Well-preserved nannofossil assemblages in late Albian and early Cenomanian hemipelagic sections from Ocean Drilling Program (ODP) Leg 171B and pelagic sections from Deep Sea Drilling Program (DSDP) Leg 77 preserve a record of the early history and adaptive radiation of the calcareous nannofossil genus Prediscosphaera. In the mid Cretaceous, Prediscosphaera underwent several phases of adaptive radiation and rose to become one of the most important calcareous nannofossil genera in the Mesozoic. The genus spans across key mid Cretaceous Oceanic Anoxic Events and may therefore lead to better understanding oceanic conditions during these occurrences. Several 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 tended to remain at low abundance levels until a significant disruption in the pelagic realm resulted in a decline of the dominant species, providing space for new species to rise in abundance and increase in size. These disruptions, resulting in newly evolved species, correspond to significant oceanic changes associated with Oceanic Anoxic Events, suggesting that major changes in deep water mixing and the structure of the surface water mass drove the early adaptive radiation of Prediscosphaera.