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Re: Stego/Ankylo limbs (long)

On Sun, 4 Feb 1996, Rob Meyerson wrote:

> >> hind legs.  The front limbs wouldn't have to do much more than steer.  
> >> So they wouldn't have to be efficent in locomotion.
> >
> >Wrong.  Unless the front legs have some sort of wheels or runners on 
> >them, they need to be just as efficient in locomotion as the hind legs, 
> >or at least be able to keep up.
> However, since the main thrust is being provided by the hindlegs, the forelegs
> can afford to lose a little efficency for the sake of stability.  A decent
> trade-off in my book.

But if you produce a lot of thrust with the hindlimb and the forelimb 
can't keep up, the animal is going to fall on its face!

> >> >        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.
> >> 
> >> However, the reinforcements I mentioned only match up (and therefore, only
> >> apply) if the humerus is held horizontal to the shoulder articulation.
> >
> >I fail to see how that is true.
> The feature I point out is the reinforcement process located on the upper
> humerus, which lines up with a similar process on the scapulacoricoid.  These
> two processes match up only when the humerus is held horizontally out from the
> shoulder attachment.

Then the "reinforcement procees" must actually be something else!  Sorry, 
but given the trackway data, there is simply no way ceratopians could be 
holding their humeri horizontally.

> >> >        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?
> >> 
> >> Actually, by holding the feet out to the side, it spreads the total weight 
> >> of
> >> the animal over a wide area.  This would reduce the total load on the 
> >> forefeet.
> >
> >No, it does not.  The weight of the animal is distributed over the area 
> >of the parts of its feet in contact with the ground.  No more; no less.  If 
> >the animal slopes down sharply towards the forelimbs, these must bear a 
> >large 
> >portion of the animal's total weight, and, as mentioned above, the animal 
> >sinks into the sand.
> Sorry, your wrong on this one.  Set this one up as a physics problem regarding
> the justification of stresses (I can't think of the real term, but this is 
> close
> enough).  With the feet under the body, the stress diagram looks like this:
>          |
>          | w
>          |
>          V
> In this case, the force on the feet (w) equals the mass times the 
> acceleration due to gravity (simple Newtonian stuff).
> With the forelimb held off to the side, the diagram looks like this:
>         --------->|
>          \ A      |
>           \       |
>            \      |
>             \     | x
>           H  \    |
>               \   |
>                \  |
>                 \ |
>                  >V
> Where:
>         H = Hypotenuse (with a value equal to w).
>         A = The angle between hypotenuse and horizontal.
>         X = Total load applied to the feet.
> Trigonometry says that X = H.cos A.  Since the cosine of any angle (provided
> A < 90) is always less than one, then X < H.  Therefore, the total load 
> applied
> to the feet is less with the feet held out to the side.

Fascinating.  How do you get gravity to act obliquely like that?

Check your diagrams again.  Weight is weight.  It always acts downwards.

What you have done is *increased* the total force on the legs 
and also introduced a horizontal component to the force, which, incidentally, 
will serve to shove the legs out from under the body, causing them to slip 
sideways, particularly on a slippery surface like sand!

This is why we build columns vertically, not slanted.

> Rob
> ***

Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447

"If you can't convince them, confuse them." -- Harry S. Truman