LOCOMOTOR MODULES AND THE EVOLUTION OF AVIAN FLIGHT

The evolution of avian flight can be interpreted by analyzing the sequ ence of modifications of the primitive tetrapod locomotor system throu gh time. Herein, we introduce the term ''locomotor module'' to identif y anatomical subregions of the musculoskeletal system that are highly integrated and act as functional units during locomotion. The first te trapods, which employed lateral undulations of the entire body and app endages, had one large locomotor module. Basal dinosaurs and theropods were bipedal and possessed a smaller locomotor module consisting of t he 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 pr imary innovation, alterations in the terrestrial system were also sign ificant. We propose that the primitive theropod locomotor module was f unctionally and anatomically subdivided into separate pelvic and cauda l locomotor modules. This decoupling freed the tail to attain a new an d intimate affiliation with the forelimb during flight, a configuratio n unique to birds. Thus, the evolution of flight can be viewed as the origin and novel association of locomotor modules. Differential elabor ation of these modules in various lineages has produced the diverse lo comotor abilities of modern birds.