A new approach to the study of evolution based on quantitative faunal analysis and the spatial distribution of taxa in the oceanic water column is presented. Middle Eocene to Miocene planktonic foraminiferal turnovers have been examined based on the numerical abundances of species and their relative depth habitat as inferred from oxygen isotope ranking of species. Results indicate that most turnovers occurs in the surface and intermediate water faunas, that all but one of the faunal turnovers coincide with accelerated global cooling, and that all faunal turnovers occur during intervals of increased productivity in the δ13C record. This apparent correlation implies that faunal turnover dynamics are mediated by intrinsic changes in the thermal structure of the oceanic surface waters over time. Nevertheless, there is no obvious linkage of specific environmental parameters to the proliferation or decline of specific faunal elements. The incorporation of these types of data into the story of the evolution of the planktonic foraminiferal fauna promises to yield new insights into the nature of both biotic and abiotic factors that have played a role in the origin and maintenance of planktonic foraminiferal diversity.