DINOSAUR RUNNING EVOLVED IN EPISODIC BURSTS

Many of the greatest evolutionary transitions in the history of animal life involve changes in locomotion. 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 moment arms of dinosaur hindlimb retraction using Bayesian phylogenetic methods. The moment arms define the velocity ratio relating mm. caudofemoralis longus and iliotibialis lateralis, which retract the femur in non-avian and avian dinosaurs, respectively. Fast animals, like gazelles, have high velocity ratios while animals that rely on torque, like elephants, have low velocity ratios. We use moment arm measurements to transform the phylogenetic branch lengths of dinosaurs, to detect evolutionary rate shifts through the tree. We find evidence that theropod dinosaurs had higher than average velocity ratios, sauropods had lower than average velocity ratios, and ornithischians maintained primitive velocity ratios throughout their evolutionary history despite advancements towards quadrupedalism and large body size. The evolution of the dinosaur velocity ratio follows a punctuated equilibrium model of evolution and included bursts of increased evolutionary rate prior to the rise of various dinosaur groups; including Ceratopsidae, Coelurosauria, and Paraves. We also found an order-of-magnitude rate increase, prior to the origin of modern birds and flight, which is associated with a shift in the muscles used to retract the femur during locomotion. Our results will help clarify how key innovations in locomotion evolve: whether they are slowly modified from ancestral forms or shift rapidly as well as redefine how we view the evolution of running and flight in dinosaurs and birds.