RECURRENT MORPHOTYPES IN THE DENTAL ELEMENTS OF SWEETOGNATHUS CONODONTS REVEALED BY GEOMETRIC MORPHOMETRICS

Conodonts are a hyper-diverse group of predators and scavengers that have an astonishing 300-million-year evolutionary record from the late Cambrian to Late Triassic. It has been hypothesized that a major contributor to diversity in the Sweetognathus lineage, a Permian group of shallow water taxa, is the appearance of recurrent morphotypes via parallel evolution. A major hurdle with implementing geometric morphometrics to test this hypothesis has been the clear lack of homologous landmark positions on their dental elements.

In this study we rendered 3-dimensional models of their dental elements using a computer tomographic scanner and computed 404 cross-sections transecting their food-processing surfaces. 50 equally space semi-landmarks were then placed around the circumference of each cross-section before procrustes superimposition and hierarchical clustering of principal component scores was combined with a revised and expanded phylogeny of multiple Permian Sweetognathus conodont lineages to test the above hypothesis.

Our research has found that the feeding organs of multiple species in the Sweetognathus lineage repeatedly developed adaptions, based on the mapping of dental morphologies across the Sweetognathus phylogeny, allowing them to occupy new trophic niches within their isolated communities. The further incorporation of widely-used ecological metrics such as limiting similarity and morphological overlap was also able to demonstrate that radiation events were facilitated by disruptive selection on extreme phenotypes that resulted in increased diversity, and directional selection, limiting competition. The outstanding diversity and fossil record of conodonts, as well as the reproducible and robust morphometric protocol proposed here, illustrate some of the mechanisms whereby diversity was achieved in the Sweetognathus group, demonstrating that a major contributor to marine diversity in Palaeozoic marine trophic networks is the emergence of recurrent morphologies in geographically and temporally isolated communities of conodonts.