CARRANO, M. T.: Homology and the evolution of non-avian dinosaur locomotion

Non-avian dinosaurs diversified throughout the Mesozoic from a single bipedal, parasagittal-limbed ancestor. Their 140-my radiation spanned over three orders of magnitude in body size and included eight major clades. In spite of this, diversification within the dinosaurian locomotor apparatus appears to have been comparatively restricted. I analyzed dinosaur hind limb and pelvic anatomy within a cladistic framework in order to quantify these patterns of change. Using methods to analyze both discrete and continuous characters in a phylogenetic context, I identified where homoplasy constitutes parallelism and might thereby be used to infer similar selective pressures on hind limb function. Finally, I evaluated the resulting phylogenetic patterns in light of terrestrial locomotor biomechanics. These analyses revealed repeated, independent derivations of several morphological features that have potential relevance for hind limb locomotor function. These include anteroposterior expansion of the ilium, medial reorientation of the femoral head, and elevation of the femoral lesser trochanter. Such parallel changes appear to reflect the enlargement of several major hind limb muscles, as well as a trend towards switching their predominant function from abduction-adduction to protraction-retraction. The timing of these changes suggests that they occurred subsequent to the initial divergence of Dinosauria into its constituent lineages. In addition, many “avian” characteristics are shared with more basal theropod dinosaurs, and several were acquired convergently in otherdinosaurian clades.