Parasitic gastropod bioerosion trace fossil on Cenomanian oysters from Le Mans, France and its ichnologic and taphonomic context

We describe and name Loxolenichnus stellatocinctus Breton and Wisshak igen. et isp. nov., a bioerosion trace fossil on an Upper Cenomanian oyster from Le Mans (France). This trace is attributed here to a parasitic gastropod. The characteristics of this ichnospecies are a combination of one or several, vertical or oblique, complete penetrations, and an asymmetrical attachment etching (fixichnion) with a diagnostic set of stellate grooves increasingly distinct towards the margin of the trace. By including two former Oichnus ichnospecies, Loxolenichnus halo comb. nov. and Loxolenichnus taddei comb. nov., Oichnus, is now constrained to pure predation traces (praedichnia). The numerous oysters collected from the Marnes à Pycnodonte biauriculata Formation show associated epibionts and encrusters as well as borers and scrapers. Encrusters comprise 24 taxa while bioerosion trace fossils comprise 17 ichnotaxa ranging from very rare (< 0.1%) to quite abundant (81%). The taphonomic history leading to an ex-situ condensation of these oysters is complex. Both the Gnathichnus and Entobia ichnofacies are represented on the shellgrounds, presumably alternatingly.