Skull evolution and the terrestrialization and radiation of tetrapods

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Our proposal brings together world class expertise and cutting-edge methods to answer a key question in the history of life: how did vertebrates conquer the land? We address this question by testing four key hypotheses derived from long-standing assertions that selection acted upon the skull to drive adaptations for improved terrestrial feeding during the water to land transition. Our methods offer a means to shift away from analogy-driven assertions of evolutionary history towards rigorous testable hypotheses founded upon mechanical principles, and will set a benchmark for future studies in evolutionary biomechanics. For the first 200 million years of their history, vertebrates lived an aquatic existence. Between 385 and 350 million years ago they evolved a host of anatomical features that ultimately enabled vertebrates to conquer land. This reorganization of the vertebrate skeleton created the basic tetrapod body plan of a consolidated head with mobile neck, arms and legs with digits and air breathing lungs. This plan has persisted, subject to modification, ever since and is shared by all terrestrial vertebrates. It was proposed over 50 years ago that tetrapods modified their skull bones and jaw muscles to create a stronger and 'more efficient' structure, capable of forceful biting for feeding on land. This reorganization is seen as key to their subsequent radiations, enabling tetrapods to expand into new ecological niches by feeding on terrestrial plants, large prey and hard or tough food. It has been proposed that these modifications came at the cost of reduced hydrodynamic efficiency and a slower bite, and could only be achieved by the loss of suction feeding and the evolution of rib-based breathing, thus freeing the skull from its roles in aquatic locomotion, drawing prey into the mouth and pumping air into the lungs.These ideas have been perpetuated in textbooks for decades, yet are based on out-dated simple line drawings of skulls and jaw closing muscles, and remain to be tested. We now have a rich and informative fossil record that documents changes in skull shape across the water to land transition. However, until now, we have lacked the means to test these hypotheses in a quantitative, rigorous way. In this proposal we will determine how changes in skull form and function enabled vertebrates to feed in a terrestrial environment and document the sequence of evolutionary changes and trade-offs that lead to their conquering of land. We will integrate principles from palaeontology and biology to reconstruct skull anatomy in 14 fossil tetrapods. Mathematical and mechanical principles will then be used to test the hypothesis that changes to skull anatomy resulted in tetrapod skulls evolving from hydrodynamically streamlined broad, flat skulls that could deliver a rapid (but weak) bite to strongly built skulls that could produce a more effective, forceful bite. New evolutionary modelling methods will assess how selection for skull strength or hydrodynamic efficiency shaped the evolution of the tetrapod skull. Our project will produce methodological advances that can be applied more broadly to evolutionary transitions and radiations, and to address long standing questions linking form and function. Palaeontologists, anatomists, biomechanists, evolutionary and developmental biologists and engineers will benefit from this work, which will establish new international collaborations. Its visual aspect and focus on early tetrapods will appeal to the general public, offering engagement opportunities and generating media interest. Members of our team are leaders in developing and validating methods for reconstructing and simulating the musculoskeletal anatomy and function of fossil organisms and have been involved in developing new methods for modelling how function has shaped form in deep time. The time is therefore ripe to apply our knowledge and skills to one of the key events in the history of life and our ow