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On 4/22/2011 5:20 PM, Jason Brougham wrote:
" ...the forelimbs of alvarezsaurids were greatly abbreviated and were certainly not
used in locomotion..."
pg 115, Chiappe et al., 2002 L.M. Chiappe, M.A. Norell and J.M. Clark, The Cretaceous,
short-armed Alvarezsauridae: Mononykus and its kin. In: L.M. Chiappe and L.M. Witmer,
Editors, Mesozoic Birds: Above the Heads of Dinosaurs, University of California Press,
Berkeley (2002), pp. 87–120.
If Chiappe did not specifically mention or consider the constrained
conditions in which I posit locomotive uses for the forelimbs, then this
citation is not relevant, is it?
On 4/23/2011 4:09 AM, Tim Williams wrote:
But the heavy-duty musculature indicates
the forelimbs were capable of applying a hefty force for some reason.
A locomotive use is logically attractive from the evolutionary
perspective -- it explains both "excessive" musculature and progressive
truncation of the forelimbs, as the generic haplocheirid morphology
specializes to access constrained areas such as nests, burrows, hives,
holes, hollow logs and carcass interiors with the head and neck.
Very short strong arms would be especially useful in extricating the
front half of the body from cul-de-sacs and tight spaces.
Is there a specific mechanical reason this idea has been ignored? Were
these animals completely incapable of moving their arms in such a way
that the forces applied to the trunk were at some angle other than
absolutely perpendicular to the spinal column?
I note that even in that case the arms would still be useful in the
activities mentioned, although their utility would be reduced.
On 4/22/2011 12:52 PM, Jason Brougham wrote:
Now, it may appear that the processes for muscular attachment have enlarged
from the ancestral state but they haven't. Not at all. They have only become
RELATIVELY larger as the rest of the bone became smaller at a faster rate. I
think that this has confused many workers, who may say that the limbs have
become MORE robust when, in fact, they have only become wider relative to
length as, overall, they reduced to a tenth of their ancestral length.
Very interesting -- makes the whole question disappear. I like it, and
thanks for the links. But haven't other theropods (and birds) reduced
their forelimbs w/out creating a similar illusion? And don't such
genetic events necessarily occur within species rather than among them?
I also read the Linhenykus paper this morning and checked the measurements on
the forelimb. Linhenykus weighed the same as a large pigeon (C. livia), 450
grams. It's hand was 17mm long, the same as your thumbnail, and the rest of the
arm was about equal in length, so the whole pectoral limb was the same length
as the last segment of your thumb. Wouldn't it take several kilograms of force
to break the concrete - hard matrix of a termite mound? If so, if those tiny
arms could somehow produce that much force, wouldn't the arm of Linhenykus push
it strongly away from the termite mound before the mound cracked? The little
fellow would be doing pushups!
Your implied point -- "The idea that these animals were using these
limbs as tools to alter external objects is er, um, COUGH!,
unconvincing..." seems unassailable to me...