RE: T-rex arms

[Tim Williams <twilliams_alpha@hotmail.com>]

Dann Pigdon wrote:

> I have trouble with this use for the forelimbs. I don't care how strong the 
> forelimb muscles of a 'rex were - surely trying to hold back a struggling 
> multi-tonne hadrosaur (or worse, a sauropod!) would rip an arm from its 
> socket?

Here's what Carpenter and Smith (2001) have to say:

Abstract: "Although proportionately the forelimb is very small, the 
mechanical advantage reveals an efficiently designed force-based system (vs. 
a velocity-based system) used for securing its prey during predation.  In 
addition, the M. biceps is shown to be 3.5 times more powerful than the same 
muscle in the human, the straight, columnar humerus provides maximum 
strength to mass ratio to counter the exertion of the M. biceps, and the 
thick cortical bone indicates bone selected for ultimate strength. Such 
mechanical adaptations can only indicate that the arms were not useless 
appendages, but were usted to hold struggling prey while the teeth 
dispatched the animal. _Tyrannosaurus rex_ was therefore an active predator 
and not a mere scavenger, as has been suggested."

Janice Boyd wrote:

> I don't understand why people fret over those little T-rex arms.  Terror 
> Bird (Phorusrhacos longissimus) didn't have arms for grasping.

You're right.  But I can offer two things in response to that.  Firstly, 
phorusrhacids are secondarily flightless birds and therefore evolved from 
flying animals in which the forelimbs (wings) became dedicated to flight.  
Secondly, although _Phorusrhacus_ did not use its forelimbs for predation, a 
related species (_Titanis walleri_) did have arms that might have been used 
for grasping.  The arms of _Titanis_ may have played a role in predation 
(albeit minor, probably).

> If you have a big mouth maybe you don't need arms to be a successful 
> predator.  Why couldn't the T-rex arms have been on their way to becoming 
> vestigial and really have had very little purpose, like hind legs on 
> primitive whales?

See above.  The arms were certainly small, but this does not mean they were 
useless, or even weak.  Also, the tyrannosaurid jaws still did most of the 
hard work to sieze and dispatch prey; the forelimbs just helped.

> If there is a good reason for worrying about T-rex arms, please explain 
> (besides:  "well, they're there")

Well, they *are* there.  And these arms were endowed with a hefty 
musculature, strong bones, and were tipped by two sharp claws.

Cheers

Tim