Morphometric analysis and taxonomy of the Idiognathodus sagittalis Kozitskaya, 1978 group and the I. swadei–I. heckeli–I. turbatus lineage (Conodonta, early Kasimovian, Pennsylvanian)

The conodonts Idiognathodus sagittalis Kozitskaya, I. turbatus Rosscoe and Barrick, I. heckeli Rosscoe and Barrick, and Swadelina subexcelsa (Alekseev and Goreva), have been proposed as biostratigraphic markers for the global Moscovian–Kasimovian stage boundary. Idiognathodus sagittalis was named the earliest and has been used in conodont zonations from the Eastern Europe Platform for decades. However, the taxonomy and phylogeny of this important species remain unresolved. New specimens of I. sagittalis, I. swadei, I. turbatus and I. heckeli from South China were compiled with published data from other regions to build a globally comprehensive data set for morphometric and taxonomic analysis. A series of quantitative analyses were used to explore morphological variability between species to determine if they can be differentiated. Landmark-based geometric morphometric analysis was used to study P1 element variation from the oral view and slope measurements of the rostral lobe were used to quantify the elevation of the lobe relative to the platform. The analysis shows that the four taxa are morphologically distinct and that I. sagittalis comprises a group of four unique morphotypes that display similar variations in platform development to that observed in the I. swadei–I. heckeli–I. turbatus lineage. The I. sagittalis group differs from the other species by having smaller and more ventrally shifted lobes, adcarinal ridges that are longer, straighter, and wider, and lobes that can be topographically lower than the platform. Although the lower elevation rostral lobe is considered characteristic of I. sagittalis, the measured slope angles span a broad range of values that form overlapping distributions for I. sagittalis group and the I. swadei–I. heckeli–I. turbatus lineage. Despite this variability and the overlapping distributions, I. sagittalis does have more specimens with steeper sloped (i.e. lower elevation) lobes than the other species. More work is required to resolve the taxonomy and evolution of the I. sagittalis group and Sw. subexcelsa, thus, the First Appearance Datum (FAD) of I. heckeli is currently the best available species to mark the Global Moscovian–Kasimovian boundary due to its global distribution, its thoroughly documented evolutionary pathway, and its clear diagnosis that is further clarified here.