In this dissertation, I investigate macroevolutionary patterns and dynamics in planktonic foraminifera and accompanying changes in their pelagic environment. My research focuses on processes that structure diversity in the open sea, including the response of oceanic ecosystems to the largest biotic disturbance of the Cenozoic, the Cretaceous-Paleogene mass extinction.

I begin by investigating the determinants of community similarity in modern planktonic foraminiferal communities. Here, I find that community similarity is primarily determined by the environmental similarity between communities, and not geographic distance. On a global scale, planktonic foraminifera provide an exception to the rule that geographic proximity is the strongest predictor of community similarity.

Patterns of morphological evolution in planktonic foraminifera also suggest that different evolutionary processes may predominate in the open ocean. I investigate a classic case of gradual evolution in the Globorotalia plesiotumida-G. tumida lineage of planktonic foraminifera using more sensitive numerical techniques and find evidence for a cryptic speciation event and a more rapid evolution of G. tumida than previously hypothesized.

New analytical approaches were also developed in order to ask questions at a higher stratigraphic resolution at the Cretaceous-Paleogene (KPg) boundary than is currently possible. To this end, Lagrangian advection-diffusion models were modified to fit iridium anomalies deposited by the KPg impactor.

Insights into the recovery of pelagic ecosystems from the KPg mass extinction were obtained from high-resolution geochemical, grain size, and taxonomic records of the early post-extinction interval. Barium proxy records were used to investigate boundary-related changes in export productivity, and supported inferences from benthic foraminifera in describing geographically and temporally heterogeneous changes in export productivity following the KPg impact. When productivity was considered along with planktonic foraminifera community composition and relative flux, this evidence suggested that an alternative pelagic ecosystem may have thrived in some locales in a post-extinction world.