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Re: Bipedal apatosaurs and stegosaurs



Nick Longrich has demonstrated quite effectively that diplodicids and
stegosaurs carry most of their weight on their hind limbs (and kind of
threw me for a
loop ... for a while).  This demonstration is then used to suggest the idea
that these animals reared while feeding, either habitually or consistantly.
I suggest this is not necessarily the case.

The fact that these animals carried their weight on their hind limbs is no
real suprise.  They both used their tails as weapons, and to be able to
wield them effectively they would need to shift their weight onto their
hind limbs (something that was reinforced by evolution).  To use Bakker's
analogy of stegosaurus as a dinosaurian ninja, the hind limbs would be
planted, while the front limbs would be free to orient the animal according
to the movements of the attacking theropod (showing that the tail is being
primarily selected for its defensive/offensive abilities).  This would
provide a very effective defence.

However, this changes nothing in regards to the feeding posture debate.
IMHO, it actually helps support the claim that these animals didn't rear
while feeding.  To use stegosaurus again, if the animal was feeding
bipedally, it's tail would be used for balance.  Since the tail is
otherwise occupied, it would be unavailable for the animal's defence,
leaving it vulnerable to a hungry theropod.  If theropods were primarily
ambush predators, then precious seconds would be lost in reorienting the
stegosaur for combat.  It seems far easier for the animal to be feeding
quadrepedally, then simply shifting it's weight to ward off the attack.

It seems to me that there is a tendancy to try to see *all* dinosaurs as
being bipedal at one time or another.  What's next, bipedal ceratopians?


Rob Meyerson
Orphan Vertebrate Paleontologist

***
"All right, we'll call it a draw."
                        -MPATHG