PENNSYLVANIAN CONODONT BIOSTRATIGRAPHY IN THE MIDCONTINENT REGION, USA AND BEYOND

Almost any sample of Pennsylvanian marine rock from near the base of the system to its top will yield a conodont fauna dominated by species of the Idiognathodus - Streptognathodus plexus. The range in species morphology is extraordinary and their diversity is seductive, promising a taxonomy reflecting these great differences and a refined and precise biostratigraphy employing them. In spite of this great promise and potential, detailed and long-time study of the Idiognathodus - Streptognathodus plexus has failed to produce the desired results. Reasons for this include a high level of both heterochronous and isochronous homeomorphy, frequently repeated iterative evolution, and tremendous ecophenotypic variation, commonly as an ecocline within a single stratigraphic unit. The reality of a phylogenetically-based taxonomy continues to elude us for this plexus. This is true even as the specific names of its members multiply in an ever-expanding nomenclature that can be referred to as "neogunnellism," which rewards these peculiar quirks in morphologic diversity, but also fails in its stated goal of producing a workable and useful biostratigraphy.

The distribution of other conodont taxa suggests viable alternatives. In Desmoinesian rocks, a primary zonation based on species of Neognathodus can be readily supplemented by species of Gondolella and Diplognathodus. For the younger Missourian and Virgilian rocks, species of Gondolella and Diplognathodus continue to be usable, supplemented by the distribution of Idioprioniodus, Ubinates, and Hindeodus spp.