Confuciusornis
The orientation of the shoulder joint was sideways, instead of angled upward as in modern birds; this means that Confuciusornis was unable to lift its wings above its back. Like Archaeopteryx, it was thus incapable of the upstroke required for flapping flight. This conclusion is not established at present. Peters sees Confuciusornis as capable of flapping flight but specialising in soaring flight. The bird's weight has also been debated.