A serpulid-Anodontia-dominated methane-seep deposit from the upper Miocene of northern Italy

A limestone deposit with an unusual fauna is reported from the late Miocene of northern Italy (Ca' Fornace site). The petrography of the carbonate and its distinct carbon isotope signature (with delta C-13 values as low as -57.69 parts per thousand) clearly identify this limestone as an ancient methane-seep deposit. The dominant faunal elements are serpulid tubes belonging to Protis, and extremely inflated, medium-sized shells of the lucinid bivalve Anodontia mioinflata sp. nov. Also common is the small bathymodiolin Idas aff. tauroparva, plus some large specimens of the lucinid Lucinoma, and poorly preserved, medium-sized specimens of a possible vesicomyid bivalve, an arcid bivalve, small gastropods of the genera Laeviphitus (Elachisinidae), Anatoma (Scissurellidae), as well as desmophyliid and caryophyllid scleractinian corals. This faunal assemblage is quite distinct from the typical Miocene seep faunas in northern Italy, which are dominated by large bivalves of the Lucinidae (Meganodontia), Vesicomyidae (Archivesica), and Bathymodiolinae, possibly due to a shallower depositional depth of the Ca' Fornace site.