RE: T-Rex Arms race

[Tim Williams <twilliams_alpha@hotmail.com>]

Andrew Simpson wrote:

> If you look at a rendering of a full grown T-Rex and try to imagine full 
> usable arms they suddenly seem awkward. It's also another few hundred 
> pounds of bulk to support that may have little
> real useful application if the mouth can do what it needs to do.

Just to give some perspective on this.  Obviously _T. rex_ did not use its 
arms to the same degree as other, more longer-armed theropods such as 
_Allosaurus_ or _Velociraptor_.  What we are seeing is different predation 
strategies at work.  The reduction in size of the forelimbs in 
tyrannosaurids certainly implies a reduction in function, and this is 
correlated with changes to the craniodental anatomy.  So the change in 
forelimb size was probably functionally correlated with a larger role of the 
head for grabbing and holding prey.  _T. rex_ did not use its forelimbs to 
grab prey - they were far too short (of course).  But the forelimbs might 
have been brought into play when the struggling prey didn't have much fight 
left in it, courtesy of those powerful jaws.

> Snakes and sharks (and all fish) get by without arms.

Their limbs are fully occupied swimming, just as it was for their ancestors.

> They might have been a hinderence once in adulthood as They might have been 
> easy to rip off
> and thus cause horrid wounds. Smaller limbed adults might have been more 
> successful and so the evolutionary change.

I would say that the reduction of the forelimb in tyrannosaurid forelimb 
concerns a changeover in function rather than a total loss of function.  If 
the forelimbs were vestigial, why did the muscles or upper arm bones not 
atrophy?  The forelimbs were so short that they could probably be kept out 
of the way when _T. rex_ was lunging ang seizing prey.

> Mate grabbers if anything.

Mating claspers is one hypothesis, but as far as we can tell both sexes 
retained forelimbs.  Then again, maybe tyrannosaurs were sexually 
adventurous.  :-)

Cheers

Tim