A new caenogastropod from the upper Rhaetian of Lombardy: Palaeobiogeographical history and implications for the Early Jurassic gastropod recovery

摘要

A new gastropod genus and its type species, namely Ederazyga fanchini gen. et sp. nov., are described from the upper Rhaetian deposits of Lombardy (northern Italy) and tentatively placed into the family Zygopleuridae. The first appearance of Ederazyga is recorded in the lower Carnian deposits of Southern Alps and the stratigraphical distribution of the genus ends almost at the Triassic/Jurassic boundary. Its occurrence in Norian beds of the Nayband Basin (central Iran) suggests an eastward extension of the distribution during that time. Ederazyga is probably one of the Alpine gastropod taxa appearing in this area after the formation of the basin. The genus is possibly related to a group of Early Jurassic, medium to large Zygopleura-like species that are well represented in the Sinemurian and Pliensbachian carbonate platform deposits of the Mediterranean region and in the Hettangian to Pliensbachian successions of the European epicontinental shelf. This group shows an apparent species radiation in these areas testifying to the gastropod recovery following the Late Triassic decline in biodiversity. Ederazyga fanchini is shown to be congeneric with Cerithium? lateplicatum Klipstein, 1843, which is the type species of Camponaxis Bandel, 1995. The definition and diagnosis of Camponaxis was based on specimens that are clearly different, at generic and higher taxonomic levels, from the holotype of C.? lateplicatum. They belong to a distinct species here named Camponaxis bandeli sp. nov. Several species have been subsequently ascribed to Camponaxis following its original diagnosis. Therefore, we invoke ICZN Art. 70.3.2 in order to preserve the unambiguous identity of the genus and to ensure its nomenclatural stability. Camponaxis bandeli is fixed as the new type species for Camponaxis and C.? lateplicatum is here assigned to Ederazyga.