Encrustation and bioerosion on Middle Miocene bivalve shells and echinoid skeletons: paleoenvironmental implication

Abundant host bivalve shells (oysters and pectinids) and echinoid skeletons were collected from the Middle Miocene Marmarica Formation exposed at northern Western Desert of Egypt. The specimens are unique in that they comprise encruster and bioeroder trace fossils. Pectinids are among the most heavily encrusted specimens, while oysters represent the highly bioeroded ones. Although pectinid and ostreid shells contain both epi- and endobionts, echinoderm skeletons show considerable signs of encrustation and bioerosion traces are missing. The preserved cemented component of an encrusting community consists of balanoid barnacles, oysters, serpulid worms and cheilostome bryozoans. Epibionts preferentially colonize valve exterior. Balanoid barnacles represent the most common encrusters, and are probably the most abundant encruster in the terms of area occupied and skeletal biomass. Bioerosion traces in shells are predominately those of clionaid sponges (Entobia), bivalves (Gastrochaenolites) and polychaete worms (Maeandropolydora). Although Gastrochaenolites is exclusively occurred in the exterior surface of the oyster specimens, Entobia are mostly common on the exterior surfaces of the host (oysters and pectinids) and on the interior of a number of oysters as well. Processes of encrustation and bioerosion are positively correlated with primary productivity and negatively with the sedimentation rate and energy.