Giant Viviparidae (Gastropoda: Architaenioglossa) from the Early Oligocene of the Nanning Basin (Guangxi, SE China)

The rich freshwater mollusc fauna from the Lower Oligocene Gongkang Formation (Yongning Group) of the Nanning Basin (Guangxi Province, SE China) has yielded shells of two species of giant viviparid gastropods, which are described as Margarya nanningensis n. sp. and Macromargarya aliena n. gen. n. sp. The genus Margarya Nevill, 1877 has so far been thought to be endemic to the Pleistocene to Recent freshwater lakes of Yunnan Province (SW China), and is first described from the Oligocene of Guangxi herein. With a maximum height of more than 100 mm, Macromargarya aliena is among the largest freshwater gastropods reported to date. Moreover, M. aliena displays a strange ontogeny. An early, very low-trochospiral growth stage is followed by a rapid shift of the coiling axis and whorl proportions, leading to regular viviparid morphology in adulthood.