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Re: Evolution of tyrannosauroid bite power



Look, there is just no wiggle room here; re-allocation of mass has to occur. 

One may argue numbers, but the phenomenon is inescapable, as is gravity. Given 
constants such as materials and gravity -- as a "balance beam" enlarges, the 
upright will necessarily be comprised of a larger percentage of the total mass. 
Period.

Further, once optimal mass distribution has been achieved at any given mass (as 
will occur in evolutionary scenarios), the relationship is continuous and 
non-linear as size increases.

If lifestyle requires a head of a certain absolute size, then the mass has to 
come from somewhere else, once minimum head size is reached. Period. No wiggle 
room whatsoever. Note that a gram of arm is "worth" less than a gram of head 
due to decreased leverage (ie, distance from the balance point). Therefore, arm 
reduction is accelerated relative to reduction of head size (such as might 
occur in  herbivores). 

Don

----- Original Message ----
From: Graydon <oak@uniserve.com>
To: dinosaur@usc.edu
Sent: Sunday, December 30, 2007 10:01:30 PM
Subject: Re: Evolution of tyrannosauroid bite power

On Sun, Dec 30, 2007 at 05:12:20PM -0800, don ohmes scripsit:
> Re-allocation of mass is what answers all three questions. The load
> had to be lightened somehow, and the arms were what could be spared,
> due to lifestyle. Just a cartoon, though, not currently testable (I
> think), other than with logic. Ask me what the arms were useful for,
 I
> promise a fuzzy answer neither critically involving, nor totally
> eliminating, predation. Getting up comes to mind ...

Bipedal dinosaurs are balance beams; if it's really important to have
the mass up forward, you can always accept being a bit slower and
 having
a larger tail to balance forward mass.  So the idea that there was
compelling selection for lighter forearms doesn't have obvious merit,
especially since T. rex arms are highly derived relative to the
ancestral tryannosauroid condition.

In the case of T. rex forearms, they're not, proportionately, strong;
very strong for their size, but not strong for the size of the animal
 or
the probable prey animals.

One probable reason for bite-and-leg dominance in T. rex predation is
that they *are* big animals; having forelimbs large enough to be useful
in direct predation would require very large and powerful forelimbs.

So instead they have dinky forelimbs that clearly underwent
proportionately high loads.  Could be mating related (but then we'd
expect to see disproportionate development/injury in males) or it could
be a prey position sensor.  It more or less couldn't be a 'hold the
prey' mechanism because even a thousand pounds of force isn't worth
anything when a five ton Edmontonosaurs thinks it's about to die.  It
can generate *lots* more force than that with its legs.  But it *could*
be a 'where is the Edmontonosaur I've just crashed into, bit once, and
which is now a)under my head/out of my direct line of vision and
b)vigorously concerned to get away.

-- Graydon