Bioerosion and encrustation signatures on Crassostrea gryphoides (von Schlotheim) from the Miocene Raghama Formation, northwest Saudi Arabia: description and paleoenvironmental implications

The present study describes, for the first time, the composition and distribution of bioerosion and encrustation assemblages on Crassostrea gryphoides (von Schlotheim) of the Middle Miocene Raghama Formation in northwest Saudi Arabia. The bioerosion assemblages, both epi- and into-bioeroders and encrusters, and their preferential distribution on left/right valves (n = 123), as well as their external/internal surfaces and areas were evaluated. Field and laboratory investigations allowed the recognition of six ichnospecies, belonging to five ichnogenera: Entobia ovula (Bromley and d'Alessandro), Gastrochaenolites lapidicus (Kelly and Bromley), G. ornatus (Kelly and Bromley), Caulostrepsis taeniola (Clarke), Maeandropolydora decipiens (Voigt), and Oichnus simplex (Bromley) which were attributed to sponge, bivalve, annelid and gastropod activities. The presence of Gastrochaenolites borings on the external surface, and the orientation of many of the Caulostrepsis borings towards the lateral edges of the left valves, suggests a dominant bioerosion activity during the life span of the C. gryphoides. Encrustations made by fouling organisms (oysters, balanomorphs, bryozoans, and serpulids) were, in comparison, scarce.