Individual species of marine calcareous phytoplankton are known to occur in blooms in today's ocean and to have dominated fossil assemblages. Interest in the study of marine phytoplankton-ocean-climate interactions has increased because of the potential influence of phytoplankton species like Emiliania huxleyi on global climate and on the global carbon cycle. There is evidence that Gephyrocapsa caribbeanica, which is closely related to E. huxleyi, was globally dominant in the late Pleistocene (480--262 ka). Morphological analyses of Gephyrocapsa coccoliths reveal that only two of six Holocene morphological associations occurred during this time interval. We examine three potential causes for the dominance: preservation, environment, and evolutionary adaptation. We conclude that evolutionary adaptation was the likely process responsible for Gephyrocapsa dominance, although there are indications in the mid-Brunhes for warmer climates than today. ¿ 1998 American Geophysical Union