THE APPLICATION OF GEOMETRIC MORPHOMETRIC ANALYSIS TO RESOLVE MORPHOLOGICAL VARIATION AND INTERPRET EVOLUTIONARY PATTERNS IN THE UPPER PENNSYLVANIAN ‘ASYMMETRICAL IDIOGNATHODUS CLADE’

The Idiognathodus magnificus, I. eudoraensis, and I. simulator groups comprise a distinct clade of Missourian-Virgilian Idiognathodus species with a unique style of P₁ element asymmetry named simulator-style asymmetry. In simulator-style asymmetry the dextral element has a much wider rostral platform in the ventral area than the sinistral element, and the two elements are more morphologically distinct in larger elements than in smaller elements. Platform landmark analysis (PLA) is a landmark-based morphometric procedure that uses eigen analyses to study statistical variation in the platform morphology of the Pennsylvanian conodont genus Idiognathodus. PLA has been previously used to characterize the older I. magnificus group and the younger I. simulator group. Here, PLA is used to describe morphological variation in the intermediate I. eudoraensis group, to compare the three species groups, and to interpret the morphological evolution of these Idiognathodus species. The asymmetrical Idiognathodus clade first appears in the middle Missourian I. confragus Zone, represented by I. magnificus, following a major extinction of older Idiognathodus species. In the later Missourian I. eudoraensis Zone, the I. eudoraensis group appears with derived I. magnificus group species. The I. eudoraensis fauna represents the development of the eccentric groove on the platform surface and a morphological radiation of the asymmetrical Idiognathodus clade. The overlying I. simulator group occurs in the early Gzhelian I. simulator Zone, and contains similar morphological variations as the I. eudoraensis group, but the I. simulator group species differ from the older species by having significantly shorter adcarinal ridges of different length, smaller lobes, more pronounced P₁ element asymmetry, and a more caudally shifted eccentric groove.