Evolutionary patterns and process of the Gassendiceratinae (Ammonoidea, Hemihoplitidae, Upper Barremian)

International audienceThe family Hemihoplitidae (Ancyloceratoidea, Ammonitina) is a key group for the North Tethyan Margin in the Upper Barremian due to its rapid diversification. The Gassendiceratinae contains three genera,Gassendiceras,Pseudoshasticrioceras and Imerites, which have close phyletic links; they range from the Vandenheckei Zone to the Giraudi Subzone. These genera are crioconic to ancyloceratic ammonites; their size varies from large (Gassendiceras) to small ( Imerites, which bears an early turricone), and the ornamentation is characterized by the succession of several specific ontogenic stages. The palaeontological approach leads to preferentially retain the characters that have proven to be an important source of phylogenetic informations. The anagenetic evolution ofGassendiceras is modulated by the interaction of heterochronies of the development (acceleration of ontogenetic development) associated with late novelties that fit the expression of a peramorphic process of palingenesis type. This evolutionary channelling leads to the expression of more and more innovative and original morphologies of the derived forms with respect to ancestral forms, but which are also more gracile. This evolution could be interpreted as the result of a constant selection of the most peramorphic individuals (and therefore the more gracile), which were favored at the expense of the most robust. It seems reasonable that this selection was to adapt to the environmental changes due to the transgressive sequence (increased water depth disadvantaging the most robust morphologies) that occurs during the Vandenheckei Zone and part of the Sartousiana Zone on the North Tethyan Margin.Pseudoshasticrioceras evolved fromGassendiceras. But conversely to the latter, thePseudoshasticrioceras lineage is characterised by a neotenic process up toP. bersaci. The later speciesP. autrani is characterised by a strong size decrease and a progenesis. The presence of a helical early stage in Imerites is not a sufficient argument to classify it into the Heteroceratidae. Its ornamental features and most of all the succession of its ontogenetic stages attest an origin within the Gassendiceratinae. The turricone is an early novelty in thePseudoshasticrioceras- Imerites lineage. The successive species are: (1)P. autrani, (2) I. dichotomus and (3) I. giraudi. This evolution is characterised by an acceleration of the ontogenetic stages and by the expression of an inflexion point from which the adult ornamentation is delayed. All these transformations/ novelties at the origin of the genus Imerites fromPseudoshasticrioceras are perhaps linked with the eustatic variations and changes in the marine environment, considering the fact that the Dichotomus Horizon corresponds to the start of an important phase of marine transgression in the S-E of France. These changes are associated with a faunal turnover with the dominance of the Heteroceratidae, which probably led Imerites to an increase of the ecological competition with the neighboring morphological species (i.e. small sized with an early turricone, too), ultimately leading to their rapid extinctio