High Resolution Biostratigraphy and Geochronology of the Vaca Muerta Formation, Neuquén Basin In this work we present new results from studies of ammonites and calcareous nannofossils in selected surface sections of the Vaca Muerta Formation that have yielded a precise biochronology on this unit. Furthermore, these studies have been correlated with the classical biozonations of the Tethys in the case of the ammonites and have also adjusted events of calcareous nannofossils recognized in the Neuquén Basin with those of the Tethys. Geochronological U-Pb dating performed at both the Vaca Muerta and the underlying Tordillo Formation has provided absolute ages of both lithostratigraphic units. These data contrast markedly with the currently accepted ages for this interval by the International Commission on Stratigraphy (ICS) and the Geological Time Scales (GTS 2012) and are about 5 million years younger. Finally, we stress the importance and need for detailed biostratigraphic studies that detect tectonic disturbances showing as an example the classical locality of La Yesera in northern Neuquén.