RE: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis

["Mike Milbocker" <mmilbocker@psdllc.com>]

Jaime A. Headden wrote:

<  Similarly, the preacetabular ala os most ankylosaurids has an area
largely 2-3 times that of the postacetabular ala, if not more, given the
position of the acetabulum is well posterior to the two-thirds point of
the iliac length and the bredth of the preacetabular ala exceeds that of
the postacetabular ala in all ankylosaurs I have seen to date, especially
in ankylosaurids and the "polacanthids" such as *Mymoorapelta,* and even
nodosaurids as in *Sauropelta.* Exactly how was it determined that these
ilia were caudally-heavy, rather than cranially heavy?>

I have looked at both Ankylosaurus and Euoplocephalus, and it is certainly
true what you say about the pre- and postacetabular ala. However, the
dividing point is far in front of the acetabulum, shifting the mass
distribution caudally. If you look at the ankylosaurids from above, you see
the ilium fills in where the lateral extent of the vertebral bodies/ribs
diminish. This creates a flared, almost rocket fin appearance because the
more lateral curvature is much more gradual. In side profile, the area is
about equal cranial/caudal. So in the case of ankylosaurids, my
understanding is that they serve as a confirmation of the hypothesis.

Regards, Mike Milbocker

-----Original Message-----
From: Jaime A. Headden [mailto:qilongia@yahoo.com]
Sent: Tuesday, April 27, 2004 4:10 PM
To: dinosaur@usc.edu
Cc: mmilbocker@psdllc.com
Subject: RE: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis


Mike Milbocker (mmilbocker@psdllc.com) wrote:

<The drawing you sent(above)has the head on the left side. I think you'll
agree the left side of the ilium in the drawing is heavier/taller than the
right side. Therefore, Alvarezsaurus is Saurischian - which agrees with
orthodoxy. The palaeos drawing shows it strongly Saurischian, if the mass
ratio is any measure.>

  Based on comparison to other alvarezsaurs, all alvarezsaur ilia known
have a slender pubis peduncle and a broad, short, and nearly absent or
fully absent ischiadic peduncle, and the postacetabular ala is broader and
longer and deeper than the preacetabular ala. This is true in
*Alvarezsaurus,* and the reference citing this as being reversed is wrong.

  Similarly, the preacetabular ala os most ankylosaurids has an area
largely 2-3 times that of the postacetabular ala, if not more, given the
position of the acetabulum is well posterior to the two-thirds point of
the iliac length and the bredth of the preacetabular ala exceeds that of
the postacetabular ala in all ankylosaurs I have seen to date, especially
in ankylosaurids and the "polacanthids" such as *Mymoorapelta,* and even
nodosaurids as in *Sauropelta.* Exactly how was it determined that these
ilia were caudally-heavy, rather than cranially heavy?

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making
leaps in the face of adversity, that a simple skip is so hard to do.  We
should all learn to walk soft, walk small, see the world around us rather
than zoom by it.

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




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