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Re: (sigh)sauropod necks again--long! -Reply




>>> "Matthew BONNAN" <Z966341@wpo.cso.niu.edu> 05/25/99 01:26am >>>

<<It appears that some sauropods (diplodocids in particular) did have
their centers of gravity over their hips, which would appear to be a
decent location for a "pivot" for rearing up.  I don't think the
neck/head blood pressure thing is as big of a deal as some make it out
to be because sauropods have tiny brains, unlike giraffes which have
much larger brains.>>

I think we have to be careful with this argument.  You may well be right, but 
it worries me.  Although it would not take much oxygen to fully supply the 
brain, a sauropod brain is presumably made of pretty much the same stuff as a 
giraffe's.  That is, the neurons die just as fast from lack of oxygen.  So, the 
circulatory system doesn't need to deliver very much blood, but it must deliver 
constantly.  There's no reason to think that the sauropod brain could withstand 
 oxygen deprivation any longer or better than a giraffe, and the sauropod quite 
possibly had a circulatory system that was slower to respond to stress (because 
of, perhaps, a lower basal heart rate and larger circulatory volume).  

The smaller size of the brain makes the problem simpler, but it does not 
eliminate the need for a mechanism which rapidly counteracts *changes* in 
pressure which might have been considerably greater than a giraffe usually has 
to deal with.  

<<The key to understanding rearing, in my humble opinion, is
understanding what is going on at the hips and the tail.  Can the hips
rotate back far enough to allow the sauropod to go tripodal?  Could
the tail support the sauropod tripodally?  Could a sauropod stand on
just its hind legs?  What of the caudofemoralis longus muscle which
runs off the tail and inserts on the middle of the back of the femur
on a landmark called the fourth trochanter?>>

Which raises another a priori biophysical problem. (Yes, it rears its ugly 
head.)  The benefit from rearing up has to be quite high because the price of 
falling would be steep.  It's the tumbling T. rex problem in spades.  A mouse 
can fall many times its height.  A human can usually fall its own height, but 
not always (I dislocated an elbow once that way).  A sauropod?  Don't even 
think about it.  If it reared up, the mechanism would have to have been be 
nearly 100% stable and the benefit very high or the behavior simply wouldn't 
have been worth the risk.

  --Toby White