Upper Bathonian ammonites from Chacay Melehué (Neuquén Basin, Argentina) and the chronostratigraphy of the Steinmanni Zone

It is here presented the study of a collection of Upper Bathonian ammonites sampled bed-by-bed from a new section of Chacay Melehué (Neuquén Basin, Argentina). Most of the specimens are macroconch females moderately well preserved, belonging to the following genera: Oxycerites, Alcidellus, Choffatia, Neuqueniceras, Iniskinites, Stehnocephalites, Lilloettia, and Eurycephalites. The new species Choffatia irregularis and Eurycephalites belgranoi are described. The microconch of Iniskinites crassus is described for first time. From the occurrence of ammonites very close to Eurycephalites gottschei the stratigraphic range of the genus is extended downwards into the upper part of the Steinmanni Subzone. The Steinmanni Zone is subdivided in two new standard subzones: Steinmanni Subzone and Gerthi Subzone. The base of this latter is the crassus Hz. (emended).