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Re: Ceratopian jaws (my last bow)



At 02:42 PM 12/16/96 -0500, Stan Friesen wrote:

>From: "Van and Kathy Smith" <vksmith@dwx.com>
>
> > It seems to me that greatly elongated jaw muscles would be primarily an
> > adaptation to drive the jaws over a greatly extended range.  
> >... Perhaps ceratopians adapted to eat or chew through very large
> > things -- tree bark perhaps, or maybe they would actually chew down
> > trees to get to the leaves.
>
>Something like this has been sugggested before.
>
>However, I am skeptical.  If an animal as abundant as _Triceratops_
>was biting down tree, then any forest would be converted into an
>open parkland.  

I think we are being misled here by assuming that long fibers on the skull
muscles were used to increase the animal's bite. In order to determine the
likely function of any jaw muscles, you have to look at the entire jaw
mechanism to see how it operates. One of the important things to consider
with respect to any jaw muscles is not only the length of its fibers or its
overall strength, but the angle at which it operates. A muscle whose chief
job is to exert a strong bite will do so if the chief lines of force it
generates operate at a right angle to the jaw hinge. Muscles that may have
arisen from the frills of some of the horned dinosaurs seem to be, if they
exist at all, lined up to insert on the jaw at a much shallower angle, and
therefore would have dissipated a great deal of their force in a simple
bite.  
 
A muscle inserting at a shallow angle, though, may be particularly well
suited to draw the lower jaw backwards against the upper jaw, something
that might have been extremely useful if the animals needed to produce
grinding or sawing movements to deal with whatever plants they were eating.
In birds, which have a kinetic jaw system, there are muscles that do this
by operating mainly around the hinge between the quadrate and the braincase
rather than the hinge between the mandible and the quadrate. As I have not
looked at the jaw mechanism in horned dinosaurs, I cannot say if there was
a free joint at this point in their skulls. However, it would still seem to
me that we will not be able to resolve the problem of exactly what these
dinosaurs were doing with their jaws without a careful analysis of the
entire mechanism, and to speculate as we are doing here without such
information is, it seems to me, idle. I would be interested in knowing if
anyone has ever done a study of this sort. 

--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net