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Re: tiny-armed theropods

David Marjanovic <david.marjanovic@gmx.at> wrote:

> ...although, in ornithomimo- and segnosaurs alike, the head + neck is still
> longer than the forelimbs.

The necks of ornithomimosaurs and therizinosaurs are decidedly long.

And yeah... I also prefer the name 'segnosaurs' over 'therizinosaurs'.
 ;-)  But I guess the latter has priority.

Jason Brougham <jaseb@amnh.org> wrote:

> I see, so you are suggesting that all theropods would benefit from reduction 
> of their pectoral limbs if
> they could find a way to do without arms in feeding and brooding and whatnot, 
> because it would
> make them more agile.

This was the essence of Carrier et al's (2001) paper on rotational
inertia in theropods (J Exp Biol. 2204: 3917-26).  The resulting
agility while turning might also have driven the evolution of the
forelimb-folding mechanism in maniraptorans, conferred by the
semilunate carpal joint.  (I don't buy GSP's notion that the
semilunate carpal evolved as a flight-related/flapping feature.)

> It's just that dispensing with the arms' uses must have been impossible for 
> many theropods,
> possibly for a large or even an innumerable set of interacting reasons.

I've wondered about this.  It is likely that bipedality preceded
predatory behavior in dinosaur evolution.  The study of Martinez et
al. (2011) further indicates that the ancestral body plan for
dinosaurs included a proportionately long forelimb (~45% hindlimb
length) and a sharp-clawed manus capable of hyperextension.  Theropods
show a shift to a more raptorial/grasping manus (elongate penultimate
phalanges of manus, etc).  Nevertheless, it appears to me that once
dinosaurs became bipedal it was a case of: "So, what do we do now with
these appendages?"