Middle Miocene evolution and stable isotope ratios of Globorotalia (Fohsella) in the western equatorial Pacific

Isotopic and faunal evidence suggests that this change in depth stratification was associated with an expansion of the thermocline in the western equatorial Pacific. After adapting to a deeper water habitat at 13.0 Ma, the G. (Fohsella) lineage became extinct abruptly at 11.8 Ma during a period when isotopic and faunal evidence suggest a shoaling of the thermocline. Following the extinction of G. (Fohsella), the ecologic niche of the lineage was filled by the Globorotalia (Menardella) group, which began as a deep-water form and later evolved to an intermediate-water habitat.

We suggest that the evolution of G. (Fohsella) and G. (Menardella) were tightly linked to changes in the structure of the thermocline in the western equatorial Pacific.