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Re: A fundimental conundrum concerning gigantic theropods



Mickey Rowe writes;

>> Actually it is the unstability that makes being bipedal so cool.  It
>> allows one to turn on a dime (just watch kids playing tag), far
>> easier than a quadreped ever could.
>
>Rob, can you provide *any* evidence whatsoever that the above is true?
>How many of your "kids playing tag" play against deer or dogs or any
>other quadruped?

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.  The bipedal nature of
ornithopods and theropods would have given the animals similar
advantages.

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.

>> Further, being quadrepedal has a disadvantage in terms of being a
>> high speed creature, in that the front limbs get in the way.  When a
>> cheetah is running, all it's forward thrust is provided by the hind
>> limbs.
>
>This is totally false.  Cheetahs most assuredly use their front limbs
>for power during a high speed run.

The point is that the power provided by the front limbs is minor
compared to the power of the hind limbs.  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.  Think of it
like putting a Camero engine in the front of a dragster (with the
usual engine in its ususal place): what difference is the camero
engine going to be to the total speed of the dragster, especially at
max speeds?

IMHO, this is why being bipedal is an advantage; the animal gets its
front limbs out of the way, and lets the hind limbs concentrate on
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).

Rob Meyerson
Orphan Vertebrate Paleontologist

***
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