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



You wrote:

>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.
>
That certainly makes a lot more sense. If you reduce the arms before you
have a evolved a "sledge-hammer" head, there is no advantage by the means of
killing another dinosaur.
>
>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.
>
Too bad not so many Abelisauroids are known from a skeleton with a skull,
just Carnotaurus, "Aucasaurus" and Majungatholus. Do you see the way I'm
getting at? Abelisaurinae skeletons are more fragmentary and contains either
just a skull (Abelisaurus) or postcranial material (Indosuchus), so the
exact proportions are not known in this group. And just Carnotaurus
secondarily reduced the size of it's head, it is more general in the
Carnotaurines, for instance, in Majungatholus, so the shortening of the
skull (and probably the reduction of the hand) had something to do with
their unique lifestyle. Unfortunately no evidence is available for this
lifestyle, but I surely doubt the lifestyle described in the novel "the Lost
World." :)
>
>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.
>
Saving weight was probably secondary, it is just the function of the arms
were serving were taken over by the skull and that is holding ánd killing.
Those smart Carnosaurs and Tyrannosaurs whacked two flies in a single blow!