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RE: tiny-armed theropods
I'd like to interject a little bit here to note a small thing:
It is actually unreasonable to show that the reduction of arms simply reduces
mass over all to allow greater pitch alleviation during accelleration, when
some of these taxa are expanding the size and mass of their heads and necks in
comparison. This is most evident when considering that the scapulae of
smaller-armed theropods tends to be more robust and larger (and fused!) then
those of larger-armed theropods. This is evident especially in the two key
clades being discussed here, *Abelisauroidea* and *Tyrannosauroidea*.
Slender, "light" crania and neck structure with larger arms tends to contrast
directly with massive, broad or exagerrated skulls, robust necks, and smaller
arms, while the scapulocoracoid is developed inversely to the arms. In
dromaeosaurs, this may be comparable to the slenderness and gracility of the
scapula relative to the shortness and robusticity of the arm, especially in
regards to the humerus, while in animals with "normal" arms, the shoulder is
"normal." *Velociraptor mongoliensis* versus *Mahakala omnogovae,* or more
rather, predicting that *Rahonavis ostromi* will have a gracile skull to
contrast is huge arms, while all other unenlagiine dromaeosaurs have long,
robust skulls and short, puny arms. Shoulder anatomy directly influences
basal-cervical musculature and the support of the head entire to the body, as
well as reinforcing the anterior body cavity, aspects that may be directly
related to higher use of the head during acquisition than the forelimbs (which
require higher mobility that is being lost).
Note also that in some taxa being discussed, the legs are very short and
robust (abelisaurs) or where seemingly adapted to short-burst movement
(tyrannosaurs, abelisaurs?, *Balaur*), indicating anatomy that favors not
pursuit runners, but ambushers, where momentary bursts of speed are favored,
and balance is less of an issue. Hutchinson has given a few talks and presented
a few papers in which it is modeled that tyrannosaurs should work better in
some cases at a crouch, and this implies (at least) that a typical tyrannosaur
at large size would not be galloping around the landscape hitting things with
its head; it would be lunging, banging, and likely bashing into things with
jaws and skull designed to withstand massive torsion and compression, and it is
simply, indeed, to consider that they were rather more ambush predators than
anything else. Construction of the rest of the body (especially the shoulders)
support this model simply by enforcing the prehensile organ of choice (the
jaws) while minimizing the non-prehensile arms.
Jaime A. Headden
The Bite Stuff (site v2)
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion
> Date: Fri, 7 Oct 2011 13:17:05 -0400
> From: firstname.lastname@example.org
> To: email@example.com
> Subject: Re: tiny-armed theropods
> On 10/7/2011 12:38 PM, Jason Brougham wrote:
> > I see, so you are suggesting that all theropods would benefit from
> > reduction of their pectoral limbs if they could find a way to do without
> > arms in feeding and brooding and whatnot, because it would make them more
> > agile.
> Not quite. I am observing that such reduction would allow "more"
> acceleration (*not* agility) in theropod taxa generally, given the base
> anatomical assumptions (low and cranial CM and a balance-beam biped).
> Given the wide spectrum of viable lifestyles, I would not expect extreme
> accelerative ability to be an overwhelming factor in all theropod species.
> > It's just that dispensing with the arms' uses must have been impossible for
> > many theropods, possibly for a large or even an innumerable set of
> > interacting reasons.
> That is true. Potential selective compromises abound, and appear to me
> to have occurred. And many theropods did indeed reduce their arms, and
> apparently their crania where possible...
> Makes that big old T. rex head even more interesting.