A new rissoid gastropod with lecithotrophic development from the Miocene of Paratethys

Abstract

The results of detailed morphological studies on Rissoa sobieskii from the badenian of the eastern central Paratethys are presented. its paucispiral protoconch with a bulbous embryonic shell indicates it to have had a lecithotrophic (non-planktotrophic) larval development, apparently lacking even a short free-swimming larval stage. this contrasts with all other known Miocene Rissoa species, which have shell features indicating a planktotrophic larval stage. the peculiarity of R. sobieskii is also supported by its characteristic narrowing of the aperture and almost complete lack of teleoconch sculpture. the time coincidence of the appearance of the non-planktotrophic species with distinct changes in salinity of the badenian basin suggests that the alteration (“switching”) of the type of early ontogeny from planktotrophic to non-planktotrophic development could have been an environmentally induced phenomenon.