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Re: Jobaria and the Elephant Commit Suicide




Matthew Bonnan wrote:

> Okay, but more to the point: are these animals bending their knees while
> rearing or are they straight?  If straight, the weight would be borne in
> compression.  If bent, then tension and some torques plus compression will
> be happening to the femur.  If the figures they have suggest it can bear 3
> times its weight in rearing, that's all good and fine: but in what position
> are the femora?  This can make a big difference.  Sereno et al have shown
> pictures of sauropods rearing with bent knees, and from what I saw at their
> website this was the case with Jobaria.  Bones do well in compression, but
> don't do as well in tension which is what is going to happen if their legs
> are bent while rearing.  Safety factors in all animals are pretty high, but
> they don't normally attempt to do things that would push these factors.
> Again, how far the vertebral column could rotate up is dependent on a
> variety of factors.  If you are using the femur as the pivot point in
> rearing, the further you tilt that vert column back, the further the pubis
> rotates forward and thus the more the pubic adductors are stretched.  This
> would seem to limit the amount of rearing that would happen.  Plus, while
> the insertion site for the caudofemoralis would remain fixed, it's origin on
> the tail would move -- what happens there has still been largely
> underinvestigated.

I have a couple of questions.  Is it clear how distal on the pubis and femur the
pubic adductors are attached?  If they were attached close to the acetabulum
there would not be as much movement and thus less stretch. Either way , has
anyone looked at to what degree the pubis has to rotate on the femoral head for
the animal to become bipedal, and how much greater the movement is than when the
animal simply retracts its leg while walking?  Also, how dissimilar is the pubic
musculature of a prosauropod such as Plateosaurus?  Could it rear up?

I have always thought that elephants bend their knees and hips when rearing, not
so much as a response to their joint morphology, but as a way of shifting their
center of gravity backwards.  With some sauopods, their centers of gravity are
over the hips thus making the leg bending unnecessary, although I suppose if the
hip adductors do get stretched  toward the end of the rearing it may necessitate
some hip flexion.  I would expect a Triceratops, if it could rear up, would be
forced to bend its legs like an elephant since it has a similar center of
gravity.  All criticisms welcome (I have developed a thick hide along with my
gray hair).--Ken Clay, M.D.