Life in near-anoxic conditions: A case study of the ichnology and infaunal ecology of Silurian graptolitic black shales from Sardinia, Italy

A trace fossil assemblage from the Silurian (Llandovery) black shales of the Genna Muxerru Formation is reported for the first time. The ichnofossil record is abundant, well-preserved and comprises Alcyonidiopsis, Cochlichnus, Phymatoderma, Planolites (large morphotype), Planolites (rough morphotype) and a ‘Small chondritid’. Sedimentological and ichnological evidence indicates that the bioturbated black shales were deposited in outer shelf (or deeper) settings with severe depletion in oxygen. The seafloor provided a high food supply for the infauna. The palaeoenvironment of the Genna Muxerru Formation is an ancient analogue of modern oxygen-minimum zones. Burrow fills are apparently massive in macroscopic view, whereas observations under the petrographic and stereoscopic microscope show that a lifestyle dominated, i.e. that of pellet-filling sediment-feeders. As such, the Genna Muxerru Formation provides not only a new ethologic reference for low-oxygen settings but also a methodological benchmark for ichnological analysis of black shales. Bioturbation in black shales may be commoner than expected but its recognition requires microscopic observation.