Borers and epizoans on oyster shells from the upper Tortonian, Lower Chelif Basin, NW Algeria

The three oyster lenses of the upper Tortonian of Djebel Touaka site which are described herein contain three species, Crassostrea gryphoides (Schlotheim), Ostrea lamellosa (Brocchi) and Hyotissa squarrosa (De Serre). The density of shell packing between the lenses is dissimilar. Most of the specimens are disarticulated and poorly fragmented; they exhibit a random distribution and orientation, without any distinct sorting. Bioerosion and encrustation are featured on both surfaces of left and right valves. The identified ichnogenera are Entobia (Bronn), Gastrochaenolites (Leymarie, 1842), Caulostrepsis (Clarke, 1908), Trypanites (Mägdefrau, 1932) and Maeandropolydora (Voigt, 1965). Encrusters are scared, represented by juvenile oysters/other bivalves, bryozoans and barnacles. The coexistence of borings on both sides of valves means that they probably occur not only while alive, but they keep happening after death. The oyster beds were deposited in a foreshore to shoreface environment, from the combined action of wave currents and sedimentation rate.