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Rearing sauropods

>> ON THE PRESSURE EXERTED ONTO FORELIMBS when a rearing sauropod moved back
to its
>> quadrupedal state, I feel it unlikely that much exertion of the forelimbs
>> have been needed. To rear into a bi/tripodal stance, a big dinosaur would
>> probably not need to 'push off the ground' with its forelimbs. Simply leaning
>> back would tilt the sacrum, the counterbalancing of the tail immediately
>> the thorax and neck. Lizards that rear in threat displays 'push up' with the
>> arms, but their tails aren't far enough off the ground to provide a levering
>> effect for the upper body. 
>     How does it "lean back" without using its forelimbs?  It seems to me 
>like to only way to shift the animals balance backward would be to push 
>off with the forelimbs.  If it isn't using these, exactly what is it 
>flexing or tilting or whatever to shift the center of gravity?

When an animal rears up (it's easier to see when the animal is large, and thus
slower-moving), it drops its haunches and humps its back as its back muscles
begin to lift its front.  As the front rises the animal draws its forelegs
inward and
leans back further.  All three motions (haunch-drop, back-hump, and
forelimb-retract) serve to shift the center of gravity of the animal to a
better position for lifting it upward.  At most, the forelimbs provide only
a small initial boost to start the rearing process.

Some earlier replies on this subject are more specific re. muscle names etc.

Leslie Gertsch
Research Assistant Professor
Colorado School of Mines