LATEST CRETACEOUS FAUNAL CHANGE IN THE WESTERN INTERIOR SEAWAY

The Late Campanian-Early Maastrichtian represents a critical phase during the Late Cretaceous greenhouse interval when mean global temperatures began to decline, oceanographic circulation altered, and water chemistry changed. Despite being moderately well documented, very little is known about the temporal faunal dynamics as well as the ecological and structural changes associated with these environmental fluctuations. To assess the faunal dynamics, in this study we examine the structure of marine habitats in the Western Interior Seaway through four biozones spanning the Baculites eliasi to B. clinolobatus zones in eastern Wyoming. Here, fossiliferous concretions were examined from offshore clay-rich facies of the Pierre Shale. Both changes in the taxonomic composition of assemblages and the relative abundance of the various species reflect ecological change through the study interval. Species richness and abundances were relatively low during the time of B. eliasi, an interval dominated by inoceramids and/or baculitids. During the early B. baculus age, species richness reflecting a broad array of macroinvertebrate groups increased, and is dominated by the epifaunal inoceramids and a lesser number of ammonoids, nautiloids, bivalves, gastropods, and scaphopods. Interestingly, baculitids are almost completely absent from this interval. Species richness and abundances continued to increase among the benthic faunal components during the late B. baculus and early B. grandis intervals. In contrast, nektic diversity remained unchanged. During the late B. grandis time and for most of the subsequent B. clinolobatus age species richness and species abundances declined sharply and resulted in a faunal composition similar to that of the time of B. eliasi. In the latest B. clinolobatus time, prior to the deposition of the Fox Hills Formation in the study area, faunal diversity increased. Inoceramids and baculitids became extinct shortly after this interval within the Western Interior as demarcated by their absence within the subsequent Hoploscaphites birkelundae biozone.