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Re: Stego/Ankylo limbs



At 10:19 AM 2/2/96 -0500, Rob Meyerson wrote:
>> V.S. wrote:
>>        What about the elbows and wrists -- do they too show reinforcements?
>>The stress on the elbows would be at least as great as the stress on the
>>shoulders in a sprawling posture.
>
>The elbows of _Triceratops_ are strongly built structures, they could easily
> withstand the added stress of being placed away from the body.
       
        Certainly the elbows of a Triceratops are strongly built structures
-- the animal was obviously built for charging -- but what reinforcements do
you see for lateral stresses produced from a sprawling posture?

        As  Nicholas Pharris pointed out, the front limbs on a horse are not
straight but slightly flexed at the backwardly pointing elbows.  The reason
for this is very important.  As the animal walks or runs the leg is first
extended as the horse reaches out in front of itself, then flexes primarily
at the elbow, storing energy like a spring, and then extends, releasing the
energy and propelling the animal forward.  This entire process occurs in the
vertical plane containing the motion vector of the animal.  
        In your sprawling Triceratops, as the leg is extended, the energy
stored will be released in force vector NOT lying in the plane described
above, but rather askew, propelling the animal sideways as well as forwards.
This is not only an obviously inefficient mode of locomotion, but jarring to
all the joints in the forelimbs, since the opposite front limb must stop the
lateral motion. 
        Additionally the wrists would not only have to absorb the punishment
I describe above, but would be under much stress as the forearm would both
pivot and twist under the range of motion necessary for your sprawling model.  
        Finally, the reinforcements you mention in the shoulder are
consistent with a large charging animal that would have to change course
quickly or lunge sideways.

You state in another reply:
       
> When they migrate to North America, the "dune buggy" design, which 
> gives them an immense  stability (definately an advantage) on the "stable" 
>  land surface, which helps them to be an extremely successful group.  
> This success is marked by an increase in size.
> ... 
> Take a look at a dune buggy sometime.  The similarity in forms between
> the ceratopian forlimb system and a dune buggy front axle assembly are
> remarkably the same.

        You definitely need to elaborate on your "dune buggy" analogy.  You
have already discounted the wide-gauge "stance" of a dune buggy, by
acknowledging ceratopian trackways showing that the forelimbs are no further
apart than the hind limbs.  The only other similarity I can see (correct me
if I wrong -- I'm no dune buggy expert) is that the front tires slant away
from the car slightly, giving a visual appearance similar to a sprawling
Triceratop's forearms, but the reason the tires slant outward is to allow a
lot of vertical play in the wheels as the car  rolls over irregular surfaces.
        By sprawling, not only are you moving the Triceratops closer to the
ground, but you are placing more weight on the forelimbs making them sink
even deeper into the sand.  You are not suggesting "dune surfing" are you?
        One thing is almost certain: ceratopians were very good walkers
(from evidence of vast migrations).  They were probably also decent runners
(chargers) judging from their armaments and coeval similarities with modern
rhinos.  Of all of the major physical characteristics of such highly
successful migratory animals, limb structure would have to be refined and
efficient.  For such a large, mobile and quite possibly agile collection of
creatures, the forelimb structure that you describe is primitive,
inefficient, and ill suited for them.

V.S.

Van Smith
vksmith@ix.netcom.com