FUNCTIONAL MORPHOSPACE OF FEEDING STRUCTURE IN ECHINOIDS – NEW INSIGHTS FROM 3D METHODS

The evolution of feeding strategies in echinoids reflect broader macroevolutionary trends that saw echinoids diversify into a wide range of morphologies, ecologies, and habitats over the last 300 million years. Crown group echinoids evolved a distinct structure, the perignathic girdle, which supports the musculature associated with the jaws, or Aristotle’s Lantern. This structure likely originated in the Permian and is underpinned by distinct developmental pathways that underly the divergence of the two crown group clades, the cidaroid and euechinoids. Though the position and morphology of the perignathic girdle has traditionally been used for taxonomic classification of these clades, the breadth of morphological variability and the associated functional performance of girdle shapes has not been explored. Here we present a preliminary analysis of the morpho-functional landscape of perignathic girdles in a cohort of 9 taxonomically and ecologically diverse echinoids, representing both fossil and living forms. We combine 3D geometric morphometric data, structural analysis using finite element modelling, and a high-resolution phylogeny to quantitatively explore girdle functional morphospace through time and space. We demonstrate notable differences in the stress distribution and stress peaks of the tested girdle shapes, suggesting that structural differences exist that can be acted upon by selective pressures. We find convergence of girdle morphology in phylogenetically distinct lineages that exhibit similar habitat preferences, suggesting that selective pressures acted to drive morphological evolution of certain girdle shapes. Preliminary results suggest that the perignathic girdle of echinoids is not as phylogenetically conserved as previously believed, and that girdle morphology and function is responding strongly to adaptive pressures. This highlights the need for a comprehensive exploration of 3D perignathic girdle functional morphology in a phylogenetic context to test for adaptive evolution in echinoid feeding strategies.