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Re: I knew it!
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
manner. In more recent years a number of researchers have argued that most
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
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
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.
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."
Having not yet seen this article and figure, I cannot comment intelligently
"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."
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
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
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
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