Two ammonite species under the same name: Revision of Deshayesites deshayesi (d’Orbigny, 1841) based on topotype material (Lower Aptian, Lower Cretaceous, Northeast of France)

Abstract

After a review of the current state of knowledge about the classical Lower Aptian (Lower Cretaceous) ammonite index-species Deshayesites deshayesi (d’Orbigny, 1841) and the problems concerning the acceptation of this species, a topotype sample of 159 Deshayesites from the Argiles à Plicatules Formation (Paris Basin, northeastern France), including the lectotype of the latter species, is studied statistically. According to the Deshayesitidae evolutionary pattern highlighted by Bersac and Bert (2012), the present study shows that the whole studied population from the Argiles à Plicatules Formation belongs actually to a single species with a large intraspecific variability under the Dipolar Variation. Nothing supports the hypothesis of polyzonality of the level that provides the Deshayesites within the Argiles à Plicatules. The characteristics of the Deshayesites studied relate the latter to Deshayesites annelidus Casey, 1963 rather than to what is usually considered to be D. deshayesi. Consequently, D. deshayesi is revised, D. annelidus is treated as a junior synonym of D. deshayesi, and the species previously usually considered as D. deshayesi has to be renamed in D. multicostatus Swinnerton, 1935. Accordingly, it is proposed to move the limit of the D. deshayesi Zone downward.