It has been widely assumed, despite few tests, that pelagic organisms speciate and diversify by invading unoccupied parts of the water column. Radiation is thought to occur when populations become isolated across oceanic fronts or across boundaries between vertically stratified water masses. Extensive convergent evolution in planktonic foraminifera and other planktonic groups suggests that the same habitats are repeatedly invaded by different lineages over geologic time. For example, globular, spiny-walled planktonic foraminifera repeatedly give rise to compressed, smooth-walled species ornamented with a peripheral carina or keel. We propose to test the popular model that explains the iterative evolution of keeled planktonic foraminifera in terms of invasion of progressively deeper parts of the water column. Our study will produce del 18 O isotopic rankings for species in two groups of globorotaliids that independently evolved the keel: Fohsella and early members of the Menardella/Globoconella clade. Isotopic differences between surface-dwelling planktonic foraminifera, benthic foraminifera, and members of these two lineages will be used to estimate changes in depth habitats. Since these lineages independently evolved from globular ancestors, we can observe whether similar changes in depth habitat occurred in concert with similar morphological changes in both lineages. These results will permit us to test the hypothesis that surface-dwelling species radiated by invading deeper parts of the water column. Our findings will show if the repeated convergent evolution of foraminifera can be explained in terms of changes of depth habitat within lineages. We can also determine whether or not morphological evolution is correlated with ecological evolution in these marine plankton. In addition, this study will document the ecological context in which a major radiation unfolded.
