Review of conodonts across the Devonian/Carboniferous boundary: implication for the redefinition of the boundary and a proposal for an updated conodont zonation

The Devonian/Carboniferous boundary definition is under revision because of problems recognized in the last few years in regard to the index taxon and the type section. The distribution of conodont species of Bispathodus, Branmehla, Palmatolepis, Polygnathus, Protognathodus, Pseudopolygnathus, and Siphonodella have been investigated in order to evaluate their stratigraphic potential for the redefinition of the Devonian/Carboniferous boundary and a revised biozonation of the uppermost Famennian and lowermost Tournaisian. For the definition of the boundary it is necessary to find criteria that guarantee stability in the future, and do not significantly change the present stratigraphic position. Only a few conodont species first appear close to the present boundary and most of these do not have the characteristics of a stratigraphic index taxon, since all are quite rare. Only the FAD of Protognathodus kockeli and the FAD Siphonodella bransoni (= S. duplicata M1) can be considered as conodont markers, although both these possible positions for the DCB boundary have advantages and problems. We propose an updated biozonation scheme across the boundary based on the FADs of Bispathodus ac. aculeatus, Bispathodus costatus, Bispathodus ultimus, Protognathodus kockeli, Siphonodella bransoni and Siphonodella duplicata.