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Biped advantage

    I'm beginning to think Rob Meyerson was on the right track after all:
    When mammals broke into the big animal scene in the Paleocene, did 
they start out with spines that were much more flexible than those of 
archosaurs?  How important spinal flexibility to fast mammals quadrapeds 
BESIDES carnivores?
     I'm trying to understand why the fastest dinosaurs seem to have been 
bipeds (with the possible exceptions of ceratopsians), while all the 
speedy mammals are quadrapeds.  If the first big mammals inherited a 
fairly flexible spine but dinosaurs did not, this seems to imply to 
me that mammals quadrapedal mammals would be more predisposed to speed 
than bipedal mammals, with the situation reversed for dinosaurs.  A biped 
with a flexible spine can get an extra boost out of the front legs, while 
an animal with a stiff spine cannot, and if it can't, the front legs are 
indeed more of a hinderance to speed than a help.  This might also imply 
that quadrapedality with a flexible spine allows slightly FASTER average 
speeds then bipedal running. 
     Could THIS be the reason theropods and ornithopods became bipeds?  
Lacking a flexible spine, reducing the mass of the front limbs and 
shortening the torso (also reducing mass) allowed them faster speeds than if 
they stayed quadrapeds?     
     I think perhaps Rob Meyerson was right about bipeds being more 
speedy after all, its just that the principle applies only to dinosaurs, not 
mammals.  SOMEONE must have relavant hard data about this, one way or the 

LN Jeff
"What are you, an archaeologist?"
"I'm a paleontologist....we dig deeper."