BIOEROSION ON SCALLOP AND OYSTER SHELL BEDS: AN EXAMPLE OF THE GNATHICHNUS ICHNOFACIES FROM THE PLIOCENE OF THE ROUSSILLON (SE FRANCE)

Bioerosion trace fossil assemblages in mineral substrates where assigned to the Trypanites Ichnofacies in the classical Seilacherian Ichnofacies model. Later on, the model was completed with the Entobia Ichnofacies, constituted mostly by dwelling borings produced in longly-exposed substrates (e.g. rocky shores); and the Gnathichnus Ichnofacies, characterized by epigenic structures produced on briefly-exposed substrates (e.g. shell beds). Both are known from Jurassic-to-Holocene examples, but while examples of the first are numerous, only a handful of descriptions of Gnathichnus Ichnofacies can be found in ichnological literature.

The Zanclean (Lower Pliocene) shell beds from the surroundings of Nefiach (Roussillon) offer a good opportunity to analyze this Ichnofacies. These beds contain abundant Pectinidae (Pecten benedictus and Aequipecten scabrella) and Ostreidae (Ostrea cuccullata and other species). Valves are generally disarticulated with some fragmentation. No size sorting is recognized and some shells occur in a concave-up attitude. These taphonomic attributes suggest very limited transport in a moderately-energetic setting.

Oyster and scallop shells bear abundant and diverse bioerosion trace fossils. Most frequent ichnotaxa are Gnathichnus pentax and Radulichnus isp., produced by the grazing activity of, respectively, regular echinoids and gastropods or polyplacophorans. Other common epigenic structures are fixation scars of small foraminifera (Cibicididae). The ichnoassemblage is completed by the presence of shallow-tier sponge and annelid borings (Entobia, Maeandropolydora and Caulostrepsis) and rare gastropod drill holes (Oichnus simplex) and durophagous scars. The assemblage is, thus, dominated by Pascichnia and Fixichnia and complies with the features that characterize the Gnathichnus Ichnofacies. This bioerosion assemblage contrasts with others found in neighbouring and contemporaneous rocky shores clearly identifiable as Entobia Ichnofacies.