MID-CRETACEOUS EVOLUTION OF THE CALCAREOUS NANNOFOSSIL GENUS PREDISCOSPHAERA IN THE WESTERN NORTH ATLANTIC

The calcareous nannofossil genus Prediscosphaera provides an opportunity to document paleontological evolution in response to environmental change. Prediscosphaera underwent several phases of adaptive radiation in the mid-Cretaceous and is a premier biostratigraphic indicator for the late Albian to late Maastrichtian. The genus spans key mid-Cretaceous oceanic anoxic events (OAEs), which may lead to better understanding oceanic conditions during OAEs and how similar genera responded to periods of shutdown of oceanic circulation. Well-preserved nannofossil assemblages in late Albian and early Cenomanian hemipelagic sections from Ocean Drilling Program (ODP) Leg 171B preserve a record of the early history and adaptive radiation of the calcareous nannofossil genus Prediscosphaera. Twelve distinct taxa are recognized and differentiated based on ultrastructural and morphological variations. Data indicate that species evolved, rose to dominance, then rapidly declined to extinction. Newly evolved species generally remained at low abundance levels until a significant paleoceanographic change resulted in a decline of the dominant species, creating space for new species to rise in abundance and increase in size. These oceanic disruptions correspond to significant oceanic changes associated with oceanic anoxic events, which suggests that major changes in deep water mixing and the structure of the surface water mass drove the early adaptive radiation of Prediscosphaera.