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Re: Evolution of tyrannosauroid bite power



My engineering muses predict that _within_ mechanically analogous
terrestrial bipeds, as size increases, structural issues arise, and
mass re-allocation (mass flowing from high to low) must result. 


Within the sequence comprised of T.rex and its ancestors, they (and I)
concluded that an absolute size limit was probably being approached as T. rex 
evolved because mass
re-allocation appeared to be occurring, and further that a relatively large and
powerful
head was absolutely essential to life style, because it is relative arm
length that decreased (even while the arms remained strong), rather than 
relative head size.

Other massive short-armed theropods supported this conclusion.


On the other hand --




You seem to be saying that short relative arm length in T. rex could well be 
more a matter of 'convenience' than any
size-related necessity for mass re-allocation, and that there is no
bio-mechanical reason T. rex could not have had significantly longer
relative arm length, _and_ kept it's massive head, had it found long arms 
advantageous
in it's daily life. 



Or, re-stated, no size constraints were approached, longer arms weren't
needed, extremely short arms were really quite handy for T. rex, and
Dryptosaurus is enough of an outlier (compared to "derived
tyrannosaurids") to support that idea. Further, the morphology of
therizinosaurs is more evidence that mass re-allocation due to
size-constraint was not occurring, ie, there was plenty of room left to grow 
(to 9000 kg, at least, according to wikipedia). 

Is the above a fair reading of your views? 

If so, because the issue of
size-limits constraining allometry within designs is one I find critical, I 
have questions --



Do you think the data
exist to do a reasonable job of plotting arm length and skull length vs body 
mass in carnivorous theropods generally? If the data exists, I would like to do 
so. 



Or has it been done? If so, could I please have a reference(s), as
I intend to make the arduous, time-consuming, expensive, technologically 
unnecessary and possibly futile trip to the
library soon, anyhow? 


In any case, could you clarify somewhat as interlineated below, when you have 
time? 


Don 

----- Original Message ----
From: Tim Williams <twilliams_alpha@hotmail.com>
To: dinosaur@usc.edu
Sent: Sunday, December 30, 2007 2:16:16 AM
Subject: RE: Evolution of tyrannosauroid bite power


Don Ohmes wrote:


>
> I'm sorry; I should have written "tyrannosaurids" instead of
 "theropods". "Theropods" does cast 
> the net a little wide ... my mistake.

No problem.  Even here, in the Tyrannosauridae, I think the situation
 is more complicated.  For example, _Dryptosaurus_ (a late-surviving
 basal tyrannosaur) was fairly large, 

==> 6.5 m, 1.8 tons -- from wikipedia, but arm length = ?

but its forelimbs were not nearly as
 reduced as those of derived tyrannosaurids.  

==> "derived t's" equals what species? length, weight, arm length = ?

Among tyrannosaurs, 

==> tyrannosaurs = what species?

the
 highly abbreviated forelimbs appears to be restricted to the latter clade, 
which took this trend to the extreme.

Cheers

Tim


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