Palaeoecology of the Pinna nobilis biofacies along the stirone river (early pleistocene, northern Italy)

The Stirone riverbanks (Northern Italy) host a famous Pliocene-Pleistocene marine succession, interpreted as a mosaic of shallow marine palaeoenvironments. One of the most remarkable occurrences is fossil Pinnidae in life position, here identified as Pinna nobilis Linnaeus, 1758. It is an endemic Mediterranean bivalve, commonly reported from within Posidonia or Cymodocea meadows, or more rarely from other unvegetated shallow sedimentary bottoms, and even under hypoxic conditions. Few reports are found in the fossil record because of the low preservation potential of this large, semi-infaunal, fragile nacreous bivalve. Although already mentioned in the literature, no detailed palaeoecological investigation has been conducted on the mollusc association of the Pinnidae biofacies outcropping at the Stirone River. Four replicas of bulk sediment (2 l each) have been collected from the Pinnidae biofacies, in order to conduct a palaeontological and palaeoecological analysis, aimed at defining the composition and structure of the mollusc fossil assemblage, for the reconstruction of the associated palaeoenvironmental setting. The associated sediment, rich in tiny plant frustules, is a poorly sorted fine to very fine sand, with a bimodal distribution of sand and mud together with a coarse bioclastic fraction. The mollusc fossil assemblage is formed by a mixing of species which are related to both infralittoral vegetated and unvegetated bottoms, together with species with an affinity for both mud and coarse detritic sediments. One of the most common and abundant species is Corbula gibba, a bivalve considered of high ecological importance due to its opportunistic nature and reported as tolerant to elevated organic mud input, bottom hypoxia and unstable sedimentation rate. The fossil assemblage and the associated sediment point to a likely colonisation by Cymodocea, although the occurrence of mosaic facies of unvegetated and vegetated bottoms, with both Cymodocea and sparse Posidonia, cannot be ruled out. The bottom was strictly infralittoral (shallower than 15 m of water depth), with pulses of mud by fluvial transport, supporting the mixing of infralittoral and mud-loving deeper species, and the ubiquitous occurrence of C. gibba.