Bipedalism, flight, and the evolution of theropod locomotor diversity

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Abstract
The evolution of theropod flight has been characterized as a shift from one to three locomotor modules. Basal theropods, which were terrestrial bipeds, had a single locomotor module composed of the hind limb and tail. In birds, aerial locomotion was acquired with the origination of the wing module and a decoupling of the hind limb and tail into separate pelvic and caudal modules. This increase in modularity is thought to have granted birds more locomotor “options” than non-avian theropods. More specifically, an aerial locomotor system could have eased constraints on the hind limb and allowed specialization for habitats and lifestyles unavailable to non-birds. If so, bird hind limbs should be more disparate than those of non-avian theropods. We addressed this hypothesis by visualizing one aspect of limb design, the proportions of the three main segments, using ternary diagrams. Our results show that avian hind limb proportions are much more disparate than those of non-avian theropods. This broad range of limb design correlates with a radiation in locomotor diversity founded on three locomotor modules. We propose that birds have reached regions of proportion morphospace that are off limits to bipeds with only one locomotor module. In comparison, the limbs of non-avian theropods are conservatively proportioned. Despite great variation in body size, theropods other than birds do not exhibit specializations for locomotion other than terrestrial bipedalism. Although other aspects of size and shape need to be analyzed, the relationship between modular flexibility and morphological disparity appears to play an important role in theropod locomotor evolution.