Conodont assemblages from the Bashkirian-Moscovian boundary interval in the Naqing section, South China have been studied in detail. A landmark-based geometric morphometric analysis to study the evolution of Diplognathodus ellesmerensis is presented. This analysis helped recognize a distinct new species, D. benderi sp. nov., which represents the direct evolutionary ancestor of D. ellesmerensis. The potential of using the First Appearance Datum (FAD) of D. ellesmerensis as the Bashkirian-Moscovian boundary marker is re-evaluated via biostratigraphic correlations of conodonts and other fossil groups between different palaeogeographic basins. The FAD of D. ellesmerensis from within the D. benderi sp. nov.–D. ellesmerensis lineage is an excellent marker to recognize a global Bashkirian-Moscovian boundary because of its global distribution, an abundance of supplementary marker species at similar stratigraphic levels, and the close stratigraphic proximity of the FAD to the traditional Bashkirian-Moscovian boundary, thus largely preserving the original concept for the base of the Moscovian Stage.