Abstract

Macrofauna abundance has been quantified across the Breistroffer interval (OAE 1d, Latest Albian) in the Vocontian basin (southeast France). This interval is characterized by a high abundance of ammonoids showing significant morphologic disparity. Twenty ammonoid genera are recognized and assigned to seven morphostructural groups. The aims of this study are to: (1) interpret abundance variations of ammonoid taxa in terms of taphonomic processes, sedimentary dilution, and paleoenvironmental factors; and (2) discuss the habitat and mode of life of some Late Albian ammonoids with respect to their morphology and abundance variations.

Most variations in ammonoid absolute abundance are not the result of post-mortem shell transport. High abundance within the Breistroffer interval is due partly to a low accumulation rate, which is related to a maximum-flooding stage. The different trends observed among ammonoid taxa can be interpreted partly in terms of response to changes in trophic conditions inferred from the study of calcareous nannofossils. Heteromorphs, which are dominant during mesotrophic conditions, could have been more competitive than involute/evolute planispirals (normal coiling) when paleoenvironmental conditions become more unstable. Lechites (orthocone), the most abundant taxon, probably inhabited epipelagic, distal paleoenvironments. This genus, interpreted as a vertical migrant in the water column, would have been able to move up when trophic conditions were good in surface waters, and down in order to avoid oligotrophic surface waters and to exploit food-rich layers in deeper waters. Turrilitoides and Mariella (torticones) may have been quasiplanktic, and inhabited mainly neritic paleoenvironments. These forms could have occupied more distal paleoenvironments when mesotrophic conditions prevailed in the surface water column. Anisoceras and Hamites (quasiplanktic U-shaped heteromorphs) mainly lived in distal, epipelagic habitats, and could have been more competitive compared with the torticones when oligotrophic conditions prevailed in surface waters. Among involute/ evolute planispiral ammonoids, Mortoniceras probably had a deep-nektonic mode of life and inhabited the lower part of the epipelagic zone. A stylized panorama of some Late Albian ammonoid habitats is proposed.

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