Aerodynamic Model for the Early Evolution of Feathers Provided by Propithecus (Primates, Lemuridae)

The initial selective context for the evolution of feathers has remained enigmatic, and recent research has focused inappropriately on the first known bird, the late Jurassic Archaeopteryx, which already had a fully developed, modern avian wing. Thus, speculative preadaptive scenarios have been envisioned to explain the initial evolution of feathers, with scarce attention paid to their primary role as aerodynamic structures. The lemurs known as sifakas, with their "gliding membranes" and rearward projecting brachial hair, provide a crucial, yet to date overlooked, analogy that illustrates parsimoniously how feathers could have evolved directly from brachial scales as aerodynamic features, with each successive minor evolutionary stage fully adaptive.