Retention of the flight-adapted avian finger-joint complex in the Ostrich helps identify when wings began evolving in dinosaurs

Abstract

The functional anatomies of avian finger joints have never been compared with those of the basal avian Archaeopteryx lithographica or maniraptoran theropod dinosaurs. These startling oversights are due to unfamiliarity of the joints outside of highly specialised studies of bird-wing biomechanics. Here fleshed to skeletonised repeated-measures analyses of finger-joint ranges of motion (ROMs) in a basal palaeognath, the Ostrich Struthio camelus, were utilised as a model to begin filling in these knowledge gaps. Results verified that all of the flight-adapted, avian-specific finger joints are retained in the flightless, yet large and well-differentiated fingers of the Ostrich. Moreover, Ostrich finger joints still possess flight-restricted ROMs within the semi-pronated plane of the manus. The osteological markers of these highly conserved flight-adapted characters in Ostrich show that prior descriptions have unknowingly demonstrated that the main avian finger-joint complex began evolving at the base of Pennaraptora (Oviraptorosauria + Paraves). This detailed functional analysis provides non-embryological support for the hypothesis that maniraptoran theropods shared digital homologies with birds, and supports recent pronation studies indicating that pennaraptoran theropods had evolved avian distal wing joints stiffened for aerial locomotion.