That’s what the slappy hands look like to me.

Incidentally, the ancestral condition is the clapping one, and it is preserved in many animals. Why do sauropod go to so much trouble of shifting the radius fully to the front of the ulna (Matt Bonnan’s papers comes to mind)? And then there is Plateosaurus, wholly incapable of pronation to any significant degree. And the Otozoum track with the supinated hands….. Plenty of evidence.

Bonnan, M.F. 2003. The evolution of manus shape in sauropod dinosaurs: implications for functional morphology, forelimb orientation, and sauropod phylogeny. Journal of Vertebrate Paleontology, 23(3): 595-613.
Bonnan, M.F. and A.M. Yates. 2007. A new description of the forelimb of the basal sauropodomorph Melanorosaurus: implications for the evolution of pronation, manus shape and quadrupedalism in sauropod dinosaurs; pp. 157-168 in Barrett, P. M. and D.J. Batten (eds.), Evolution and palaeobiology of early sauropodomorph dinosaurs. Special Papers in Palaeontology, 77.
Bonnan, M.F. and P. Senter. 2007. Were the basal sauropodomorph dinosaurs Plateosaurus and Massospondylus habitual quadrupeds?; pp. 139-155 in Barrett, P. M. and D.J. Batten (eds.), Evolution and palaeobiology of early sauropodomorph dinosaurs. Special Papers in Palaeontology, 77.

more Plateosaurus
http://www.app.pan.pl/article/item/app20090075.html

Otozoum:
E. C. Rainforth. 2003. Revision and re-evaluation of the Early Jurassic dinosaurian ichnogenus Otozoum. Palaeontology 46(4):803-838
(hope that is the right one, I am not able to access my PDF collection right now).

Senter P, Robins JH (2005) Range of motion in the forelimb of the theropod dinosaur Acrocanthosaurus atokensis, and implications for predatory behaviour. J Zool 266: 307–318.

It’s not that I want to draw them slapping, since I’ve way most often drawn them with palms facing in–better to GRAB with!!!!–but broad claims like this are better served by proof, rather than some photos of a few tracks.

While older illustrations used to depict dinosaurs with “flapping” hands, I’ve noticed that more recent ones, perhaps increasingly more commonly after the Chinese fossil “boom” of the late 90s, they were frequently drawn with “clapping” hands, which I thought to be motivated by the increase in popular knowledge (at least to the extent it can be said to be popular) of the link between dinosaurs and birds, and the consequent more bird-like depictions of theropod dinosaurs. Perhaps even a mistaken intuition, when using birds as a reference, assuming/guessing/betting on a trait to be ancestral when it might be more bird-specific (it’s not uncommon to see that, like pterosaurs with yellow beaks (which I think is actually worse), even though they actually nailed regarding the feathers in some dinos). Actually, I think I recall reading G. S. Paul pointing something related with the hand positioning as an argument in favor of his “dromaeosaurs as secondary flightless birds” theory; if I’m not misremembering it, and I might well be, the clapping hand would be a bird-specific adaptation to flight, retained in related flightless birds thought to be “just” dinos.

Whatever were the explanation, secondary flightless dinos or just some exaptation, I was almost “sure” that what I would find when I looked about more deeply was that the “slapping” position was primitive (after all, they came from quadrupeds at some point), guessing that it wouldn’t be much unusual in theropods, and that the “clapping” position would be more specific to the ones closer to birds/potentially flightless birds. I just wanted to look about how exactly paleontologists know that, and when in the phylogenetic tree the change happened. How surprising to find that the reality may be almost as close to the complete opposite as it could be.

I’ve been aware of this position for procompsognathus and all such, but does this apply to the large carnosaurs… particularly TRex. He’s always represented with two tiny dribble-fingers!