Evolutionary convergence in conodonts revealed by synchrotron-based tomographic microscopy

The conodont fossil record is well known for its morphological diversity, but the iterative evolution that characterizes conodonts often avoids providing reliable phylogenetic frameworks among species, making unclear if the diagnostic characters of the taxa are indicative of common ancestry or evolutionary convergences. To distinguish homologies from analogies in conodonts, the most reliable method is by studying the ontogenetic development of the single taxa. Until now, the reconstruction of the ontogenetic stages was based on the study of separate individuals at different age from single populations. Nevertheless, the only unequivocal way to describe the ontogenesis of a conodont is to describe it from a single specimen. We achieve this objective by using Synchrotron Radiation X-ray Tomographic Microscopy applied to P₁ elements of species belonging to the Upper Triassic genera Carnepigondolella and Epigondolella. Our analysis provided internal tomographic information for the reconstruction of the conodont ontogenesis. We focused our study on the posterior platform, where an accessorial node develops behind the cusp. This node resulted in an autapomorphy of the genus Epigondolella from previous cladistic analyses and, thus, a diagnostic character for the elaboration of phylogenetic models. The microtomographies showed that this character is instead an evolutionary convergence. These results suggest the revision of the Late Triassic conodont phylogenetic relationships, showing that ontogenesis can be used as a criterion for discriminating homology from homoplasy in conodonts, and demonstrating that Synchrotron Radiation X-ray Tomographic Microscopy is a powerful and reliable tool to investigate conodont ontogenesis, evolutionary processes and phylogenetic relationships.