Chemosymbiotic bivalves from the late Pliocene Stirone River hydrocarbon seep complex in northern Italy

Seven species of chemosymbiotic bivalves are described from the late Pliocene Stirone River hydrocarbon seep complex in northern Italy, including one new species and two in open nomenclature. The known species are the solemyid Acharax doderleini, the lucinids Lucinoma persolida and Megaxinus ellipticus, and the vesicomyid borropodon aff. perplexum; in open nomenclature we report two lucinids, including the largest species of Lucinoma known from the Italian Pliocene to date, and a strongly inflated, large Anodontia sp. The most abundant species at the Stirone seep complex is the lucinid Megaxinus stironensis sp. nov. This Pliocene seep fauna differs from that of the well-known Miocene "Calcari a Lucina" seep deposits by lacking large bathymodiolin mussels and vesicomyid clams; instead, the dominance of the lucinid Megaxinus stironensis gives this fauna a unique character. We speculate that at the Stirone seep complex, Megaxinus had occupied the ecological niche that Meganodontia occupied at the Miocene "Calcari a Lucina" seep sites in the Mediterranean basin, and that the dominance of Megaxinus could be a wide-spread feature of Pliocene chemosynthesis-based ecosystems in Mediterranean Pliocene.