On the northern bank of the Rio Atuel, Mendeza province, Argentina, the marine fossilfferous Mesozoic is now known to begin with Hettangian-lower Sinemurian strata. Traditionally, the marine Jurassic sequence in the area was believed to start with upper Sinemurian levels. Representatives of the genera Psiloceras Hyatt, Caloceras Hyatt, Alsatites Haug, Waehneroceras Hyatt, Schlotheimia Bayle, Sulciferites Spath, Badouxia Guex and Taylor, Vermiceras Hyatt, Coroniceras Hyatt, Agassiceras Hyatt, Euagassiceras Spath, and Arnioceras Hyatt thus indicate the presence of beds equivalent to the Planorbis, Liasieus, Angulata, Bucklandi, Semicostatum, and Turneri zones of the standard international chronostratigraphic scale. Hence, in Argentina, the marine Hettangian, Sinemurian, and lowermost Pliensbachian have been proved only in the neighborhood of the Rio Atuel region, where marine Triassic could also be represented. Elsewhere in Argentina, the oldest Jurassic ammonite-bearing levels usually belong to the upper lower or the upper Pliensbachian. Regionally, these findings imply a substantial modification to previous paleogeographic reconstructions for the Early Jurassic of South America.En la margen norte del rio Atuel, Mendoza, Argentina, la sucesi6n fosilifera marina del Mesozoieo comienza con niveles del Hettangiano y Sinemuriano inferior. Tradicionalmente se habia considerado queen la regi6n el Jur~sico marino comenzaba con el Sinemuriano superior. Representantes de los g~neros Psiloceras Hyatt, Caloceras Hyatt, Alsatites Haug, Waehneroceras Hyatt, Schlotheimia Bayle, Sulciferites Spath, Badouxia Guex y Taylor, Vermiceras Hyatt, Coroniceras Hyatt, Agassiceras Hyatt, Euagassieeras Spath y Arnioceras Hyatt, indican la presencia de estratos equivalentes alas zonas de Planorhis, Liasieus, Angulata, Bucklandi, Semicostatum y Turneri de la Escala Cronoestratigrafica Internacional. Asi en Argentina el Hettangiano, Sinemuriano, y Pliensbachiano basal marinos solamente han sido comprobados en proximidades de la regi6n del rio Atuel, donde tambi~n podrian hallarse representados niveles de Tri~sico marino. En el resto de Argentina los niveles m~s antiguos portadores de amonites del Jur~isico corresponden generalmente al Pliensbachiano inferior alto o Pliensbachiano superior. Desde el punto de vista regional estos hallazgos determinan una modificaci6n substancial en la paleogeografia del Jur~sico temprano de America del Sur.Facultad de Ciencias Naturales y Muse
