Gastropods in deep time and the early Modern Evolutionary Fauna

The species richness and shell size distribution of major clades and functional groups among gastropods, a key element of the Modern Evolutionary Fauna (MEF), underlines the dominant role at modern tropical latitudes of carnivorous Caenogastropoda and Heterobranchia, including small-sized ectoparasites and micrograzers. The escalation hypothesis emphasises prey-predator interactions as gastropods’ macroevolutionary drivers during the Mesozoic Marine Revolution, but overlooks the significance of the highly-diversified smaller species. The early Mesozoic roots of the Neogastropoda, active predators particularly diversified since the Cretaceous and eminent extant members of the MEF, are poorly understood. I revise the tropical fossil record of the Permian-Triassic mass extinction (PTME), the major in animal history, and the Triassic rise of the MEF. The study suggests that non-carnivorous species dominated the gastropod fauna immediately before and after the PTME; Permian micrograzers mainly fed on sponges and waned during the rise of the MEF; ectoparasites and micrograzing carnivores diversified in the second part of the Middle Triassic; larger predators are lacking throught the interval. Patterns of gastropod species richness, size and form, the fossil record of reef-builders and other benthic invertebrates, and an analysis of stem neogastropods jointly highlight a Middle Triassic revolution of small-sized gastropods triggered by the emergence of scleractinian corals and the diversification of echinoderms. Habitat heterogeneity and new food sources offered niches for the early radiation of modern gastropod clades, pointing the Triassic as a laboratory to understand macroevolutionary processes in the wake of a major biotic crisis.