Fibular reduction and the evolution of theropod locomotion

Since Hampé's classic developmental experiments in the mid-twentieth century^1,2, the reduced avian fibula has sparked sustained curiosity^3-6. The fibula transformed throughout dinosaur evolution from a columnar structure into its splint-like avian form, a change long thought to be of little biomechanical consequence^3,6. Here we integrated comparative three-dimensional kinematic analyses with transitional morphologies from the fossil record to refute this assumption and show that the reduced fibula serves a crucial function in enabling extreme knee long-axis rotation (LAR). Extreme LAR is fundamental to avian locomotion and is regularly exploited by living birds to execute complex terrestrial manoeuvres⁷. We infer that the evolution of this capacity was preceded by restriction of the knee to hinge-like motion in early theropod dinosaurs, driven by the origin of a mid-shank articulation⁸ that precluded ancestral patterns of tibiofibular motion. Freeing of the fibula from the ankle joint later enabled mobilization of this initially static articulation and, in doing so, established a novel pattern of tibiofibular kinematics essential to the extreme levels of LAR retained by modern birds. Fibular reduction thus ushered in a transition to LAR-dominated three-dimensional limb control, profoundly altering the course of theropod locomotor evolution.