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I knew it!
I used to illustrate my bipedal dinosaurs with the hands facing palm
backwards. In this arrangment the radius and ulna are crossed in the normal
manner. In more recent years a number of researchers have argued that most or
all dinosaurs could not cross the radius and ulna, in which case the palms
faced inwards unless the elbow is sprawled laterally so the palm can face
backwards or downwards. Restorations showing the hands facing backwards have
been derided as bunny rabbit poses.
I've always been skeptical about this because it makes no functional sense
for animals that used their hands for manipulation to have such limited lower
arm and hand rotation.
In Kobayshi and Lu's new description of complete ornithomimid Sinornithomimus
specimens they restore the skeletons with the hands in the bunny pose. Was
figuring they were going to get in trouble with the authorities on this. Until
wandering through the text and photos when I came across the articulated
humerus, radius and ulna, which as they observe clearly show the radius and
strongly crossed with all joints in full articulation.
A Gallimimus specimen shows the articulated radius and ulna uncrossed. So
these theropods could rotate the radius and ulna. Probably ~90 degrees. A
functional adaptation. But less than humans which can rotate the hand 180
degrees so the palm faces forwards.
The hadrosaur radius and ulna can only articulate crossed, which is why the
palms face strongly backwards in trackways. So the notion that all dinosaurs
had permanently uncrossed lower arms is incorrect. Am very suspicious of
arguments that this was true of any dinosaur, except perhaps those with such
arms that it does not matter.