NEARSHORE-OFFSHORE DIVERSITY GRADIENTS IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

Although the Campanian-Maastrichtian biota of the Western Interior Seaway (WIS) has been extensively investigated, very little is known about either paleoecological dynamics or temporal and spatial changes in community structure during this interval. In this study we investigated the community composition and trophic structure of nearshore and offshore habitats in the WIS by analyzing faunas from one upper Campanian biozone (Baculites eliasi zone) in Wyoming. Fossiliferous concretions were analyzed from sandy nearshore facies of the Lewis Shale and from more clay-rich offshore facies of the Pierre Shale. Changes in taxonomic composition of assemblages, relative abundance of species, and differences in niche occupation reflect ecological gradients across the WIS. Taxonomic diversity and evenness were relatively low in the muddy-offshore environments which show a high dominance of either pelagic Baculites and/or epibenthic suspension-feeding inoceramids. Other taxonomic groups and adaptive types are usually rare, suggesting stressful environmental conditions in the offshore area. In comparison, sandy, nearshore facies either contain a high diversity fauna or are barren, probably reflecting different environmental conditions between nearshore localities. Where the sandy, nearshore assemblage is present it shows higher species diversity and evenness, as well as a greater number of filled niches. The fauna is represented by a diverse array of macroinvertebrates, including bivalves, gastropods, scaphopods, ammonoids, crustaceans, serpulids, and bryozoans. The decrease in diversity from nearshore to offshore environments in the WIS is in contrast to the slightly increasing or steady diversity seen with increasing depth in most modern open-shelf settings. This suggests a fundamental difference between the extrinsic environmental conditions that influence diversity in epicontinental seas versus those present on continental shelves.