Pliensbachian gastropods from Venetian Southern Alps (Italy) and their palaeobiogeographical significance

Abstract: The Pliensbachian gastropods described by De Toni in 1912, coming from an isolated boulder at the foot of Mt Vedana (eastern margin of Trento Platform, Venetian Southern Alps, Italy) are revised. The fauna consists of 13 species representing nine families and eight superfamilies. Despite the low number of species, the assemblage represents the most diverse Early Jurassic gastropod fauna known for the Venetian Southern Alps. The boulder yielding the material was thought to derive from the upper part of the Early Jurassic Calcari Grigi Group, a carbonate platform unit extensively cropping out in the Mt Vedana area. The sedimentological analysis indicates a prevalently bioclastic wackstone‐floatstone, reflecting a lime‐muddy deposit undergone to an early consolidation. This and the high content of ammonoids, which is unusual for the Calcari Grigi Group, are typical aspects of a condensed pelagic sediment, presumably a fissure filling at the top of the carbonate platform succession. Palaeobiogeographical comparisons show that the fauna is composed of species occurring exclusively in pelagic limestones of the western Tethys. By contrast, it shows no relationships with the coeval faunas of the adjacent Trento platform and of the other western Tethyan carbonate platforms. These lines of evidence and the facies analysis would testify to the Pliensbachian drowning of the eastern margin of the Trento platform. In the wider context of the palaeobiogeographical history of Early Jurassic western Tethyan gastropods, the species from Vedana belong to a faunal stock which is typical for pelagic, mainly postdrowning sediments. Thus, appearance and diffusion in space and time of this stock were probably regulated by the direction, rate and pattern of the Neotethyan rifting. A new subgenus, Proarcirsa (Schafbergia) subgen. nov., and three new species, namely Ataphrus (Ataphrus) cordevolensis sp. nov., Guidonia pseudorotula sp. nov. and Proarcirsa (Schafbergia) zirettoensis sp. nov. are erected.