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Re: Evolution of tyrannosauroid bite power

----- Original Message ----- From: "Graydon" <oak@uniserve.com>
Sent: Monday, December 31, 2007 4:01 AM

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 ...

The almost complete lack of a range of motion of the arms makes that particular explanation impossible. Newman (1970) has the poor animal pronating its forearms and hyperextending its wrist... a partial tendon avulsion is preferable!

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.

But why would a mere sensor have this kind of claws that converge towards each other so that anything the claws have been stung into can only escape if it rips its own flesh apart, if it rips the tyrannosaur's hand apart, or if the tyrannosaur graciously decides to pull the claws out itself? Wouldn't that be selected against?

----- Original Message -----
From: "Michael Habib" <mhabib5@jhmi.edu>
Sent: Monday, December 31, 2007 4:22 AM

Abstract: "[...]"

An interesting set of observations, but I see some possible flaws in their conclusions. [...] For example, one reason that the small limbs have high bending strength is that their short length gives them a small moment arm. In some animals, such shortening is an adaptive trend to increase bending resistance, but in this case it could be a side-effect of how non-avian theropods reduce forelimbs. There is a tendency for the length to reduce to a greater degree than the breadth, forming short, stout forelimbs. Such stout limbs are strong, technically speaking, but it may be some kind of intrinsic effect.

But why should it be? Have a look at the table I posted 10 hours ago. Why can't a humerus as long as mine be as thin as mine and have as laughable muscle attachment sites as mine? Why can't a radius or ulna half as long as mine be half as thick as mine? Why does a 2nd metacarpal about as long as mine need to be as broad at the distal end as my articulated mc II and III together? And what about *Carnotaurus* and *Aucasaurus*, where the humerus _is_ more like mine and the forearm and hand are forgettable?

Similarly, while thickened cortical bone often indicates
selection for high ultimate strength, thickened cortical bone may also
be a result of secondarily reduced limb breadth.  Ratites, for example,
have relatively thick-walled humeri.

Which are very long and thin.

The second problem is a more kinematic one: as best I can tell, there
is no way for a large tyrannosaur to hold struggling prey and engage
the jaws at the same time.  The neck is simply not long or flexible

But the prey could be big enough. :-) In fact, I have a poster of Brian Franczak's 1991 painting of three *T. rex* attacking an *Edmontosaurus*. One of them bites into the upper half of the tail at a point about a third of the length of the tail behind the legs, and the left arm only escapes being rammed into the ventral tail musculature by being impossibly pronated and perhaps impossibly extended.

It is also worth noting (and I cannot take credit for this observation)
that many of the taxa with very reduced forelimbs actually have quite
large coracoids.

True, but not so large in comparison to body size, right?

So, the first question is, "why were the arms so little (1m)?" The
second question is, "why were they still useful (ie, not vestigial)?"
The third question "If they were useful, why weren't they bigger?"

The only caveat to throw in here is that the arms may, in fact, have been vestigial after all (see above).

They really, really, really don't look like vestigial arms -- neither what one should expect a vestigial arm to look like, nor what the arms of *Carnotaurus* and *Aucasaurus* actually look like.