Sabellid tubes from the deep shelf off Liguria (Italy): A modern analogue for distal Skolithos Ichnofacies?

Dense associations of vertical trace fossils are typically used as a proxy for nearshore palaeoenvironments. Here we challenge this assumption by reporting dense associations of vertical tubes occupying vast areas of the deep shelf off Liguria (Italy) at depths ranging from 56 to 85 m. The tubes, produced by the suspension-feeding sabellid Bispira riccardi, are J-shaped and extend for ∼5–10 cm above the seafloor. Optical microscopy, SEM imagery and elemental maps evidence that the tubes consist of (1) an outer agglutinated layer consisting of clay- and silt-sized particles, (2) an intermediate mineral layer enriched in manganese, and (3) an inner, sulfur-rich organic layer. The tubes serve to obtain shelter and resuspend food particles, representing a modern reference for understanding palaeoethology of morphologically similar trace fossils. The Ligurian ichnoassociation is comparable to the Skolithos Ichnofacies, but it departs from the typical nearshore environment of the Ichnofacies. The characteristics of the studied ichnoassociation are explained by the significant turbidity of the Ligurian shelf, which favours domichnial, suspension-feeding strategies that are typical of the Skolithos Ichnofacies. As such, the sabellid tubes from the Ligurian Sea are palaeontologically important to identify similar current-swept, high-turbidity settings in the fossil record.