Abstract
Birds still share many traits with their dinosaur ancestors, making them the best living group to reconstruct certain aspects of non-avian theropod biology. Bipedal, digitigrade locomotion and parasagittal hindlimb movement are some of those inherited traits. Living birds, however, maintain an unusually crouched hindlimb posture and locomotion powered by knee flexion, in contrast to the inferred primitive condition of non-avian theropods: more upright posture and limb movement powered by femur retraction. Such functional differences, which are associated with a gradual, anterior shift of the centre of mass in theropods along the bird line, make the use of extant birds to study non-avian theropod locomotion problematic. Here we show that, by experimentally manipulating the location of the centre of mass in living birds, it is possible to recreate limb posture and kinematics inferred for extinct bipedal dinosaurs. Chickens raised wearing artificial tails, and consequently with more posteriorly located centre of mass, showed a more vertical orientation of the femur during standing and increased femoral displacement during locomotion. Our results support the hypothesis that gradual changes in the location of the centre of mass resulted in more crouched hindlimb postures and a shift from hip-driven to knee-driven limb movements through theropod evolution. This study suggests that, through careful experimental manipulations during the growth phase of ontogeny, extant birds can potentially be used to gain important insights into previously unexplored aspects of bipedal non-avian theropod locomotion.
Highlights
Based on multiple lines of evidence, it is widely accepted that birds evolved from bipedal theropod dinosaurs [1,2,3,4]
At the end of the stance phase, the knee joint was more extended in the experimental group (102.062.1 deg) than in the control group (83.366.0 deg). This resulted in reduced range of knee flexion during the stance phase in the experimental subjects compared to the control group (E: 30.163.4 deg; C: 41.363.1 deg)
We have shown that the addition of an artificial tail during ontogeny can produce postural and locomotory changes in chickens, consistent with the posture and kinematics inferred for non-avian dinosaurs [5,6,11]
Summary
Based on multiple lines of evidence, it is widely accepted that birds evolved from bipedal theropod dinosaurs [1,2,3,4]. It is appealing to think of birds as a model system to gain insights into aspects of non-avian dinosaur biology that are hard to study directly from fossil material, such as the relationship between limb morphology, posture, and locomotion [5,6,7]. It has been hypothesized that non-avian bipedal dinosaurs had more vertical femora due to the more posteriorly located CoM, and that their hindlimb movement was ‘hip-driven’, powered mainly by the caudofemoralis longus muscle (CFL). Their study produced unexpected results: birds with attached tails showed even more horizontally oriented femora, while no qualitative changes in kinematics were observed during locomotion compared to non-manipulated chickens. We expected adult chickens with added tails to show a more vertical femur in standing position and increased femoral excursion during locomotion as postulated for non-avian theropod dinosaurs
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