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RE: Triassic dinosaur evolution



Mark Witton wrote:


> I really, really don't have time to write this, but the heads of allosaurs, 
> dromaeosaurs and a bunch of other 'saurs ain't build for holding struggling 
> animals. 
> They may get there first, but theropod arms are surely going to be holding 
> things in place while sickle claws, raking jaws or whatever do the dirty work.
> Noteable exceptions to this, of course, as tyrannosaurs, which appear to have 
> jaws that both hold and dispatch prey. Maybe that's why their arms were 
>comparatively diminutive. 


This is backed up (among other studies) by the work of Senter and Robins on 
forelimb motion in _Acrocanthosaurus_.  The anterior reach of the forelimbs is 
fairly limited, so the initial contact with large prey would have been via the 
jaws.  Then, the forelimbs were rapidly deployed to grasp and subdue the prey - 
so it's the forelimbs that do the 'heavy lifting' (quite literally).  Againt 
smaller prey, the forelimbs could perhaps grasp first; but this means that the 
carnosaur be actually leaning over the prey, which requires that the prey be 
either very slow or very stupid - or very dead.


Mike Habib wrote:

> And, of course, there is the arm reduction
> weirdness in several abelisaurids...


Yep, the arms of abelisaurids like _Carnotaurus_ and (to a lesser 
extent)_Aucasaurus_ were too small to be used for anything.  Thus, unlike 
tyrannosaurids (where the forelimbs probably played some role in securing prey 
that was already held by the jaws), their forelimbs were nonfunctional (based 
on Senter & Parrish, 2006).  So for _Carnotaurus_, the capture and dispatching 
of prey must have been carried out by jaws and/or hindlimbs.


Cheers

Tim
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