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Re: Please remind me!



Sherry asks:
 
> Why did birds "revert"
> back to the bird hipped form? A change of distribution in posture due to
> no tails and wishbones?

To turn your question around, the "lizard-hipped" configuration is 
primitive for the Dinosauria.  Among saurischians, the 
dromaeosaurids, birds (surprise, surprise) and segnosaurs evolved a 
"bird-hipped" configuration, convergent with the ornithischians.  

Why did they do this?  Umm... actually I don't know.  Theories which 
I've heard:

For birds and dromaeosaurids it was associated with a decoupling of 
the tail from the hips.  Birds probably inherited an opisthopubic 
pelvis from their immediate ancestor - dromaeosaurids have been put 
forward as the most likely candidate by many researchers.  

For segnosaurs (and ornithischians) the backwards movement of the 
pubic shaft was an accommodation for the larger gut - herbivores
need more space than carnivores to digest copious amounts of cellulose-
rich (and nutrient-poor) plant matter.

Tim