Neritopsoidea

Abstract: The calcareous operculum of the Neritimorpha is more often preserved in the fossil record than the operculum of most other Gastropoda that is of purely organic composition. Together with the morphology of the protoconch and the shell structure, operculum shape represents a useful tool for understanding evolution within Neritimorpha. The Devonian Nerrhenidae (Nerrhenoidea) have an operculum with spiral construction resembling that present in the operculum of modern Neritoidea during very young stages of growth. The operculum of the modern and Mesozoic Neritopsis resembles that of Triassic relatives of the Cassianopsinae (both Neritopsoidea). The Carboniferous and Permian Naticopsidae (Naticopsoidea) can be connected with species from the Triassic St Cassian Formation. Hologyra thus resembles Naticopsis. The Triassic Tricolnaticopsidae is distinguished by a quite different operculum. The first Neritoidea are represented by the Triassic Neritariidae and have a similar shape of their operculum, with inner hinge, to that of modern Neritina. A strong decline in diversity of Neritimorpha occurred towards the Jurassic, and Neridomidae fam. nov. (Neritoidea) has an operculum as in Triassic Ruganeritaria. Cretaceous Otostomidae n. fam. differs in shell shape from Neritidae, while their operculum resembles that of Nerita. The shape of the operculum distinguishes Neritiliidae, Neritidae, Smaragdiidae and Septariidae from each other. The Theodoxidae have their lecithotrophic early ontogeny reflected in the simplified construction of their juvenile operculum.