Andean Lower Tithonian (Picunleufuense Zone) ammonites and aptychus from Estancia Maria Juana, Southern Neuquén Basin, Argentina

The ammonite fauna of the Lower Tithonian of the locality Estancia María Juana, southern Neuquén Basin is described for first time. The studied material was collected from the Portada Covunco Member (lower part of the Vaca Muerta Formation) which consists of finely sandy shaly marls with large concretions containing abundant crushed ammonites, aptychi, bivalves and gastropods. The ammonites belong to the species Choicensisphinctes choicensis and Lithacoceras picunleufuense transient a, indicating the picunleufuense a Horizon, which is the base of the Lower Tithonian Picunleufuense standard chronostratigraphic Zone. Middle Tithonian ammonites occur as poorly preserved specimens of Catutosphinctes in calcareous sandstones. An aptychus, collected from a concretion, among the bulk of Choicensisphinctes choicensis is of the Praestriaptuchus-type and is concluded it belongs to this ammonite. This association gives additional support to the hypothesis on the origin of Choicensisphinctes from L. picunleufuense in the Late Kimmeridgian or earliest Tithonian. The concretions of the picunleufuense a Horizon are interpreted as late diagenetic, showing that this kind of concretions are among the few marine sedimentary settings where ammonites with their aptychus in-situ could have been preserved.