Les Gregoryceras (Ammonitina) de l'Oxfordien moyen terminal et supérieur téthysien : révision systématique, biostratigraphie et évolution

In prolongation of the previous studies, the Gregoryceras Spath, 1924 (Ammonitina, Peltoceratina) species from the uppermost Middle Oxfordian (Rotoides sub-zone) to the Early Late Oxfordian (Bifurcatus zone), are revised. Sections providing most of the studied specimens in this work (Spain, Algeria, Tunisia) are described and dated based on faunal comparisons between Tethyan and Subtethyan domains. It appears that the best tool for correlations is the presence of the genus Gregoryceras, the succession of its species being similar for the two considered domains. It allows us to complete the biostratigraphic scale based on the Gregoryceras species succession, and parallel to the standard zonation. During this period the revised species of Gregoryceras are G. fouquei (Kilian, 1889) and G. pervinquieri (Spath, 1913). G. fouquei is only present in the Rotoides sub-zone (uppermost Middle Oxfordian), and not in the Stenocycloides sub-zone (early Late Oxfordian), where G. pervinquieri (Spath, 1913). A new species, G. benosmanae nov. sp. is described as the most recent species of the genus (Grossouvrei sub-zone). The evolution of the youngest species of Gregoryceras continues the peramorphocline described for older forms. Concerning a potential dimorphism, new data (size differences, umbilicus enlargement during growth) are particularly provided by G. pervinquieri.