The occurrence of the genus Tanousia Servain (Mollusca, Gastropoda, Hydrobiidae) in the Middle Pleistocene Pià nico-Sèllere Basin (Bergamo, Northern Italy)

The occurrence of the freshwater genus Tanousia Servain 1881 (Hydrobiidae) in the Middle Pleistocene lacustrine succession of the Piànico-Sèllere Basin (Bergamo, Northern Italy) represents a significant biostratigraphical discovery. The Tanousia were recovered from the basal part of the carbonate varved bed (BVC), in siltysandy slump sediments, and from the lower and middle parts of the La Palazzina Member (MLP) of the Piànico formation. The BVC unit, in which a dated tephra layer occurs, accumulated during an interglacial phase, as shown by pollen analyses. The MLP Tanousia-bearing layers have been laid down during cool-temperate climate oscillation. The Piànico population of Tanousia displays great morphological variability but is similar to T. runtoniana (Sandberger, 1880), T. stenostoma (Nordmann, 1901) and T. cf. T. stenostoma (Nordmann, 1901), from the Bavelian and Cromerian interglacial deposits of late Early and early Middle Pleistocene age in Northern Europe. As for these species, the Tanousia population from Piànico appears to indicate fluvial conditions under a fully temperate climate.