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Re: Evolution of tyrannosauroid bite power



One more shot and a question from the peanut gallery; suppose there is a guy 2m 
tall that has arms that appear very strong, but are only 20 cm long. Does one 
assume that longer arms would be less useful than short arms, and that he would 
refuse longer arms if they were offered? Not likely. The likely assumption is 
that he can't _get_ longer arms for reason(s) not related to their potential 
usefulness. 

Carpenter (et.al.)'s idea (from D. M.) that the arms became shorter to become 
stronger is so
 counter-intuitive I am tempted to accept it simply on grounds of
 Universal Irony. However, I cannot conceive of anything a 1m meathook can do 
that
 a heavier, 1.5m meathook couldn't do a little better, except stay out of the 
way on a 20 km
 hike. A classic selective compromise between general scaling trends
 magnified by an active locomotive lifestyle (resulting in arm reduction),
 and general arm usefulness (including or even mainly predation), is
 still best-fit for strong-yet-extremely-dinky arms on very-large-and-active 
bipeds in my personal cartoon. 

However, Carpenter, et al, have definitely shown that the arms of the big old 
thing were such that they would have been more useful to "T. rex the predator", 
than to "T.rex the scavenger". I personally don't see it as _definitive_ proof 
of predatory lifestyle, but it is at least strong support for the assumption. 
Not that I am a 'scavenger-ian', anyway.

So anyhow, on to my main question -- for those that _do_ accept that Carpenter, 
et al, has supplied definitive proof of a predatory lifestyle; can Carnotaurus 
and others with extremely-dinky-and-also-very-weak arms now be safely accused 
of being 'dumpster-divers'? 

Don

----- Original Message ----
From: Michael Habib <mhabib5@jhmi.edu>
To: DML <dinosaur@usc.edu>
Sent: Tuesday, January 1, 2008 1:06:55 AM
Subject: Re: Evolution of tyrannosauroid bite power

> 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?

All good questions.  So either tyrannosaur arms are not entirely 
vestigal, or there is some unusual constraint or developmental 
mechanism that results in a unique morphology under extreme reduction.
  
After all, the forelimbs of Carnotaurus and Aucasaurus don't look much 
like that of a ratite, so it appears that we already have two reduction
 
morphologies; why not three?

>> Ratites, for example,
>> have relatively thick-walled humeri.
>
> Which are very long and thin.

Absolutely; but the humeri of abelisaurids with reduced forelimbs are 
not long and thin.  So, are short, stout forelimbs always a sign of 
high loads, or can they be the result of particular patterns of 
vestigal reduction?

> 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.

True, it could be.  Which is one reason that the question gets 
complicated :-)

>
>> 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?

Actually, my understanding is that the coracoids are relatively larger 
(compared to body dimensions) than in many species with more robust 
forelimbs.  The plot thickens...

>> 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.

But do we know what vestigal arms really should look like?  
Intuitively, we would tend to expect gracile little splints (and this 
is what tends to happen in birds), but intuition may be leading us 
astray on this one.  The arms of Carnotaurus and kin certainly look 
very different, but they also are not the thin-shafted splints that we 
might more easily identify as vestigal structures.  So, the possibility
 
remains that some historical constraints/developmental systems can 
produce stout vestigal long bones.  I'm not arguing that I'm convinced 
either way, merely that it's difficult to say with confidence because 
extreme reduction may impose unusual morphological constraints and 
patterns of change.  There is also the somewhat "middle road" 
possibility, which is that the arms had little remaining use in 
tyrannosaurs, but were still subjected to impact loads during prey 
capture, and thus were under selection to be strong enough not to 
shatter terribly and lead to infection.

Cheers,

--Mike H.


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu