The P aleozoic evolution of the gastropod larval shell: larval armor and tight coiling as a result of predation‐driven heterochronic character displacement

SUMMARY E arly and middle P aleozoic gastropod protoconchs generally differ strongly from their corresponding adult morphologies, that is, most known protoconchs are smooth and openly coiled, whereas the majority of adult shells are ornamented and tightly coiled. In contrast, larval and adult shells of late P aleozoic gastropods with planktotrophic larval development ( C aenogastropoda, N eritimorpha) commonly resemble each other in shape and principle ornamentation. This is surprising because habitat and mode of life of planktonic larvae and benthic adults differ strongly from each other. Generally, late P aleozoic to R ecent protoconchs are tightly coiled. This modern type of larval shell resembles the adult shell morphology and was obviously predisplaced onto the larval stage during the middle P aleozoic. The oldest known planktonic‐armored (strongly ornamented) larval shells are known from the late P aleozoic. However, smooth larval shells are also common among the studied late P aleozoic gastropods. The appearance of larval armor at the beginning of the late P aleozoic could reflect an increase of predation pressure in the plankton. Although there are counter examples in which larval and adult shell morphology differ strongly from each other, there is statistical evidence for a heterochronic predisplacement of adult characters onto the larval stage. Larval and adult shells are built in the same way, by accretionary secretion at the mantle edge. It is likely that the same underlying gene expression is responsible for that. If so, similarities of larval and adult shell may be explained by gene sharing, whereas differences may be due to different (planktic vs. benthic life) epigenetic patterns.