Complex sentence: He argued that early birds, including "Archaeopteryx", lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes; since the downforce on which WAIR depends is generated by upstrokes, it seems that early birds were incapable of WAIR.
Split into simpler sentences: He argued that early birds, including "Archaeopteryx", lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes. Since the downforce on which WAIR depends is generated by upstrokes, it seems that early birds were incapable of WAIR.