The limpet form in gastropods: Evolution, distribution, and implications for the comparative study of history

The limpet form - a cap-shaped or slipper-shaped univalved shell - convergently evolved in many gastropod lineages, but questions remain about when, how often, and under which circumstances it originated. Except for some predation-resistant limpets in shallow-water marine environments, limpets are not well adapted to intense competition and predation, leading to the prediction that they originated in refugial habitats where exposure to predators and competitors is low. A survey of fossil and living limpets indicates that the limpet form evolved independently in at least 54 lineages, with particularly frequent origins in early-diverging gastropod clades, as well as in Neritimorpha and Heterobranchia. There are at least 14 origins in freshwater and 10 in the deep sea, with known times ranging from the Cambrian to the Neogene. Shallow-water limpets are most diverse at midlatitudes; predation-resistant taxa are rare in cold water and absent in freshwater. These patterns contrast with the mainly Late Cretaceous and Caenozoic warm-water origins of features such as the labral tooth, enveloped shell, varices, and burrowing-enhancing sculpture that confer defensive and competitive benefits on molluscs