The Dichotomus Horizon: proposal for a new biochronologic unit of the Giraudi Zone of the Upper Barremian of southeastern France, and considerations regarding the genus Imerites Rouchadzé (Ammonoidea, Gassendiceratinae)

International audience ; Recent revisions of the genus Imerites Rouchadzé make it possible to introduce a new biochronologic horizon to define more precisely the lower boundary of the Giraudi Zone: the Dichotomus Horizon. Using the concept of 'interval zone', this new horizon maintains the current lower boundary of the Giraudi Zone as accepted by authors, and thus contributes to the stabilization of the Barremian zonal system. This stabilization is also strengthened by abandonment of the use of "Crioceras" cristatus d'Orbigny (nomen dubium) that ought not be used as an index species in detriment of Imerites giraudi (Kilian). The classification, origin, and intraspecific variation of the genus Imerites Rouchadzé are examined.