Functional constraints on tetrapod feeding ecology were released long after terrestrialisation

The radiation of tetrapods during the Devonian and Carboniferous was associated with a transition from aquatic to terrestrial environments, with attendant changes in feeding ecology that are poorly characterized. Using a theoretical morphospace and functional optimality approach, we characterize the functional evolution of tetrapod mandibles, finding an antagonistic relationship between the strength, rotational efficiency and the height of jaw morphologies. We further show that the regions of morphospace occupied by the jaws of aquatic, semi-aquatic and early terrestrial tetrapods are optimised within this trade-off. As terrestrial amniotes radiated, they explored broader regions of jaw morphospace that are suboptimal with respect to the trade-off. We interpret this as a release of functional constraint on the jaw morphology of later branching terrestrial herbivores, with new functional demands driving evolutionary innovation. While feeding in aquatic and terrestrial environments is fundamentally different, the functional requirements of the lower jaw do not change. Instead, the jaws of aquatic species perform adequately on land.