Metapolygnathus parvus Kozur, 1972 (Conodonta): a potential primary marker for the Norian GSSP (Upper Triassic)

The base of the Norian Stage (Carnian/Norian boundary, Upper Triassic) is still awaiting a formal designation by the International Commission on Stratigraphy. At present, two stratigraphic sections, Pizzo Mondello (Sicily, Italy) and Black Bear Ridge (British Columbia, Canada), have been proposed as GSSP (Global Stratotype Section and Point) candidates for the base of the Norian, but a bio- or physical primary marker event has not been yet selected by the Working Groups. We show here that conodonts represent the best choice to define the Carnian/Norian boundary. In the Upper Triassic, in fact, conodonts have been studied in great detail in the last 15 years providing a very high resolution biostratigraphic tool. We illustrate here the phylogenetic lineage Metapolygnathus praecommunisti - Metapolygnathus dylani - Metapolygnathus parvus, demonstrating that this lineage is present in both the GSSP candidate sections. Thus, we propose the First Occurrence (FO) of Metapolygnathus parvus (the last representative of its lineage) as a potential primary marker to define the base of the Norian Stage. Metapolygnathus parvus is in fact the most reliable and useful biomarker, because this species is morphologically simple and easy to recognise and its First Appearance Datum (FAD) is globally recognisable within its phylogenetic lineage. The FO of M. parvus also coincides with an important global conodont faunal turnover (known as T2), that corresponds to the disappearance of the typically Carnian conodont genus Carnepigondolella and the rise of the genus Metapolygnathus. In addition, the FO of M. parvus occurs very close to a brief positive δ13Ccarb shift, documented in the two GSSP candidate sections and in the uppermost part of the magnetozone PM4n (ca. at 3.20 m below the top) at Pizzo Mondello.