Morphology and relative frequency of planktic foraminiferal species in relation to oxygen isotopically inferred depth habitats

In modern low-latitude open oceans, planktic foraminiferal species with complex morphologies and/or thickened shells usually dwell in the subsurface and deeper waters; morphologically simple species with globular chambers and an umbilical aperture, tend to be surface water dwellers. This has led to the widely held belief that the relative depth habitats of extinct forms can be deduced from their test shape, with planoconical or keeled species dwelling below globigerinids. Yet, oxygen isotope ratios have shown some globorotaliids (e.g. Morozovella) to have been surface dwellers. ermining the relative depth position of many different fossil groups from their oxygen isotope ratios, I found no relationship between test shape and preferred depth habitat. Many keeled and/or planoconical forms (Globotruncana, Contusotruncana, Morozovella, Fohsella, and Turborotalia), as well as unkeeled groups (Acarinina, Globigerinatheka, some Jenkinsella and Globoconella) were surface or subsurface dwellers. Deep dwellers include the keeled Planomalina, Globotruncanita, and Planorotalites, the unkeeled Catapsydrax, some Jenkinsella and most Paleogene globigerinids. y the most common group shows relatively low oxygen isotope ratios, indicating that surface-dwelling species are more abundant and/or had a shorter life-cycle with a larger shell output. The tests of most inferred surface dwellers are relatively susceptible to dissolution, with the globigerinathekids and ‘Acarinina’ senni as notable exceptions. The low-diversity Oligocene faunas, dominated by globigerinids, followed the extinction of the morphologically diverse surface-dwelling part of the Late Eocene fauna, not the elimination of deep-dwelling species.