The ammonites of the Les Ferres Aptian Basin (Lower Cretaceous, Southeast of France): the genus Toxoceratoides (Ancyloceratina, Helicancylidae)

Abstract : Toxoceratoides (Ancyloceratina, Helicancylidae) is a lower Aptian (Lower Cretaceous) small heteromorph ammonoid mainly known by small samples of fragmentary specimens. In the present work, we study a sample of 145 specimens of this genus from the Deshayesites forbesi, Deshayesites deshayesi and Dufrenoyia furcata zones (lower Aptian) of the vicinity of the village of Les Ferres (southeastern France). The taxa recognized (in stratigraphic order) are: Toxoceratoides sp., Toxoceratoides royerianus (d’Orbigny, 1842) and Toxoceratoides rochi Casey, 1961a, Casey, 1961b. Their ontogenetic sequence is described; the intraspecific variability of Toxoceratoides rochi is considerable and it concerns adult size, coiling, duration of the ontogenetic stages and ornamentation. No sexual dimorphism is recognized. The main evolutionary pattern of the genus consists in the replacement of primitive ribs (here named ‘royerianus ribs’) by derived ribs (‘rochi ribs’) on the flexus and retroversum. The genus Toxoceratoides could be used as a biostratigraphic tool.