The operculum of Otostoma Retzii (Nilsson, 1827) (Gastropoda, Neritidae; Late Cretaceous) and its phylogenetic significance

Abstract

The Neritimorph gastropod genus Otostoma d'Archiac, 1859 (p. 871), with its hemispherical shell and typical axial ribs and lamellae, is a characteristic fossil in (sub)tropical, nearshore deposits of the late Early and Late Cretaceous ages (Saul and Squires, 1997; Kiel, 2002; Kiel et al., 2002; Bandel and Kiel, 2003). The genus survived Cretaceous/Paleogene boundary perturbations and appears to have become extinct during the Middle Eocene (Glibert, 1973; Saul and Squires, 1997). Its placement in the Neritimorpha Golikov and Starobogatov, 1975 is clear from its typical hemispherical shell and D-shaped inner lip with a row of denticles. Long considered linked to Recent Nerita Linnaeus, 1758 (see Wenz, 1938-1944; Kase, 1984; Squires and Saul, 1993; Bandel and Kiel, 2003), assignment of Otostoma to the Neritoidea Rafinesque, 1815 has recently been corroborated by documentation of dissolved internal walls in Otostoma divaricatum (d'Orbigny, 1842) from the Campanian (Upper Cretaceous) of northeast Spain (Bandel and Kiel, 2003). However, axial (collabral) ornament as seen in Otostoma is unknown in modern neritids, which usually are either smooth or spirally sculptured. Here we describe the operculum of Otostoma retzii (Nilsson, 1827) for the first time; it closely resembles that of Nerita and many species of Neritina Lamarck, 1816, yet differs significantly from those of other neritimorph clades, including Neritopsidae Gray, 1847 and Neritiliidae Schepman, 1908.