Late Campanian-Early Maastrichtian Faunal Dynamics in the Western Interior Seaway

Early-diagenetic limestone concretions in the Upper Cretaceous Pierre Shale are an important source of paleontological and paleoecological data for the Western Interior Seaway (WIS). The goal of this study was to examine changes in the species richness, species evenness, and trophic structure in nine concretions spanning the Upper Campanian Baculites eliasi through Lower Maastrichtian B. clinolobatus biozones of northeastern Wyoming. Each concretion was carefully dissected to determine the molluscan composition and abundance of the incorporated fossils. The fossil assemblages were derived from a shallow-marine offshore habitat. Variation in both taxonomic composition of assemblages and relative abundance of a species reflects ecological change through the interval studied. Species richness and evenness was relatively low during the time of B. eliasi and increased throughout the time of B. baculus. The early B. baculus biozone is marked by an absence of ammonoids and an abundance of suspension feeding inoceramids. In addition to the inoceramids, low numbers of infaunal bivalves, scaphopods, and gastropods were also present. Both species richness and species evenness amongst bivalves, gastropods, and ammonoids continued to increase, reaching their maximum values during the B. baculus through B. grandis ages. This greater diversity suggests improved environmental conditions. The bivalve, scaphopod, and gastropod species from this interval began to occupy previously unfilled trophic (e.g. detritivores) and habitat niches (e.g. infaunal habitats). Both species richness and species abundance declined sharply during the B. clinolobatus age, indicating stressful environmental conditions. These conditions might have contributed to the extinction of the inoceramids (excluding Tenuipteria) and the endemic species of Baculites in the WIS.