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Re: Dino Knees



Jeffrey Martz writes;

>    My impresssion of the ways birds walk is that they don't move thier 
>femurs as much as a quadrapedal mammal.  They hold it with the femur 
>cocked forward, and most of the movement occurs at the knee and ankle, 
>rather than the hip socket.  Is this so?

An accurate observation, but I think that this can be expanded for fast running
quadrepeds too.  I once saw an x-ray generated footage of a horse at full
gallop, and the range of motion of the femur seemed to be rather small when
compared to the total movement of the leg.

>If so, is it to keep balance 
>since the center of gravity is set forward?  In other words, in a 
>dinosaur with a long tail, would there be more femur movement since the center
>of gravity is set back more toward the hip socket?

I don't think it has to do with balance (WARNING, speculation ahead).  The
theropod limb is constructed like this:

                                 *
                                  /
                                 /
                                / 
                               /  
                               \   
                                \   
                                 \
                                  \
                                   \
                                   /
                                __/

Where "*" is the center of gravity.  When the animal is just standing, the foot
is placed directly below the center of gravity.  I suspect that the main reason
for this type of limb construction is that the leg and upper foot acted as a
shock absorber between the ground and the body.  To return to my horse
comparison, this type of limb arrangement can be found in animals that are
rather fast on their feet.  To *really* go out on a limb (no pun intended), one
could say that this would mean that, like horses, theropods not only could run
fast, but did so frequently; possibly another point for the high-metabolism
theropod model.

Rob

***
Evolution = God's kingdom with a facelift.