Tubi calcificati giganti nell'Oligocene dei Colli Berici (Italia settentrionale): ambiente di deposizione e affinità tassonomica

Tube-shaped shells or skeletons are a common shape for marine calcifying organisms, belonging mostly to the phyla Annelida or Mollusca. Genus Kuphus, a special shipworm bivalve of the family Teredinidae, lives in neritic soft sediments instead of wood. We found a large amount of large (up to 35 mm in diameter) tube-shaped shells in a shallow-marine carbonate deposit of lower Oligocene age in the Berici Hills, our study area. Carbonate petrography showed that the host sediment is a skeletal grainstone, whose skeletal composition is dominated by articulated red algae and benthic foraminifera (foralgal), implying deposition in the photic zone of a typical Cenozoic Mediterranean carbonate platform. This skeletal association suggests a seagrass meadow depositional environment. Petrographic and in-situ chemical analyses showed that calcareous tubes were originally made of calcite, with a double-layered structure. An excess of sulfur (S) in the tube calcite may be related to sulfur bacteria in the sedimentary environment or within the tubes. The stable isotopic composition of carbon in calcium carbonate of the tube walls is lighter, on average, than that of bulk sediment. In conclusion, Kuphus sp. may have cooperated with sulfur bacteria hosted in its trophosome, which were using the hydrogen sulfide (H2S) produced by the decomposition of seagrass residues in the shallow seagrass meadow environment from the Oligocene.