ABSTRACT: Idiognathodus simulator (Ellison 1941) and Idiognathodus eudoraensis n. sp. constitute twomembers of a distinct group of Late Pennsylvanian species that are characterized by an eccentric groove on the P1 element and Type IIIb symmetry,where dextral and sinistral P1 elements show different morphological features. The oldest species, Idiognathodus eudoraensis n. sp., probably developed from a species of Idiognathodus during the late middle Kasimovian. The simulator group is not closely related to Streptognathodus, which is characterized by a deep medial trough. The distinctive morphology of I. simulator [s.s.], redescribed here, in conjunction with itswidespread distribution inNorthAmerica and Eurasia, has led its first appearance to be selected as the event marker for the base of the global Gzhelian Stage. The simulator group includes other species that have been named in Eurasia, and will require additional work to adequately resolve species ranges and interpret its evolutionary history.