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RE: Sneak Peak at Yale Torosaurus Sculpture



> -----Original Message-----
> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu] On Behalf Of
> Jaime A. Headden
> Sent: Tuesday, September 27, 2005 11:10 PM
> To: dinosaur@usc.edu
> Cc: ralph.miller@alumni.usc.edu; andyfarke@hotmail.com
> Subject: Re: Sneak Peak at Yale Torosaurus Sculpture

> By comparison, birds and crocodilians with
> such
> narrow and long rostra process their food largely by grip, lift, and gulp:
> 
>   1. grip the food firmly in the jaws;
>   2. lift the head and point the snout upwards;
>   3. let the food drop to the back of the throat and gulp it down.

To play devil's advocate, I don't see why ceratopsids couldn't have done
this. . .but then again, this is a behavioral thing which doesn't fossilize
all that well!

>   We can determine by jaw mechanics and toothwear that ceratopsians
> apparently
> operated their jaws primarily straight up and down, with various degrees
> of
> side-to-side movement, which means the process of eating foot, without
> cheeks,
> in such a narrow jaw, would result is loss of food during cropping.

To play devil's advocate again, you would lose food either way, whether the
jaw was narrow or not. . . the issue is the presence or absence of cheeks.
And, narrowness of the jaws is only a matter of perspective. Between the two
maxillary tooth rows, we're still talking about a distance of about 30 cm
between the left and right batteries.

So as a sort of "compromise" solution, is it possible that the "normal" jaw
musculature formed a cheek of sorts along the posterior half of the jaw
(like what you talked about, Jaime, in some of the "cheeked" birds)?

> In
> ceratopsians and especially ankylosaurs, the toothrows are strongly inset,
> a
> condition that doesn't compare to anything else in nature save mammals and
> some
> lizards (which, as in *Moloch*, lack cheeks: See digimorph.org for CT
> scans of
> *Moloch* skulls, inset jaws, laterally placed jugals, and broadened
> mandibular
> ventral margins that support ornamental scalation).

Interesting example of something without cheeks that has an inset toothrow.
 .

And to really be an annoyance here, the toothrows are actually a little
*outset* on many mammal skulls. Looking at the dozen hartebeest skulls in my
office (long story), the teeth are about flush with the lateral surface of
the maxilla along almost their entire length! The same goes for goats.

Another thing to keep in mind is that the shape of the toothrow may not be
driven only by feeding needs. Phylogenetic baggage, functional constraints
from the respiratory system, etc., may all contribute to the shape of the
oral cavity and its associated structures.

>   I am partial to Witmer finding absolute correllates to test these
> hypotheses,
> and I am glad he's finding answers. But I do not think dismissing "cheeks"
> because one is looking for mammal-style tissues is the answer. What other
> forms
> of tissue can form a cheek-like structure and not require mammal-style
> correllates? What if simple skin covered this region and formed pockets
> lateral
> to the jaw margins? what traces would this leave?

Exactly--and how does this relate to the known traces on the skulls of
ceratopsians (and possibly ankylosaurs).

(incidentally, I just took a glance at the close-up of the skull of
Scelidosaurus featured in Barrett's paper on toothwear, in the Armored
Dinosaurs volume. It looks like, at least on the maxilla, that the
sculpturing stops short of the inset maxillary margin. Not quite as certain
as for what's going on in the dentary. . .)

So to summarize so far. . .

*If ceratopsids had cheeks, they were not homologous to those of mammals
(everyone agrees on this, I think).
*The evidence is pretty good that ankylosaurs had cheeks.
*Ceratopsids have an inset tooth row.
*Vascular impressions pretty much go right up to the teeth on ceratopsid
dentaries and maxillae. Similar impressions may do the same thing in
ankylosaurs, but not in scelidosaurs.
*In modern mammals, an inset tooth row is associated with cheeks, but an
"outset" toothrow doesn't negate the existence of cheeks.

I wholeheartedly agree with both Tom and Jaime, in that much, much more
study is needed on this.

Now back to my own dissertation. . .

Andy