The heteromorph ammonite genus Ammonitoceras Dumas, 1876 (Ancyloceratidae) in the lower Aptian (Lower Cretaceous) of the Les Ferres Aptian Basin (southeastern France)

Abstract

Ammonitoceras Dumas, 1876 is a genus of heteromorph ammonites characterized by the presence of a peculiar ontogenetic stage in its inner whorls: the Ammonitoceras stage. But in spite of its wide paleogeographic and biostratigraphic extension throughout the Aptian (Lower Cretaceous), this genus remains poorly known. In the present work we study specimens of Ammonitoceras from the lower Aptian Deshayesites multicostatus (Deshayesites deshayesi Zone) to Dufrenoyia furcata (Dufrenoyia furcata Zone) subzones of the Les Ferres Aptian Basin (southeastern France). The results are as follows: (1) representatives of Ammonitoceras from this area are regarded dimorphic with criocone macroconchs and ancylocone microconchs, (2) their ontogenetic sequence is described, (3) their intraspecific variability is significant and concerns the adult size and the duration of the ontogenetic stages, especially the Ammonitoceras stage, (4) two species are recognized: the earlier Ammonitoceras ucetiae Dumas, 1876, characterized by a brief Ammonitoceras stage on average, and the latter Ammonitoceras lahuseni (Sinzow, 1906), characterized by a longer Ammonitoceras stage on average. The sample of the Deshayesites grandis Subzone (Deshayesites deshayesi Zone) is composed of specimens too fragmentary to be identified at species level.