Geometric morphometric analysis of Idiognathodus species across the Atokan-Desmoinesian boundary in north-central New Mexico, USA

Landmark-based geometric morphometric analysis of three Idiognathodus species that occur in highest Atokan to lower Desmoinesian strata in New Mexico, I. salensae, I. obstipus, and I. amplificus, show that they represent three similar but distinctive P1 element species. Idiognathodus salensae has the most twisted element and the smallest, most restricted rostral lobe. Idiognathodus amplificus has the least twisted element and has the largest, most expanded rostral lobe. Idiognathous obstipus represents an intermediate taxon between these two end members. The sinistral elements are more clearly distinguishable between the species than the dextral elements. Other oral features of the P1 element in these species are highly variable and the analyses demonstrate plasticity in carinal length, adcarinal ridge constriction, transverse ridge disarticulation, and the size and arrangement of nodes on the lobes. These variations and the overall shape changes in ontogeny and chirality are interpreted to be intraspecific variability and are not useful for species discrimination in this group of species. The morphological groups were distinguished along canonical variance axes but not differentiated by principal component axes, highlighting the importance of using canonical variant analysis to detect specific variations undetected by principal component analysis.