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Re: I knew it!

Hi Greg:

G. Paul said:
"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 tetrapod
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."

This last instance you describe is indeed what we see in crocodylians and birds -- neither are capable of actively rotating their radius about their ulna, although some proximodistal sliding motion can occur in both crocs and birds. Because digit I (thumb) aligns with the radius and digit V (pinky finger) aligns with the ulna, if the radius does not or cannot cross the ulna, the palm of the hand will face up or inwards. Crocs "solve" this problem by having a sprawled stance in which orienting the humerus more horizontally orients the forearm such that the radius is now medial and the ulna is posterior and lateral, which in turn allows the hand to be planted palm-side down. Birds cannot actively pronate their hands either -- but this doesn't matter because they're using their hands as part of wings. If a bird brings its forelimbs together at the midline of its body, the palms of the hands face inwards.

"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."

But do we know that they actually did use their hands for manipulation to such an extent that full cross-over of the radius and its active rotation about the ulna were necessary? If theropods had manipulative hands but little forearm rotation, what might this functionally suggest? Were manipulative hands even necessary, and how does one determine this? For instance, compare just the wrist, metacarpus, and phalanges of a dromeosaur and some living vertebrate with manipulative hands side by side (e.g., Homo sapiens, chimps, squirrels, etc.) -- are the hands similar in some functional ways? I don't know, that's why I'm asking.

"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 ulna
strongly crossed with all joints in full articulation."

Having not yet seen this article and figure, I cannot comment intelligently on this.

"A Gallimimus specimen shows the articulated radius and ulna uncrossed. So
these theropods could rotate the radius and ulna. Probably ~90 degrees. A logical
functional adaptation. But less than humans which can rotate the hand 180
degrees so the palm faces forwards."

Whoa! Hold on there! The reason that some paleontologists, myself included, are skeptical of the radius crossing the ulna actively to pronate the manus in many dinosaurs is because this is not supported by the bony morphology. If one looks at the proximal end of the radius and ulna of most dinosaurs, you will soon notice the head of the radius is not circular as it is some mammals, but is instead sub-oval, oval, or even angular. It lies snugly against a flat articular face on the ulna, so it is difficult to comprehend how such a radius would rotate against the ulna. Furthermore, the "capitulum" or radial condyle of the humerus in most dinosaurs is not well-rounded as it is in mammals such as ourselves. Instead, it is typically much more shallow and lip-like -- not what one would predict if the radius were capable of movement at the elbow, but suspiciously similar to what we see in crocs and birds -- animals that don't actively pronate their hands.

This does not mean that the hands of many bipedal dinosaurs were not capable of grasping or manipulating items, but it does not follow that in order to do this they had to actively rotate their radius about their ulna, or even needed to simply cross the radius over the ulna.

"The hadrosaur radius and ulna can only articulate crossed, which is why the
palms face strongly backwards in trackways."

Not having examined many hadrosaurs personally, I cannot comment on this either. However, dinosaurs as a group seem to show less developed pronation. Rather than trying to make them "pronate" their hands by arguing that it was adaptive for manipulative hands, maybe we should consider other functional scenarios that are supported by the bony evidence. Like I said, I have not seen the dinosaur Greg speaks of, and of course there are exceptions to every rule. But based on my experience, which has mostly been with saurischians, the bony anatomy just doesn't seem to allow a lot of cross-over or active rotation of the radius about the ulna in most of them. This is not just an opinion -- this is how the bones actually fit and articulate with one another in the specimens I've examined.

"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 reduced
arms that it does not matter."

And that's where we will disagree. I'm sure you have your own thoughts, Greg. This topic is always so interesting to me -- what were the dinosaurs doing with their forelimbs???


Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
(309) 298-2155

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