ISSN 1661-5468 1 Geology Department, Faculty of Science, Alexandria University, Moharram Bay 21511, Alexandria, Egypt. E-mail : m_elhedeny@yahoo.com Revue de Paléobiologie, Genève (décembre 2005) 24 (2) : 719-733 Taphonomy and Paleoecology of the Middle Miocene oysters from Wadi Sudr, Gulf of Suez, Egypt Magdy M. EL-HEDENY 1 Abstract The Upper Rudeis Formation (Middle Miocene, Langhian) of Wadi Sudr, Gulf of Suez, Egypt contains thick-bedded deposits of oysters. The oyster shells are large and massive. The paleoecological study of these oyster beds has revealed a nearshore shallow marine origin of the Upper Rudeis Formation with storm influence during the accumulation of their oyster shells. Taphonomic observations indicate that these shells are highly altered by disarticulation, bioerosion and encrustation. Abrasion, breakage and high fragmentation of shells are generally lacking. In addition, the studied oyster shells provide some traces of predation. Thicker shells and faster growth rates in oysters may reflect the greater exposure to fully marine predation and bioerosion, as well as to the conditions associated with higher salinity. Borings occur on the external and internal surfaces of their shells during the pre and/or post-mortem. They can be regarded as relatively diverse, being represented by the following ichnotaxa ; Oichnus (O.) paraboloides BROMLEY, 1981, O. simplex BROMLEY, 1981, Entobia isp., Rogerella isp. and Radulichnus isp. Key words Taphonomy, Paleoecology, Miocene, oysters, Sudr, Sinai, Egypt. INTRODUCTION The early Middle Miocene is a time of an extensive marine transgression following a major drop of the sea level at the Burdigalian/Langhian transition (HAQ et al., 1988 ; HARDENBOL et al., 1998). In Egypt, the Miocene rocks are widely distributed covering about 1/8 of the area of the exposed sedimentary rocks. They are exposed mainly in the following areas : north Western Desert, Cairo-Sukhna area, Gulf of Suez and Red Sea coastal plain. The Miocene rocks of the Gulf of Suez have been extensively studied, especially because all known Egyptian oil fields lie in the region of the Gulf of Suez and seem to be related by source or by tectonics to the Miocene period (SAID, 1962). These rocks contain a rich macroinvertebrate fauna dominated by oysters, bivalves, gastropods and echinoids. The most dominant element of the benthic community in the Miocene of Egypt are oysters, which are particularly well represented in Middle Miocene of Sudr area. Until now, oysters from this area were only mentioned in passing by a few authors (e.g. MOON & SADEK, 1923, 1925 ; SADEK, 1959 and SAID, 1962). The morphological features of the studied Miocene oyster shells are strongly influenced by ecological factors. In addition, they display interesting traces of boring predation and many signs of post-mortem processes (e.g. encrustation, bioerosion, disarticulation) and other taphonomical features that give important indications for the post-mortal processes acting on their shells. The purpose of the present article is to discuss taphonomically and paleoecologically a number of oyster shell beds accumulated in Sudr area western Sinai, Egypt during the Middle Miocene time. MATERIAL This study is based on an oyster assemblage collected from the Middle Miocene (Upper Rudeis Formation) succession exposed at Sudr area, Gulf of Suez, Egypt (Fig. 1A&B). Oyster valves occur in great quantity in the layers of greenish marl, sandy shale and algal and coralline limestone. Their mode of fossilization ranges from exceptionally well preserved tests including microscopic surface characters to slightly abraded and fragmented test material. The occurrence of very well preserved specimens of the Middle Miocene oysters of Wadi Sudr allows a detailed paleoecological and taphonomic study of this group. They dominate a fauna, which also includes corals, pectinid bivalves, gastropods as well as other echinoid remains (spatangoids, spines of regular sea urchins). The recorded oyster species include Ostrea plicatula GMELIN, 1791 ; Ostrea vesitata FUCHS, 1883 ; Ostrea (Cubitostrea) dubertreti ROMAN, 1940 ; Cubitostrea frondosa (DE SERRES, 1839) ; Cubitostrea digitalina (EICHWALD, 1830) ; Hyotissa virleti (DESHAYES, 1900) and Crassostrea gryphoides (SCHLOTHEIM, 1813). Type material is housed