CONODONT DEFINITION OF THE BASHKIRIAN/MOSCOVIAN (EARLY/MIDDLE PENNSYLVANIAN) BOUNDARY: PRELIMINARY EVALUATION OF THE LEADING CANDIDATES

Three evolutionary events have been proposed to the pertinent IUGS Task Group to define the Bashkirian/Moscovian (B/M) boundary: the first occurrence of Declinognathodus donetzianus, the first occurrence of Idiognathoides postsulcatus, and a point along the morphologic continuum between early and late forms of Neognathodus nataliae. The proposed markers, like the traditional B/M boundary interval, occur higher in the section than the North American Morrowan/Atokan boundary interval.

There are significant problems with each proposed boundary marker. D. donetzianus has a distinctive morphology that can be recognized by non-specialists, but appears to be hampered by strict provinciality. It has not been recovered from China or North America, except in accreted terranes. Also, its numbers are low where it is distributed. Id. postsulcatus occurs in the Donets Basin and in China, but it is not reliably recovered in the Moscow Basin. It is not widely recognized in North America, with a morphology that is only subtly different from its ancestor, Id. sulcatus. Neither an ontogenetic series nor the population variation of Id. postsulcatus have been illustrated, either of which could account for those slight morphological differences. N. nataliae and its transition from early to late forms are well represented in North America, but the boundary will probably be defined in Eurasia. There, the succession of Neognathodus species is complicated by additional forms that are absent from North America, whereas those that co-occur appear to have different ranges.

My preliminary evaluation is that D. donetzianus is the least suitable taxon for definition because of its provinciality. A boundary based on the morphocline within N. nataliae would be easy for non-specialists to recognize, and it is the most widely dispersed taxon under consideration. However, Neognathodus needs additional taxonomic treatment to resolve miscommunication between international specialists regarding range and evolutionary relationships. For example, Neognathodus is widely recognized for heterochronic evolution, but ontogenetic stage is seldom considered in previous Eurasian taxonomic treatments. If Id. postsulcatus can be confidently recognized in the Moscow Basin and North America, it will be the leading candidate to define the B/M GSSP.