three perturbations that lead to a major one at the Norian-Rhaetian boundary. The ~6‰ negative shift associated to the Norian-Rhaetian boundary has been proposed as a physical marker of the Rhaetian, in association with the usual biostratigraphic markers (e.g. FAD of conodont Misikella posthernsteini, base of the Proparvicingula moniliformis radiolarian Zone). The Misikella posthernsteini taxonomic issue In 2010, the Rhaetian Working Group of the ICS (International Commission on Stratigraphy) defined the markers defining the base of the Rhaetian Stage. One the main biomarkers was the FAD of conodonts Misikella posthernsteini and Mockina mosheri (morphotype A). M. posthernsteini is the typical marker in Tethys, whereas in North America it appears later than M. mosheri A. Conodont M. posthernsteini is the phylogenetic descent of Misikella hernsteini, and the passage is determined by transitional forms. So, the FAD of M. posthernsteini is determined by the final and fully evolved form. In 2007, the Steinbergkogel section in Austria has been proposed as GSSP candidate for the Rhaetian, placing the Norian/Rhaetian boundary with the FAD of M. posthernsteini. With the proposal of the Pignola-Abriola section as GSSP candidate for the Rhaetian, the specimens defining the Norian/Rhaetian boundary in Steinbergkogel have been reclassified as M. hernsteini/posthernsteini transitional forms, following the taxonomic definition of M. posthernsteini by Giordano et al. The M. posthernsteini sensu stricto represents the entirely developed morphotype. Of course, the ages related to the FAD of Misikella posthernsteini sensu lato (interpreted as transitional forms) and M. posthernsteini s.s. are different. Following the age model proposed by Maron et al. in 2015, based on the magnetostratigraphic correlation with the Newark APTS, the FAD of M. posthernsteini s.s. is ca. 205.7 Myr old, whereas the FAD of M. posthernsteini s.l. is around 209 Myr ago. The age of 205.7 Ma is supported by recent U/Pb (Uranium-Lead) radiometric ages from Peru (205.30±0.14, 205.40±0.09 Myr, and 205.70±0.15 Myr ago), calibrated with the Last Occurrence (LO) of Monotis bivalves (marker of the Rhaetian base). References Limestone Stratigraphy