Geometric morphometrics suggests different environmental pressures on small and large Polygnathus conodonts during the recovery after the Hangenberg crisis (latest Devonian – earliest Carboniferous)

The morphology of feeding structures is driven by the nature of the food source, itself being influenced by environmental factors. However, food sources may change along growth, leading to potentially complex interactions between ontogenetic trajectories and environmental variation. The present study investigates these interactions using a geometric morphometric analysis of the platform shape of P1 elements of Polygnathus designed here as ‘unornamented’, from a post-Hangenberg succession from the Montagne Noire. Allometric trajectories vary along the record, with variable shapes for small elements but a common adult morphology. The specimens from each stratigraphic level were split into three size classes to assess environmental forcing. The shape of the small-sized class varied primarily with conodont biofacies, the medium-size class with sedimentary microfacies, while the large-size class followed a random walk unrelated to environmental variation, with little support for a role of paleotemperatures driving morphological evolution. These results agree with a change of diet along ontogeny, and with an increase of functional constraints related to occlusion in large elements. Biofacies and microfacies trace different signatures of bathymetry and distance from the coast, suggesting that post-Hangenberg unornamented Polygnathus varied in their exploitation of the water column along ontogeny. Finally, the high morphological disparity of small elements suggests less feeding specialization constraints in this size class, making small specimens more susceptible to environmental influences than large ones. These results showcase how the complexes interactions between allometric trajectories and changes in feeding behaviour may render difficult to identify underlying patterns of environmental forcing.