Re: Evolution of tyrannosauroid bite power

[Michael Habib <mhabib5@jhmi.edu>]

> 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