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Re: T.rex and elephants

Dann Pigdon wrote:

<elephantine snip> <g>

> > > Well, I can't. For one, elephants amble (not run; different gait and
> > > different mechanics) on four legs, not two. I don't have the ref to 
> > > support
> > > this, but I think it's farily safe to say that most of their weight is
> > > supported on their forelimbs, too. I don't find it easy to compare
> > > theropods and elephants in terms of anatomy, although the overall hindlimb
> > > mechanics of large theropods and elephants might be similar, who knows?
> >
> > Seems reasonable to me to say that the forelimbs are the main
> > weight-bearing limbs in elephants.  After all, you've got that great
> > awkward head and tusks up front, and not much behind to balance it.
>         I was of the opinion that the weight distribution of an
> elephant was fairly even between all four legs, with the centre of
> mass somewhere between the front and hind legs. Elephants can support
> themselves entirely on their hind legs, and in the circus they are
> taught (made?) to balance on their heads and front legs only. I
> suspect that the longer front legs may help increase the normal
> foraging height of the trunk, perhaps in a similar fashion to
> that of brachiosaurs.

_Any_ quadrupedal animal is going to have the center of mass between the
front and hind legs.  However, the exact placement of the CM is
determined by the total distribution of mass.  A heavy mass up front and
no balancing mass behind moves the CM forward, and vice versa.  Offhand
I can't think of any mammals that have the CM placed well aft of the
geometric center of the body.  Most have the CM right around the
geometric center.  A few have oversize heads and/or necks -- elephants,
giraffes, gnu, rhino -- and such animals always seem to have the CM
farther forward, with a sloping spine and forequarters noticeably larger
and more muscular than the hindquarters.  

In any case, common sense indicates that you'll be able to tell roughly
where the CM is just by looking, and my eyes tell me that elephants have
the CM distinctly forward of the geometric center of the body. 
Elephants are head-heavy.  Sure, they _can_ walk on their hind legs, but
they can't do it for long, they can't achieve an _upright_ posture (that
is, the spine held vertically or almost so -- dogs can do this,
elephants can't), and they definitely look _weird_ doing it! <g>   

>         Rhinos also have large heads, but they are held
> low. Rhinos also tend to feed lower to the ground. This begs the
> question: does the height of the food eaten control the height at
> which the head is held, or is the head held at a certain height for
> structural reasons and the diet has to conform to biology?

MHO is "sometimes one, sometimes the other, and sometimes a mixture of
both."  I doubt that giraffes would have evolved that long neck if they
weren't high browsers to start with, and I doubt that bison would have
developed the permanent downward arch to the neck if they weren't
grazers to start with.  OTOH, I also doubt that you could make a high
browser out of a rhino or a hippo.  

-- JSW