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Re: Ceratopian jaws (my last bow)



Jon Woolf writes;

>I don't understand why anyone would think that the ceratopsid frill
>served to anchor jaw muscles at all.  I recently read Peter Dodson's
>book THE HORNED DINOSAURS, in which he makes a convincing (to me, at
>least) case that the frill's primary function was display, possibly
>with a secondary protective function.  Put together, these two reasons
>seem to more than adequately explain the frill.  I don't see any need
>for the "jaw muscles" idea, and I don't see any good reason to accept
>it.  I do see a lot of questions that it raises, though.  For example:

The idea stems from observations of the evolution of the group.  The first
 ceratopian (note the missing "s"), 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.  In the
 protoceratopids, the fenestra pair seem to be a match with the openings on
 the first simple frill.  Therefore, one could conclude that the frill first
 originated as a way to help the jaw muscles somehow (perhaps as a better
 anchor site???).

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.

>     * No other type of dinosaur had enormous jaw muscles, not even
>       the ceratopsids' own ancestors.

_Protoceratops_ has a well developed frill, and most likely needed it to go
 after the available vegetation.  I understand that desert plantlife is not
 exactly chewer friendly.

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.  This idea would allow several groups of
 herbivores to live in the same area, with little interspecies competition
 (the definition of ecological stratification).  However, I have no idea if
 the paleobotany backs up this statement.

>     * Different ceratopsid genera had different frill sizes and
>       shapes, and also different sizes, shapes, and placements of the
>       frill openings.  Why such a variety of forms to answer a single
>       common requirement (jaw muscle anchorage)?

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

Since this is the last week in the semester, and will be un-subscribing
 soon, I must cut out of the discussion.  Next semester, I will be back at
 my old college to pick up a few classes I missed my first time around, so
 my address will be back to what it was last year.

Later all, [poof!]

Rob Meyerson
Orphan Vertebrate Paleontologist

***
"Happy Christmas to all, and to all a good night!"