Bioerosion in shell beds from the Pliocene Roussillon Basin, France: Implications for the (macro)bioerosion ichnofacies model

摘要

The marine Pliocene at the locality of Nefiach (Roussillon Basin, SE France) includes several shell beds constituted by oysters and scallops that bear a diverse and abundant bioerosion trace fossil assemblage. The most abundant trace fossils are Gnathichnus pentax and Radulichnus inopinatus, produced by the grazing activity of echinoids and polyplacophorans upon algae and other microorganisms coating shell surfaces. Other bioerosion traces include polychaete dwellings (Caulostrepsis taeniola and Maeandropolydora sulcans), sponge boring systems (Entobia isp.), and rare bryozoan borings (Pinaceocladichnus isp.), predation structures (Oichnus simplex and repaired durophagous scars), and foraminiferal fixation pits (Centrichnus cf. eccentricus). The trace fossil assemblage records short-term bioerosion in shellgrounds in a moderate energy setting as evinced by the dominance of epigenic or shallow endogenic structures produced in most cases by "instantaneous" behaviors. The assemblage can be assigned to the Gnathichnus ichnofacies, and it contrasts with that found in Pliocene rocky shores in the same geographic area, which are examples of the Entobia ichnofacies. The Gnathichnus ichnofacies is validated as an archetypal one and its recurrency demonstrated since the Jurassic. Entobia and Gnathichnus ichnofacies have to be used in the Mesozoic and Cenozoic as substitutes of the previously existing Trypanites ichnofacies, which is still valid in the Palaeozoic.