Meeting Abstract
Paleontologists have traditionally reconstructed the locomotion of dinosaurs and other extinct animals by manipulating their fossil bones and inferring the mobilities of their limb joints. But even if we could estimate the ranges of motion (ROMs) of joints perfectly, are we justified in assuming that all of an animal’s potential joint poses are exploited in life, let alone in locomotion? Here we evaluate the predictive power of joint mobility by determining what portion of a joint’s full passive ROM is actually used during various behaviors. We measured the passive joint ROMs of the hip, knee, and ankle of the helmeted guineafowl (Numida meleagris) and the American alligator (Alligator mississippiensis) based on manipulations of fully intact cadavers. We then measured thousands of poses used at each of these joints during locomotor and non-locomotor behaviors using XROMM, and plotted the mobilities and poses on a common ROM map in 3-D joint pose space. We found that in all the joints studied, steady forward locomotor poses form a small and uncentered subset of all possible joint poses. The centroid of each joint’s mobility — sometimes termed the joint’s “neutral pose” and thought to reflect habitual stance — has no relationship to posture or locomotion. Rather, locomotor poses often fall along the edges of cadaveric ROM envelopes in 3-D joint pose space. These results suggest that even well-estimated joint ROM, though critical for the elimination of impossible joint poses, is a poor predictor of the locomotor poses actually used by extinct ornithodirans such as non-avian dinosaurs and pterosaurs. Future analyses of hindlimb joint surface interactions during life are necessary to further constrain paleontological reconstructions of locomotion.
