Re: Quadruped theropods?

[Graydon <graydon@epiphyte.net>]

On Sun, May 21, 2006 at 06:16:19PM +0200, Vladim?r  Socha scripsit:
> Good day, just a short question: Did anyone ever hypothesised why
> theropods never gained quadrupedal stance (in any of their
> phylogenetic lines) during their 160 (or 230 if you like) million year
> long evolution? Was it simply disadvantageous and contraproductive?
> Thanks for any comment, Vlad.

Assuming you accept a monophyletic saurischia, theropods and sauropods
split early, and the quadrupeds are the sauropods; the theropods proper
are the members of the saurischian clade where the forelimb became
intimately involved in prey-capture (and probably display) functions and
lost the ability to pronate.  (The theropod wrist couldn't rotate to put
the palm of the hand in a position to face the ground.)

That being the case, the evolutionary path to a quadrupedal stance would
have been very unlikely, since anything that made a theropod a better
potential quadruped would have made it worse at feeding itself or
finding mates.

Note that the ornithsician lineages which recovered quadrupedal stances
didn't alway have an easy time of it despite the forelimb function in
feeding being somewhat less constrained by the inability of plants to
run away; see the long-running "sprawling ceratopsian" debate.

Birds did eventually find a way to get the forelimb involved in
locomotion again, but no quadrupedal stance.