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Speed (Re: A fundimental conundrum concerning gigantic theropods)

Rob Meyerson wrote... 

> I have no documented info, just things I have observed.  Think of a
> game of "keep-away": the kid with the ball can twist around, change
> directions quickly, and perform other avoidance maneuvers (to use the
> cliche of throwing one's weight around).  Or think of a martial arts
> master, as he/she can do the same things to an opponent to avoid hits
> (and to land a few of their own).  All these maneuvers are possible
> because the person is bipedal, and is able to use the inherant
> instability of their form to greater advantage.

    I think these maneuvers are possible because humans have a really 
flexible spine and a vertical upright posture, not theropod 
     A biped needs a means of balncing itself, true.  Theropods 
accomplished this with a long, fairly flexible tail.  Since apes lack 
tails, the upper body flexibility of humans in a different approach to the 
same problem.  As far as high speed turns are concerned, cats are 
quadrapeds that use thier flexible tails to change direction as sharply as a 
kid could.  
> Usually, the more stable the form, the more manuverable it becomes
> (for the same reason that Air Force aircraft are built for
> unstability).  Therefore, a deer or dog will never be able to pull off
> the moves that a human is able to do.  Granted, a dog can do some
> pretty acrobatic things (like when it goes after a frizbee), but it
> isn't going to be doing any triple backflips off the high wire.

     Lacking a flexible, vertical spine, neither is a theropod.  Consider 
the evolutionary history of humans before you start assigning the same traits 
to theropods just becasue both are bipeds.
     Anyhow, a quadraped wouldn't NEED to.  Again, human upper body 
flexibility for the sake of balance is a means of coping with a more 
unstable posture, not the REASON humans evolved bipedality.  
> The point is that the power provided by the front limbs is minor
> compared to the power of the hind limbs.

     Hold it:  You are saying that a quadraped with four legs on the 
ground has LESS potential to produce forward thrust then a biped with 
half as many legs on the ground to produce thrust?  Why?  Exactly how do the 
front legs "get in the way"?  Is it because the bigger front legs on a 
quadraped are larger and more massive and slow the animal down or what?   

>  Just compare the biceps/triceps of the forelimb with the quadreceps
> of the hind limbs: the biceps/triceps are considerably smaller than
> the quads (this is true for all quadrepedal animals).  Since the
> biceps don't have the muscles of the quads, they won't have the
> muscle power of the quads; therefore, the forelimbs simply *cannot*
> generate the same power as the hindlimbs, so then the speed provided
> by the front limbs is *going* to be less than that provided by the
> hind limbs.

    You have cause and effect mixed up:  In humans, the arm muscles 
are MUCH weaker than leg muscles because they lost thier weight bearing 
and locamotion functions.  True, the back legs are in a better position 
to produce thrust, but this doesn't mean that the relative potential of 
the front legs to produce thrust is so low that it is better to do away 
with them altogether.     
     You are going to have to provide a better explanation of how front legs 
get in the way of increased speed, especially considering the fastest 
living land aniamal is a quadraped.  Front legs do not get in the way.  
Look at a cheetah or dog running, and you will see that the front leg 
range of motion carries it BETWEEN the back legs, allowing the hind limbs 
a full range of motion.
     The only I see that your allegation that bipedality is better 
for speed could be true is if the drag produced by the extra mass of having 
larger front legs nullified the additional propulsion provided by these extra 
legs.  No, the front legs are probably not going to provide quite as much 
propulsion as the back legs do to thier placement, but the tradeoff is 
apparently beneficial on the side of speed.   

> >Their mode of locomotion is fairly unique in the way that they take
> >advantage of the elastic energy stored in their spines.  This has
> >been discussed before on this very list.
> Right.  I suggest that the cheetah has this running style not only to
> utilize the spinal energy, but also to help the hind legs by providing
> a longer stride (which would be helpful by any definition).

     Not if you are a biped.  Having an elastic spine increases the 
maximum distance between the front and back legs.  A theropod would have 
no use for this.

LN Jeff