Platyceratid gastropods - stem group of patellogastropods, neritimorphs or something else?

Abstract

A systematic study of Silurian and Devonian platyceratid gastropods provides new data on their early shell ontogeny. Typical cyrtoneritimorph protoconchs were found in Praenatica cheloti (Oehlert & Oehlert, 1887) from the Lower Devonian strata of Brittany (W France). The same protoconch type, as in Praenatica Barrande in Perner, 1903, was earlier documented in Orthonychia Hall, 1843. Both of the latter taxa have often been considered to be subgenera of Platyceras Conrad, 1840. However, Silurian and Devonian species of Platyceras from the Barrandian area develop quite different protoconchs (orthostrophic and tightly coiled), similar to those found earlier in different species of the family Naticopsidae Waagen, 1880. Current data suggests that Paleozoic platyceratids represent a diphyletic group. The nature of their protoconchs (development of a true larval shell) testifies against the hypothesis that the Paleozoic platyceratids are the stem group for the Patellogastropoda. However, the derivation of modern neritimorphs from Paleozoic "platyceratids" with tightly coiled protoconchs or from the naticopsids seems to be probable. The strongly convolute neritimorph protoconch (apomorphy of neritimorph crown-group) probably originated after the Permian/Triassic mass extinction event, but before the Late Triassic.