Locomotor Modules and the Evolution of Avian Flight

The evolution of avian flight can be interpreted by analyzing the sequence of modifications of the primitive tetrapod locomotor system through time. Herein, we introduce the term "locomotor module" to identify anatomical subregions of the musculoskeletal system that are highly integrated and act as functional units during locomotion. The first tetrapods, which employed lateral undulations of the entire body and appendages, had one large locomotor module. Basal dinosaurs and theropods were bipedal and possessed a smaller locomotor module consisting of the hind limb and tail. Bird flight evolved as the superimposition of a second (aerial) locomotor capability onto the ancestral (terrestrial) theropod body plan. Although the origin of the wing module was the primary innovation, alterations in the terrestrial system were also significant. We propose that the primitive theropod locomotor module was functionally and anatomically subdivided into separate pelvic and caudal locomotor modules. This decoupling freed the tail to attain a new and intimate affiliation with the forelimb during flight, a configuration unique to birds. Thus, the evolution of flight can be viewed as the origin and novel association of locomotor modules. Differential elaboration of these modules in various lineages has produced the diverse locomotor abilities of modern birds.