ABSTRACT: Species of the Idiognathodus simulator group, which possess an eccentric groove and asymmetrical P1 element pairs, dominate the conodont fauna of the basal Gzhelian (Upper Pennsylvanian) Heebner Shale in Midcontinent North America, but they are accompanied by two series of smaller more typical Idiognathodus P1 elements lacking an eccentric groove. Landmark-based geometric morphometric analyses show that one series of these P1 elements possesses the same pattern of P1 element asymmetry and similar platform shapes as species of the I. simulator group, and are here designated as I. boardmani sp. nov. These P1 elements occur in small numbers through the entire thickness of the Heebner Shale at the three sections studied. The second series of P1 elements, which includes the species I. tersus Ellison 1941, differs by possessing nearly symmetrical P1 element pairs,more symmetrical platform margins, an arched platform and long adcarinal ridges. Specimens of the I. tersus group were recovered only from the uppermost part of the Heebner Shale in the southernmost section studied, which indicates a different ecological preference from the species of the I. simulator group.
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