New eocene oysters and the final regression at the southern Rim of the central High Atlas (Morocco)

The oysters Ostrea (Ostrea) todraensis n. sp. and Ostrea (Ostrea) sidialiensis n. sp. are described from the uppermost marine beds south of the central High Atlas in two sections west of Tinerhir. The oysters and the codiacean alga Ovulites margaritula (Lamarck, 1801) Lamarck, 1816 indicate a final regression at the end of Middle Eocene times, i.e. earliest at the end of the Lutetian and latest at the end of the Bartonian. The oysters lived in muddy lagoonal environments sealed off from the sea by oolitic-bioclastic carbonate sand shoals and bordered in landward direction by sebkha-like redbed deposits. Taxonomy and systematic affinities of the oysters are discussed. Their shell structures and depositional history are described in respect to sedimentary infillings. Some selected aspects of biometry explain differences in shell morphometry both between different valves of the same species and equivalent valves from the different populations, illustrating the importance of size and location of the adductor muscle scar.