RE: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis

["Andy Farke" <andyfarke@hotmail.com>]

> From: "Mike Milbocker" <mmilbocker@psdllc.com>
> To: <andyfarke@hotmail.com>
> CC: <dinosaur@usc.edu>
> Subject: RE: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis
> Date: Tue, 27 Apr 2004 09:51:25 -0400
>
> The best approach would be to make a cast of
> the ilium and follow the procedure as described.

Or better yet, get 3D coordinate data for a variety of animals. This could 
be done relatively easily using CT scans or point digitization (and on a 
relatively cheap budget, particularly in the latter case).

> I suspect you are not asking me about the importance of the division of the
> dinosauria into two primary groups, rather I suspect you are asking, "What
> is the importance of the mass distribution in the ilium, functionally, in
> defining the locomotive and possibly behavioral distinctions between the 
> two
> groups."

Yep, that was exactly what I was asking. Or more precisely, I was wondering 
what the anatomical distinctions might be (because the soft tissue anatomy 
is hard enough to interpret, and locomotion and behavior are even one more 
step removed!).

> To begin to answer this question I think it would be rewarding to
> quantify the anterior/posterior ilium mass ratio for a large number of
> examples in both categories. Speaking generally, and I may be wrong here,
> theropods yield the highest ratio >>1, sauropods approx. 1, Ornithischian 
> <<
> 1, through there are borderline cases in the Ornithischia as well. Now
> speculating, this ratio appears to be related to max. stresses transferred
> from the thoracic region to the ilium via the synsacrum (hence the minor
> importance functionally of the pubis and ischium and their orientations).
> Incidentally, the position of the pubis and ischium, I would suggest, has
> more to do with tail function, e.g., whether the tail is used as a stiff
> navigational member or a flexible, possibly defensive member.

Please try testing this--it'd be an interesting relationship, and an 
important one if you can demonstrate it!

> >If you have a sample of ilia from the same taxon (or a group of related
> taxa, such as hadrosaurines), how much intragroup variation is there?
> What's the range of variation, in general?
>
> I do not know, but this variability can be quantified and statistical
> measures applied which may prove predictive.

Interesting. Can you get information on orientation of major axis, minor 
axis, etc., with the software you're using (presuming you're using a 
software package such as SigmaScan or ImageJ)? This may also be quite 
interesting, and may be even more functionally informative than simple 
location of moment of inertia.

> Photographs are preferred, and from at least 2 orthogonal directions. An 
> MRI
> would be better.

Actually, a CT scan. . .MRI doesn't deal well with fossil bone (or even live 
bone).

> The MRI might allow one to take into consideration mass
> density variations, which I suspect would remove some borderline cases 
> since
> bone density is known to be increased by stress.

Unfortunately, increased stress=increased bone density hypothesis isn't 
quite being supported by all experimental data in the general case (I was 
well reminded of this issue the other day in my functional morphology class. 
. .Brigitte Demes gave a great lecture on some research she's been involved 
in that has helped modify this classic hypothesis. It's annoying to me, 
because it forces me to rethink a lot of my comfortable old views about 
sinus placement in dinosaur skulls, etc. Apparently, it has more to do with 
the frequency of the stress, etc. At any rate, I can send you some of the 
refs we read. . .).

You're welcome--very interesting stuff to me. . .these sorts of morphometric 
methods have been pretty widely used in the anthropology realm; it's 
surprising more vertebrate paleontologists don't utilize them.

Andy
_______________________________________________
Andrew A. Farke
Graduate Student
Department of Anatomical Sciences
Stony Brook University
Stony Brook, NY

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