RE: Bipedal apatosaurs and stegosaurs?

[Robert J Meyerson <meyersrt@uwec.edu> (Rob Meyerson)]

Nick Longrich writes;

>        The short chests, long, heavy tails, hollow forward vertebrae and
>reduced forelimbs of Diplodocids all help move the center of gravity
>backwards.

Actually, I don't see the tail being all that massive; certainly not massive
 enough to radically change the CG position.  Besides, the tail verts are
 rather hollow as well, which would reduce the *tails* weight.

>As has been pointed out, diplodocid necks at best stick straight
>out, the Dicraeosaurus illustration I looked at actually had it pointed
>*down* slightly.

The necks may  *start* with a downward direction, but that doesn't mean that
 they had to stay that way.  The neck of Diplo seems rather flexible, and
 would've been able to reach above body level.  As I mentioned to someone
 off-list, the diplodicids may have gotten longer for the same reason that
 camarasaurids got bigger, they both get a better reach far more efficiently
 then they would by rearing.

>Diplodocids would make absurdly bad giraffe-style browsers because
>of the downard direction of the neck and short forelegs that only
>compound the problem. Either they grazed, or they reared.

Nature is not this simple.  Consider diplos not as giraffe-style feeders,
 but as elephant-style feeders (where the diplo neck and head are analogous
 to the  elephant trunk).  This may be more appropriate.

>These arguments apply to stegosaurus, despite the puzzlingly short
>neck of the animal. Notice that stegosaurs and diplodocids bear a
>striking degree of similarity in their overall body plans (same
>forelimb-hindlimb ratio, same big tail, same downcurved spines, same
>big hip spines, same big tail, etc.) with the exception of the
>neck.

All these anatomical features could also be explained as convergence.  Since
 both groups used their tails as anti-predator devices, we would expect both
 groups to be  well developed in the hindquarters.

Besides, a rearing stegosaur would be suicide, since the tail spikes would
 be effectively out of action while the animal was feeding.

Rob Meyerson
Orphan Vertebrate Paleontologist

***
"If anything is going to go wrong, it'll happen at maximum velocity."
                        -Red Green