Abstract
A primary study of bioerosional structures and skeletobionts associated with an almost monotypic assemblage of free-lying bryozoan colonies of Celleporaria? sp., from the middle Miocene succession of the Siwa Oasis, Egypt, revealed four groups of encrusters (serpulids, sheet-like cheilostome bryozoans, balanoid barnacles and oysters) and eight different ichnotaxa. Undersides of the colonies have basal exterior walls, which are relatively richer in encrusters than are the convex sides. The dominant bioerosional structures were due to the boring activities of non-predatory organisms. The diversity of macro-bioerosional ichnotaxa is moderate and includes Trypanites, Gastrochaenolites, Maeandropolydora, Caulostrepsis, Renichnus, Spirolites and Centrichnus. In addition, some conical borings perpendicularly penetrating the shells could belong to the predatory tracemakers of Oichnus. Despite a moderate total ichnodiversity, the observed sample diversity of ichnofossils is low (about 35% of the total specimens studied). This is explained by a relatively high sedimentation rate with only short periods of exposure on the seafloor. The dome-shaped bryozoan occurrence is interpreted to represent a parautochthonous assemblage, which may have been influenced by shallow-water processes in response to storm activity.