Very good point. It's a plesiomorphy. The extant crocodiles can't pronate
either -- they just sprawl enough that walking quadrupedally works in spite
of that. Sauropodomorphs other than sauropods at least as derived as
*Antetonitrus* were incapable of forearm pronation, too, and accordingly
must have been obligatorily bipedal, too, as was recently published. Forearm
pronation is an autapomorphy of Ornithischia, and of the "silesaurids" (if
they aren't ornithischians in the first place), and presumably of various
non-sprawling crurotarsans as well.