Biostratigraphy, taphonomy and palaeoecology of two tropical Coniacian-Santonian oyster species from Wadi Sudr, western Sinai, Egypt

The Upper Cretaceous carbonates of Egypt provide numerous opportunities to fill in palaeobiogeographical and palaeoecological gaps in tropical hard substrate communities. One such unit is the Coniacian-Santonian Matulla Formation exposed at Wadi Sudr, western Sinai. It is a fossiliferous and argillaceous limestone, marl, shale and sandstone about 110 m thick. The oysters Pycnodonte (Costeina) costei (Coquand, 1869) and Oscillopha dichotoma (Bayle, 1849) are abundant in the middle (Coniacian) and upper (Santonian) members of this formation, respectively. Biostratigraphically, they represent the most characteristic index oyster species for the Coniacian-Santonian formations of Egypt. Their palaeobiogeographic distribution confirms the strong affinity to the Mediterranean Province of the Tethyan Realm. Taphonomic observations indicate that abrasion and highly fragmented shells are generally lacking. However, the thick shells of P. (C.) costei and O. dichotoma at Wadi Sudr are highly altered by disarticulation, encrustation and bioerosion. The common encrusters include ostreids, cheilostome bryozoans, serpulid worms, foraminifera and plicatulid bivalves. Bioerosion structures include Tiers D, E, F, G and H. They are represented by traces of acrothoracican cirripeds (Rogerella), clionaid sponge (Entobia), bivalves (Gastrochaenolites) and polychaetes and/or sipunculans (Maeandropolydora, Trypanites and Caulostrepsis). The distribution and frequency of the observed taphonomic features are varied greatly between the studied shells of both species, as well as within the shells of the same species. The oysters and their skeletozoans, the associated faunal content and sedimentological framework show a high-energy, intertidal to shallow subtidal marine environment with normal salinity, low rate of sedimentation and high productivity level.