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Re: Vestigial Arms != Big Jaws (Was: Theropod limbs - how mobile?)

On Friday, May 17, 2002, at 12:20 AM, Mike Taylor wrote:

... so that more mass can go into the head and particularly the jaws,
yes.  That's what I assumed.  Makes perfect sense.

I think this might be getting cause and effect confused. It was the other way around. Tyrannosaurids were evolving larger and more powerful jaws, the result of which was less reliance on forelimbs for predation. The arms were then "free" to reduce in size.

The fly in this particular ointment, to my mind, is _Carnotaurus_.
This critter has ludicrously attenuated forearms -- I mean, it makes
_T. rex_ look like _Deinocheirus_.  Yet its head is also relatively
puny: it has one of the shortest heads (relatively) of all theropods.

Carnotaurus is a very strange critter. It is best to remember that other abelisauroids had normal (or large?) heads. Carnotaurus had secondarily reduced the size of its head, and it's arms might already have been too atrophied to re-enlarge them.

And worse, it has a long trunk and neck.  All that precious body-mass
wasted when it could have gone into the jaws!

I don't think it was so much a balance issue (being front-heavy), as you imply. If arms are no longer useful, then it is better to reduce them; not to save weight (although it might be a factor), but to save the energy to used grow them, and to get them out of the way.

So I think the simple smaller-arms-for-a-bigger-head model,
intuitively appealing though it appears, is flawed.

True, it was either a bigger-head-then-reduce-the-arms, or a reduce-the-arms-because-they're-useless-anyway, scenario.

John Conway, Palaeoartist

"All art is quite useless." - Oscar Wilde

Protosite: http://homepage.mac.com/john_conway/
Systematic ramblings: http://homepage.mac.com/john_conway/phylogenetic/