Relationships of the Triassic Eucycloidea Koken, 1897 (Mollusca, Gastropoda) to modern genera such as Pagodatrochus, Calliotropis and Euchelus, based on morphology of the early shell

摘要

The aim of this study is to document the relationship that may exist between Triassic species of Eucycloscala, Ampezzotrochus, Sabrinella, Microcheilus, Eunemopsis and Pseudoclanculus and living species of genera such as Pagodatrochus, Turcica, Calliotropis, Danilia, and Euchelus. Their shell has a nacreous inner layer and ornament of axial ribs on the early teleoconch. This ornament distinguishes from living and fossil Vetigastropoda with Trochus, and Turbo-like shells. Eucycloscalidae, containing the Triassic genera Eucycloscala and Ampezzotrochus, can be distinguished from Eunemopsis and Pseudoclanculus and the Eunemopsidae fam. nov. by folds on the inner lip and Sabrinellidae fam. nov. with Sabrinella and Microcheilus by the varix of the aperture of the last whorl. Lanascala with Lanascalidae belongs here. Neoeunema gen. nov. represents a Triassic member of Eucyclidae, which contains numerous Jurassic genera and species. Pseudoturcicidae fam. nov. differs by the ornament of the early teleoconch. The Chilodontidae form a characteristic Cretaceous group with teeth on the inner side of their outer lip. Three families of modern genera can be distinguished by the arrangement of teeth in the radula: four pairs in Pagodatrochidae fam. nov. and Turcicidae fam. nov., and three pairs in Calliotropidae, documented with Euchelus. The Seguenzioidea represents a related group having a distinctive radula with two lateral pairs of teeth and a variety of shell shapes and ornament. The early teleoconch of Seguenziidae (with slit in the aperture) and Ancistrobasidae fam. nov. (without such a slit) resembles that of the Eucycloscalidae, but has a strong spiral rib in addition to the axial ribs. The new species Microcheilus maxwelli sp. nov., Eucycloidea madagascariensis sp. nov., new genera Pseudoturcica and Heterodiscohelix, and new families Sabrinellidae fam. nov., Euemopsidae fam. nov., Pseudoturcidae fam. nov., Pagodatrochidae fam. nov. Turcicidae fam. nov., and Ancistrobasidae fam. nov. are introduced. The fossil record confirms existence of two independent branches of Vetigastropoda since more than 220 Ma, and within one of these branches parallel to the Eucycloidea the Seguenzioidea since Mesozoic times.