Palaeoenvironmental significance of sponge spicules in pre-Messinian crisis sediments, Northern Italy

The Messinian Salinity Crisis (MSC) created a substantial negative impact on the Mediterranean marine fauna and flora because hypersaline conditions were established in many marginal basins. At the northern margin of the Tertiary Piedmont Basin, Northern Italy, the Messinian successions exposed at Banengo and Moncalvo recorded the transition into and through the MSC with well-preserved nanno-, micro- and macrofossils. Siliceous sponge spicules present in these pre-MSC sediments reflect the sponge population in this area with twenty different morphological types of spicule identified, twelve of them attributable to sponge genera of the modern Mediterranean sponge fauna. The most common genera were Stelletta, Geodia and Dercitus, with fewer records of Tethya, Chondrilla, Alectona and Samus. Only two types of spicule belong to species not currently present in the Mediterranean sponge fauna and are putatively attributed to the tropical species Stelletta purpurea and Diplastrella megastellata. The sponge assemblage thriving before the onset of the MSC provided new information on the sponge diversity and palaeoenvironment of this northern sector of the palaeo-Mediterranean basin over this crucial period. These data, integrated with information derived from the associated fossils (nannofossils, benthic and planktic foraminifers, and bryozoans), indicate that the considered sites were coastal localities, with sediments rich in an organic matter likely derived from phytoplankton blooms. At both sites, the presence of patchy rocky substrates on the seafloor is testified by a diversified group of sponges typical of hard substrata. These rocks were, very likely, carbonate as suggested by the presence of spicules of typical boring sponges. In the studied sites, common epiphytic foraminifers and bryozoan segments indicate the presence of Posidonia seagrass or algal meadows.