, USA

Most studies examining faunal assemblages use their sedimentary context as a critical element in constraining and reconstructing their underlying environmental controls. This has resulted in the assumption that an absence of lithofacies change in a section should be reflected in a lack of environmental variation. This inference, however, has been placed into question by evidence that marine species are influenced by a broader range of environmental dynamics than just change in lithofacies. In this study, we examine the sensitivity of marine faunas to broadly defined environmental change within lithologically homogenous strata by examining concretionary fossil assemblages of the Baculites eliasi through B. clinolobatus biozones in monotonous, clay-rich strata of the Campanian-Maastrichtian Pierre Shale in Wyoming. We recognize five biofacies, which reflect different environmental conditions related to benthic oxygenation, substrate firmness, and water depth. Analyses of abundance patterns, raw species richness trends, and life-habit patterns display recurrent switching, upsection, between low- and high-diversity intervals. Our data reveal that samples with lower diversity show a strong relationship with intervals when water conditions were deepest, whereas higher diversity samples are associated with periods when shallow-water conditions prevailed in the study area. The distribution of taxa and diversity of the assemblages most likely reflect migrating oxygen- and substrate-controlled biofacies that were responding to changes in depth. This study shows that substantial changes in biofacies, diversity, and life habits can arise in response to variations in water depth with limited to no apparent change in lithofacies supporting the hypothesis that fossil taxa are much more sensitive indicators of environmental change than lithofacies.