Unrealized optimal designs challenge models of constraint in early vertebrate evolution

Jawed vertebrates dominate modern vertebrate diversity, whereas most jawless vertebrate lineages disappeared during the Devonian. Their decline has often been attributed partly to morphofunctional constraints imposed by rigid, heavily armored body plans. We tested this hypothesis in pteraspidomorphs by integrating three-dimensional geometric morphometrics, computational fluid dynamics, Pareto optimality, and phylogenetic comparative modeling. Hydrodynamic performance varied strongly across headshield morphospace, and later lineages tended to occupy more efficient regions. However, extensive areas containing highly optimal designs remained unexplored throughout the group's history. Evolutionary model fitting provided little support for hard phenotypic bounds preventing access to these regions. Pteraspidomorphs therefore retained substantial unrealized functional potential, suggesting that their extinction occurred before they fully explored the range of viable hydrodynamic solutions available to their body plan. More broadly, these results challenge interpretations in which intrinsic morphofunctional constraints exhausted the evolutionary potential of jawless vertebrates.