REVISION OF IDIOGNATHODUS SPECIES FROM THE MIDDLE-UPPER PENNSYLVANIAN BOUNDARY INTERVAL IN THE MIDCONTINENT BASIN, NORTH AMERICA

Efforts to establish a conodont marker for the Moscovian-Kasimovian (Middle-Upper Pennsylvanian) boundary have focused on conodonts of the genus Idiognathodus. In the Midcontinent Basin, cyclothemic deposits of the regional Desmoinesian-Missourian boundary interval provide the greatest potential for correlation at or near the Moscovian-Kasimovian boundary.  Midcontinent idiognathodids were revised using functionally significant characters of the P₁ element. Rostral lobe development and major changes in surface roughness are species level characters, whereas the pattern of ornamentation and shape of the dorsal platform are considered to be environmental variation.

Species of Idiognathodus from the uppermost Desmoinesian major Lost Branch cycle include I. expansus and I. n. sp. S. Idiognathodus n. sp. S is a member of a group of Idiognathodus defined by a characteristic expanded and well developed rostral lobe. Idiognathodus expansus disappeared at the Late Desmoinesian extinction event along with Swadelina and Neognathodus. The minor South Mound/Checkerboard cycle, the youngest marine horizon in the Desmoinesian, shows the first stage of recovery. Idiognathodus n. sp. S is ancestral to all species that developed during the recovery. Idiognathodus sulciferous evolved from I. n. sp. S and expands into the distinctive sulciferous group, which is characterized by a moderately restricted and poorly developed rostral lobe. Two subspecies of I. sulciferous occur in the South Mound/Checkerboard cycle. In the intermediate Exline cycle, the oldest Missourian marine horizon, another subspecies of I. sulciferous appeared, and I. eccentricus evolved from I. sulciferous.  The new species I. n. sp. T evolved from I. n. sp. S. No new forms appeared in the minor Critzer cycle. In the major Hertha cycle, I. n. sp. V evolved from I. species S and I. clavatulus evolved from I. eccentricus in the sulciferous group.  Specimens from the overlying major Swope cycle were also investigated to determine the validity of published names. The Eurasian species, I. sagittalis is not present in the Midcontinent but is closest to I. n sp. T.  Forms similar to I. eccentricus occur in Eurasian strata, which hold the greatest potential for correlation of a Moscovian-Kasimovian boundary between Midcontinent North America and Eurasia.