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Re: Sneak Peak at Yale Torosaurus Sculpture



Dino Guy Ralph (ralph.miller@alumni.usc.edu) wrote:

<You want cheeks?  Check out the cheeky hornbill hatchlings at 
http://avesinternational.com/softbillnurs.html>

  With all due respect to birds, birds don't have the neccessary functional
tissue or size of such to make a cheek in the sense being used here, either a
tissue that holds food or is built from a muscle that surrounds the jaw. In
birds, this small tissue in the rear of the jaw anterior to the hinge is formed
from a set of muscles, some very tiny, that aid in closing the jaw, pulling the
mandible backward, or doing so while closing the mandible, including the mm.
pterygoideus and pseudomasseter. The sheer size in birds is so small as to have
very little effect on food, which birds typically swallow after any oral
processing (including parrots).

http://biology.clc.uc.edu/graphics/taxonomy/animals/aves/Parrot/JSC%20980814%20Zoo%20Parrot%201.JPG

  Here we have a macaw that, given the benefit of the shortened rostrum and
gigantic jaw adductors, has strongly anteriorized adductor musculature that
results in a "cheek" as Ralph would have it, but all oral processing is
anterior to this set of muscles, and thus the appearance of the cheek is not
functional to mammals or what some of us would like to see in dinosaurs.

  My problem so far with Witmer's hypothesis is that, while ceratopsians have
stronglyvascularized cortical surfaces of their skulls, this doesn't explain
the inset jaw margin as being dangerous to process food interorally when the
mandible is so long and narrow? 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.

  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. 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). 

  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? I don't think we have the
answers available yet to say that *Torosaurus* had wide-open jaws as shown. It
would help to know what ceratopsians ate....

  Cheers,

Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


                
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