ACCELERATED BIOMECHANICAL EVOLUTION PRECEDES SHIFTS IN LOCOMOTION

Movement is crucial for animals to obtain food, escape predators, secure mates, migrate to new habitats, and a host of other behaviors for survival. The greatest transitions in the history of animal life depend upon shifts in locomotion, from water to land and from land to air. Yet we know little about how major innovations in locomotion are coupled with shifts in evolutionary rate. To address this issue, we analyze the primary lever arms of dinosaur hindlimb retraction using phylogenetic Bayesian methods. The lever arms define the velocity ratio that relates the mm. caudofemoralis longus and iliotibialis lateralis, which are responsible for locomotion in non-avian and avian dinosaurs, respectively. The velocity ratio is a straight forward biomechanical assessment of the speed at which the hindlimb retracts. Fast animals like gazelle have high velocity ratios while animals that rely on torque, like badgers, have low velocity ratios. We hypothesize rapid rates of change in the velocity ratio associated with major adaptive transitions, such as graviportal quadrupedality (sauropods) and flight (avialans). We use the lever arm measurements to transform the phylogenetic branch lengths of dinosaurs, which can detect rapid shifts in evolutionary rate through the tree. The rates of the lever arms along branches can then be compared in a statistical context to detect relative shifts in the velocity ratio, but also to detect significant rate shifts among groups and directional evolutionary patterns associated with body size and ontogeny. We predict that rapid evolutionary change in the velocity ratio associated with transitions to graviportal quadrupedality and flight. We also predict a decrease in the rate at which these velocity ratios change once these transitions are finalized. Our results will help clarify how key innovations in locomotion evolve: whether they are slowly modified from ancestral forms or shift rapidly; even when coupled to a complex functional system.