The upper Bathonian-lower Callovian ammonite fauna of Chacay Melehué was sampled through new exposures under close stratigraphic control. Two new faunal horizons are described: the Iniskinites gulisanoi horizon and the Iniskinites crassus horizon, lying in the lower and middle parts, respectively, of the Steinmanni Zone. The ammonite fauna contains some species not previously described: Choffatia aff. neumayri (Siemiradzki, 1899) [M&m], Iniskinites evolutus sp. n. [M], Iniskinites sp. A [M], Eurycephalites aff. gottschei (Tornquist, 1898) [M], Xenocephalites aff. neuquensis (Stehn, 1923) [m], and Xenocephalites sp. A [m]. Oxycerites tenuistriatus (de Grossouvre, 1888) is new for the Andean region. Several species are represented by large samples that allow descriptions and analyses to be made of the entire ontogeny and the characterization of the principal morphs within each species. It is then possible to propose sexual dimorphic correspondence between some nominal morphospecies. The successive species Lilloettia steinmanni (Spath, 1928), Eurycephalites gottschei (Tornquist, 1898), E. rotundus (Tornquist, 1898), and E. extremus (Tornquist, 1898) form a phyletic morphocline here interpreted as a mosaic heterochronocline.

Key words: Andes, Bathonian, Callovian, ammonites, systematics, biostratigraphy, evolution, heterochrony.

Horacio Parent [infomax@satlink.com], Laboratorio de Paleontología y Biocronología, Instituto de Fisiografía y Geología, Universidad Nacional de Rosario, Pellegrini 250, 2000 Rosario, Argentina.