Updated November 5, 2015 with a new interpretation of the pectorals,a new nesting and images of Eosinopteryx and Xiaotingia.
Sometimes it just helps
to see taxa to scale with possible sisters (Eosinopteryx in this case, Fig. 1). Smaller than Aurornis, Eosinopteryx also had a shorter snout, shorter torso, shorter tail, more robust clavicle and shorter pubis.
Godefroit et al. (2013) reported, “The straight and closely aligned ulna-radius of Eosinopteryx also means that pronation/supination of the manus with respect to the upper arm would have been limited; combined with the absence of a bony sternum and weakly developed proximal humerus, these attributes suggest that Eosinopteryx had little or no ability to oscillate the arms to produce a wing beat.”
Funny that they didn’t even mention the short coracoid.
The locked down elongate coracoid is a hallmark of flapping tetrapods (pterosaurs and birds) and an elongate clavicle does the same thing in bats.
The ulna is not bowed
in Aurornis or Eosinopteryx. It is bowed in Xiaotinigia and Archaeopteryx and more greatly bowed in subsequent flapping taxa, including oviraptorids by convergence. The coracoid is strut-like and locked down to the sternum in Xiaotingia and Archaeopteryx, perhaps by convergence because overall Archaeopteryx has proportions more similar to Aurornis.
The bowed antebrachium
produces a parallelogram in living birds that serves to automatically extend and fold the manus bearing the outer flight feathers with flexion/extension of the elbow. Prior to this, muscle power would have to extend and bend the wrist, independent of the flexion/extension of the elbow.
Godefroit P, Demuynck H, Dyke G, Hu D, Escuillié FO and Claeys P. 2013. Reduced plumage and flight ability of a new Jurassic paravian theropod from China. Nature Communications 4: 1394. doi:10.1038/ncomms2389