SEX AND SHIFTING BIODIVERSITY DYNAMICS DURING THE MESOZOIC MARINE REVOLUTION

Several studies of the fossil record of marine animals indicate that diversity-dependent processes exerted strong control on biodiversification—after the Ordovician Radiation, genus richness did not trend for hundreds of millions of years. However, diversity subsequently rose dramatically in the Cretaceous and Cenozoic, even in sampling-standardized data, indicating that limits on diversification can be overcome by ecological or evolutionary change. Here, we apply maximum-likelihood modeling to marine animal diversity curves and show that the Cretaceous-Cenozoic radiation was driven by increased diversification in animals that transfer sperm between adults during fertilization (e.g., arthropods, vertebrates, some gastropods), whereas animals that broadcast sperm into the water column (e.g., bivalves, brachiopods) have not changed significantly in aggregate richness since the Ordovician, although there has been turnover among dominant taxa. We argue that the former group radiated in part because directed sperm transfer permits smaller population sizes and additional modes of pre-zygotic isolation, as has been argued previously for the coincident radiation of angiosperms. Directed sperm transfer tends to co-occur with many ecological traits, such as motility and a predatory lifestyle, and ecological specialization likely operated synergistically with mode of fertilization in driving the diversification that began during the Mesozoic Marine Revolution. Motile predators had greater potential to specialize ecologically (relative, for example, to sedentary suspension feeders), and direct sperm transfer permitted smaller population sizes that allowed specialized taxa to survive. Plausibly, the ultimate driver of diversification was an increase in food availability, but its effects on the fauna were regulated by fundamental reproductive and ecological traits.