Diversification patterns of planktic foraminifera in the fossil record

Theories of taxonomic diversification dispute whether global diversity has an upper limit set by the strength of biological interactions or grows in unlimited fashion until catastrophic events reset the system. We analyzed a global compendium of fossil planktic foraminifera from the early Cretaceous to the present after correcting for temporal differences in sampling effort. Our results show that their morphological diversity has increased exponentially through time in two different phases. The first phase collapsed at the Cretaceous–Paleogene boundary transition. The second phase developed through the Cenozoic era and declined during the Pliocene–Pleistocene interval, a period of time that was dominated by enhanced climatic and oceanographic instability. In none of these phases diversity reached an equilibrium level. Cenozoic faunas evolved faster than those inhabiting Cretaceous oceans, perhaps as a result of a higher environmental variability. Our results indicate that planktic foraminifera conform to an exponential diversification model. However, because catastrophic events have repeatedly reset the level of biological diversity and have kept it below the maximum, we cannot confirm whether the level of diversity is potentially unlimited.