After over 15 m.y. of evolution in near-surface waters of the Western Pacific and Caribbean, planktic foraminifera in the lineage Globorotalia (Fohsella) abruptly invaded thermocline habitats at 13 Ma. Stable isotope data demonstrate that this habitat shift took 50 ka to complete, a duration similar to the fastest measured rates of speciation in marine protistans. The ecological change occurred near the end of a long series of gradual morphological transitions and demonstrates that at least some of the chronospecies in this lineage had distinctive ecologies and were probably biological species. Other than this one shift in depth distribution, skeletal evolution is largely disconnected from depth changes in Globorotalia (Fohsella). Morphological trends of globorotaliid foraminifera may not record changes in depth ecology, contrary to existing models for foraminiferal diversification.