Cursoriality in bipedal archosaurs

Modern birds have markedly foreshortened tails and their body mass is centr ed anteriorly, near the wings(1-5). To provide stability during powered fli ght, the avian centre of mass is far from the pelvis, which poses potential balance problems for cursorial birds. To compensate, avians adapted to run ning maintain the femur subhorizontally, with its distal end situated anter iorly, close to the animal's centre of mass; stride generation stems largel y from parasagittal rotation of the lower leg about the knee joint(6-12). I n contrast, bipedal dinosaurs had a centre of mass near the hip joint and r otated the entire hindlimb during stride generation(4-8,11-13). Here we sho w that these contrasting styles of cursoriality are tightly linked to longe r relative total hindlimb length in cursorial birds than in bipedal dinosau rs. Surprisingly, Caudipteryx, described as a theropod dinosaur(14,15), pos sessed an anterior centre of mass and hindlimb proportions resembling those of cursorial birds. Accordingly, Caudipteryx probably used a running mecha nism more similar to that of modern cursorial birds than to that of all oth er bipedal dinosaurs. These observations provide valuable clues about curso riality in Caudipteryx, but may also have implications for interpreting the locomotory status of its ancestors.