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Re: Ceratopian jaws (my last bow)



From: Robert J Meyerson <meyersrt@uwec.edu> (Rob Meyerson)

 > The idea stems from observations of the evolution of the group.  ...
 > and the only bipedal member of the group, was _Psittacosaurus_.
 >  In this animal, the jaw muscles appear to attach to the fenestra
 > pair located at the back/top of the head.  ...
 > 
 > An added benefit of a frill is that it is a free billboard for all
 > kinds of advertisements.  Not only is it helpful during a rut, but
 > it also is a  great species marker.  However, I suggest that this
 > is a secondary benefit to what frill provides to the jaw.

It could well be that the frill *originated* as jaw muscle attachment,
and then diversified and enlarged due to sexual selection of it as
a billboard.

Thus the large size and prominant shape variations would be due to the
display function, even though it was anatomically originally an
attachment site.

I would suggest that the largest a frill would get due purely to
use for muscle attachments is found in animals like _Protoceratops_,
and even there some secondary enlargement might be present in the males.

 > I am of the opinion that North American ceratopians were going after
 >  vegetation that was considerably tougher than hadrosaur fare,
 > resulting in a need for a good set of jaws.

The difficulty with this is that there is little evidence for abundant
high-fiber plant material in the Late Cretaceous of North America. In
fact the vegetation in question was dominated by varying mixtures of
sequoias, flowering trees, gingos, ferns, and "ur-sedges".  Over much
of the area where _Triceratops_ is found even palms are rather rare.

 >  This idea would allow several groups of herbivores to live in the
 >  same area, with little interspecies > competition (the definition
 >  of ecological stratification). ...

I agree there has to have been ecological differentiation.  But in
modern ecosystems terrestrial herbivore differentiation is often
quite subtle.

One point that is of some significance is that _Edmontosaurus_
remains are mroe often associated with swamp deposits than are
_Triceratops_ remains. This alone may be sufficient ecological
differentiation for the Lancian exemplars  Similar behavioral/
micro-habitat differentiation may have been involved during the
earlier times, when diversity in both groups was higher.

 >  However, I have no idea if the paleobotany backs up this statement.

I do not believe it does.  If people wish I can post my raw data on
the plants of the Hell Creek and Lance Formations.
[I will reserve my more detailed analysis for later use].

 > The idea of ecological stratification could explain this: different
 >  species/genera going after differing vegetation, with differing
 > degrees of toughness.

The differences in shape do not, to my eye, really match with what
differences in mechanics would require.  The differences are more
related to appearance (squared in _Chasmosaurus_, spikey in
 _Styracosaurus_ and so on - even the forward facing spikes on
_Centrosaurus_ seem to me to be more visual than functional).

swf@elsegundoca.ncr.com         sarima@ix.netcom.com

The peace of God be with you.