MORPHOMETRIC ANALYSIS OF THE CONODONT IDIOGNATHODUS SIMULATOR (ELLISON), INDEX SPECIES FOR THE BASE OF THE GZHELIAN STAGE - (UPPER PENNSYLVANIAN), FROM THE HEEBNER SHALE OF MIDCONTINENT NORTH AMERICA

Reliable discrimination among morphotypes of the conodont Idiognathodus simulator (Ellison) is required to ensure consistent biostratigraphic correlation of the base of the Gzhelian Stage. A morphometric procedure has been created to define biological landmarks and features on P₁ elements of I. simulator that are used to resolve quantifiable geometries. The procedure superimposes line segments onto a digital image of a specimen. The line segments are created by connecting a series of landmarks, each with an X and Y coordinate value. These landmarks are the central node, dorsal tip, caudal and rostral most extent of the platform ribs, and the dorsal and ventral extensions of the adcarinal ridges and lobes. Idiognathodus simulator is a species that contains a relatively high degree of asymmetry between P₁ element pairs and some morphotypes appear to be heavily or entirely chirality dependent. Five sinistral and four dextral morphotypes can be distinguished based on the presence or absence of lobe growth, the linear nature of the adcarinal ridges, platform margins, eccentric groove, and overall platform shape. The diagnostic features separating sinistral morphotypes are: 1. A detached caudal adcarinal ridge. 2. No lobe development and straight adcarinal ridges. 3. Caudal lobe development with a platform symmetrically split by the eccentric groove. 4. Caudal lobe growth with a platform asymmetrically split by the eccentric groove. 5. Caudal and rostral lobe development, with strong concave curvature of both adcarinal ridges. The diagnostic features separating dextral morphotypes are: 1. A detached caudal adcarinal ridge. 2. An elongate and curved caudal adcarinal ridge. 3. Triangular elements containing caudal lobes. 4. Elongate elements containing caudal lobes. The holotype of I. simulator which comes from the Heebner Shale, belongs to the sinistral morphotype with the detached caudal adcarinal ridge.