Systematics and taphonomy of Pliocene Gastropoda (Mollusca) from the Dahra Mountains, NW Algeria

Abstract

Palaeontological investigation of the Pliocene Tahria and Slama formations in the Lower Chelif Basin (NW Algeria) led us to collect significant gastropod assemblages for taxonomic and taphonomic purposes. There is presently a very complete inventory list of Pliocene gastropods from Algeria, with 16 species from the Zanclean Tahria Formation and 54 species from the Piacenzian Slama Formation, some of which are recorded for the first time from the Pliocene of the Mediterranean. The northwestern Algeria gastropods represent a shallow-water, fully marine fauna, sandy substrate, and show a typical Upper Pliocene Mediterranean assemblage. The gastropod shells show significant taphonomic alteration; most specimens are abraded and decalcified to a variable degree. Fragmentation was mainly a result of mechanical and biological processes. Bioerosion traces are uncommon, but when they are present, they are predominantly those of predatory gastropods (Oichnus simplex and Oichnus paraboloides), polychaete activity (Maeandropolydora isp.), and acrothoracican barnacle borings (Rogerella isp.). Integration of taphonomical, palaeoecological, and sedimentological characteristics of the gastropod-bearing beds suggests sedimentological concentrations originated from discontinuous processes of winnowing and bypassing sediments, most likely due to storm action in the shoreface depositional environment.