The furcula and the evolution of avian flight
The presence of a short furcula in Archaeopteryx suggests that this bird possessed a small, shortfibered, cranial portion of the pinnate m. pectoralis originating from the furcula and possibly from the aponeurosis between the furcula and the coracoid and cartilaginous sternum, and inserting on the cranial edge of the humerus, and an equally small, short-fibered pinnate caudal part of the same muscle arising from the presumably cartilaginous sternum and inserting on the ventral surface of the deltoid crest of the humerus. In Archaeopteryx, the cranial-most portion of the m. pectoralis protracted the wing and held it in place against the backward pressure, or drag, of the air when the bird flew. There is no basis for postulating that the caudal part of the m. pectoralis in Archaeopteryx was sufficiently large for active flapping flight, although this presumably small muscle probably held the wings in a horizontal position necessary for aerial locomotion. The muscle fibers of all parts of the m. pectoralis were short because the small distance between its origin and insertion. The combination of features in the pectoral system of Archaeopteryx indicates strongly that this bird was a specialized glider, not an active flapping flier. Avian flight started from the trees downward, not from the ground upward.