LOWER AND MIDDLE PENNSYLVANIAN CONODONT ZONATION FOR MIDCONTINENT NORTH AMERICA

Most of the important advances in Pennsylvanian conodont biostratigraphy since the early work of Lane and colleagues is dispersed among papers that address limited biostratigraphic or taxonomic issues, often on limited geographic and stratigraphic scales. A summary of these advances is now appropriate. The early work provided a detailed zonation for the Morrowan Stage that is widely accepted. However, a combination of stratigraphic and taxonomic problems has hindered development of a reliable conodont zonation for the Atokan and Desmoinesian stages.

Most conodont specialists currently rely on species of Neognathodus to zone Middle Pennsylvanian strata. We believe that Neognathodus is just as useful in Morrowan strata, but is probably less useful than generally believed for subdividing the Desmoinesian. Idiognathodus remains underutilized, but should eventually provide the best biostratigraphic resolution once its complex taxonomic relationships are worked out. Gondolella is useful in lithologies where it reliably occurs. Additional taxonomic revision of these three genera is needed, but enough useful taxa are now described and illustrated that we propose the following zonation:

Zones based on species of Neognathodus for the Morrowan through Desmoinesian are [ascending order]: higginsi, symmetricus, bassleri; a zone based on an unnamed species (spans the Morrowan-Atokan boundary), atokaensis, colombiensis, caudatus (base coincides with the Atokan-Desmoinesian boundary), asymmetricus, roundyi (sensu lato). We recognize the following very preliminary zones based on idiognathodids for the Atokan and Desmoinesian [ascending order]: incurvus (base approximates the Morrowan-Atokan boundary), incurvus descendents (informal zone for a succession bearing features derived from incurvus), amplificus (base coincides with the Atokan-Desmoinesian boundary), amplificus descendents (informal, like above), Idiognathodus sp. 3 (of Swade 1985), new genus ‘S’. ‘Naked’ gondolellids and mesogondolellids are commonly encountered as acme zones in the lower and middle Atokan respectively. Non-crenulated Gondolella species such as G. pohli characterize the middle Desmoinesian, followed by rare crenulated gondolellids, and ultimately G. magna in the upper Desmoinesian.