An early Bathonian Tethyan ammonite fauna from Argentina

Evidence for the Lower Bathonian Substage in most of the Pacific area has been lacking or controversial because index ammonoids of the Tethyan Subrealm in southern Europe and North Africa were unknown. During the Bathonian, the East Pacific margin belonged to either the Boreal Realm or the East-Pacific Subrealm of the Tethyan Realm, each with faunas distinctly different from the Eurafrican Tethyan faunas that form the basis for the chronostratigraphical standard. The first representatives of Morphoceras from South America, M. gulisanoi sp. nov., here described from a single locality in southern Mendoza Province, clearly document the Lower Bathonian Substage, probably the Zigzag Zone. Associated ammonoids are the perisphinctoidean Procerites cf. schloenbachi de Grossouvre and the oppeliid Oxycerites ( O. ) cf. aspidoides (Oppel), also typical Tethyan elements but less useful for dating.