AMONITAS y ESTRATIGRAFIA DEL AALENIANO-BAYOCIANO EN LOS ANDES ARGENTINO-CHILENOS

Abstract

The fauna studied came from differen Middle Jurassic sections of Neuquen, Mendoza (Argentina) and Atacama (Chile) provinces. The taxonomic and chronologic review of the early Middle Jurassic (Aalenian to Middle Bajocian) Hildocerataceae, including type material, recently described (Westermann & Riccardi, 1972), demonstrates the existence of about 40 species, although a number of previously described species are regarded as conspecific. Several subgenera and species were formerly known only from Europe. The Aalenian is mostly rather poorly fossiliferous, yielding locally the genera Leioceras (?), Staufenia (?), Tmetoceras, Bredya, Erycites y Planammatoceras. Several diverse ammonoid assemblages occur at the approximate base of the Bajocian, with Eudmetoceras s.s, E. (Euaptetcceras ), Fontannesia (?), Zurcheria, Sonninia s.s., S. (Euhoploceras ), Tmetoceras and the endemic genus Puchenquia Westermann and Riccardi. Superjacent Sonninia (Fissilobiceras) zitteli (Gottsche) and Pseudotoite clearly indicate the Sowerby Zone, in which S. (Papilliceras) espinazitensis Tornquist appears. The Sauzei 'Zone yields this last species, early Dorsetensia, Otoitidae and early Stephanoceratidae. The Humphriesianum Zone is established by Dorsetensia romani (Oppel ), D. liostraca Buckman, with Stephanoceras ex gr. S. humphriesianum (Sowerby).