The stratigraphy and the ammonite fauna of the Vaca Muerta and Picún Leufú formations outcropping along the transect Portada Covunco-Cerrito Caracoles (south-central Neuquén Basin) were studied by means of two representative sections. The detailed study of the lithological succession of the two sections has allowed to complete the formal subdivision of the Vaca Muerta Formation by the introduction of two new members: the Portada Covunco Member (nov.) at the base, and the Pichi Moncol Member (nov.) at the top; the middle part of the formation is currently known as the Los Catutos Member. The ammonite fauna is mainly composed by species already recorded in other localities of the basin. There are six new species, but they remain to be formally described based on better and more abundant material: Choicensisphinctes cf./aff. platyconus, Catutosphinctes aff. rafaeli, Catutosphinctes n. spp. A & B, Steueria aff. alternans, and Sutneria n. sp. A. The lowermost Tithonian (lower Picunleufuense Zone) is well represented in the section P. Covunco by the guide assemblage (Lithacoceras picunleufuense, Choicensisphinctes platyconus and Catutosphinctes guenenakenensis) at the base of the P. Covunco Mb. The Zitteli Zone (which seems to overlap the Mendozanus Zone) and the Proximus Zone occur in the middle and upper parts of the P. Covunco Mb, as recongnized by their characteristic ammonite assemblages. The upper part of the Middle Tithonian, the Internispinosum Zone, corresponds to the Los Catutos Mb and, apparently, the upper part of the Vaca Muerta Fm (Pichi Moncol Mb), although it is fossiliferous only in its lower part. The lower Upper Tithonian Alternans Zone is well documented by ammonites, whereas the upper Upper Tithonian yielded only scarce fossils which could even represent the lower Berriasian Noduliferum Zone.

The ammonite fauna of the Tithonian–Berriasian of the Vaca Muerta Formation in Pampa Tril has been recently described in detail. New important specimens and additional information are presented in this paper. The phyletic evolution of Choicensisphinctes, passing from C. platyconus to C. erinoides is confirmed, as well as the sexual dimorphic correspondence of this latter with C. mendozanus. A microconch of the genus Krantziceras is described for first time. New specimens of Substeueroceras koeneni identical to the paralectotype, along with material already described from the koeneni Hz. (Koeneni Zone), point to the fixation of this horizon as the type horizon of the species. New specimens of Subthurmannia boissieri from the Damesi Zone match clearly the range of variation of this species in Spain, thus providing an element for time-correlation with the Tethyan standard scale. Additional material from the internispinosum alpha Hz. confirms the origin of W. internispinosum from C. proximus by the inception of an evolutionary innovation in the juvenile ontogeny.

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