A palaeobiological revision of the species Ammonites flexisulcatus d’Orbigny, 1840 (Ammonoidea) from the upper Aptian of southern France

The Lower Cretaceous ammonite species Ammonites flexisulcatus d’Orbigny is revised on the basis of seventy-five topotype specimens, including the newly designated neotype, from the Cassis–Roquefort-la-Bédoule area, southern France. Because of its suture line and juvenile shell morphology, Ammonites flexisulcatus belongs to the family Desmoceratidae, and is herein assigned to the upper Aptian genus Caseyella Cantú-Chapa. Our palaeobiological study documents a morpho-dimensional dimorphism based on small-sized, robust, ribbed microconchs with a short ventral rostrum, and large, high-whorled macroconchs with a simple aperture. This species occurs in the upper part of the Epicheloniceras martini standard ammonite zone in both the South Provence and Vocontian Basin of southeastern France. Another morphotype of Caseyella, which occurs in the lower part of the Epicheloniceras martini Zone of southern France and Spain, differs by a flattened ventral region in the juveniles; pending examination, they could belong to a new species or consist of an ecological variant of Caseyella flexisulcata. These stratigraphic occurrences subscribe to previous observations suggesting that the Mediterranean Caseyella are younger than the species group of Caseyella reesidei dating the basal upper Aptian in Mexico.