Morphometric analysis and taxonomic revision of the conodont genus Idiognathodus from the Hebbner Shale (upper Pennsylvanian) of the Midcontinent Basin, North America
Hogancamp, Nicholas Jay
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Recently the conodont species Idiognathodus simulator Ellison (1941) was chosen as the marker species for the base of the global Gzhelian stage (Heckel et al., 2008). Because of this decision, the I. simulator group and closely related species will be used for the correlation of the Kasimovian-Gzhelian boundary around the world. A new morphometric procedure was developed to study the morphological variation within Idiognathodus from the Heebner Shale of the Oread cyclothem, which is the oldest Gzhelian cyclothem in the North American Midcontinent region. This new procedure combines landmark-based geometric morphometrics with eigen analyses in order to analyze the shape variation within and between groups of Idiognathodus P1 elements. The morphometric analyses reveal three morphologically distinct groups of Idiognathodus in the Heebner Shale of the Midcontinent region. These groups have been designated as the I. simulator group, the I. tersus group, and the I. pictus group. The I. simulator group contains species with asymmetrical P1 element pairs, relatively short adcarinal ridges, and an eccentric groove. The I. tersus group contains species with asymmetrical P1 element pairs, relatively short adcarinal ridges, and no eccentric groove. The I. pictus group contains species with symmetrical P1 element pairs, relatively long adcarinal ridges, and no eccentric groove. Species discrimination made within these groups is based on lobe presence and platform shape. Many different morphologies have been previously classified as I. simulator and these atypical morphotypes are differentiated by morphometric and ontogenetic analyses. The use of I. simulator Ellison (1941) is restricted to P1 elements which have asymmetric pairs, an eccentric groove, and a caudal adcarinal ridge which is isolated from the caudal platform margin. Many different morphologies resembling the I. pictus group have been previously classified as I. tersus and this has likely resulted in the incorrect stratigraphic range interpretation of I. tersus. The use of I. tersus Ellison (1941) is restricted to P1 elements which have asymmetric pairs, no eccentric groove, short adcarinal ridges, and no lobe development. It appears likely that the stratigraphic and geographic distributions of conodonts from the Heebner Shale are dependent on paleoecological and paleogeographic factors.