RE: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis

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

Jaime A. Headden wrote:

< Another caudally heavy ilium in similar token was noted by Glut in 1997
as the referred ilium to *Carcharodontosaurus* (Stromer, 1931) in which
the postacetabular is much longer than the preacetabular ala.>

First, let me thank you for your considered explanation of the Alvarezsaurus
drawings (reproduced below). I will respond to the Alvarezsaurus claims in
my response to Mickey Mortimer. I have checked the Carcharodontosaurus ilium
you cite, and you are correct the postacetabular region is much longer than
the preacetabular ala. However, I must disagree that it serves as a
counter-example to the hypothesis. Although it's tempting to use the
acetabulum as the dividing point between cranian and caudal halves, this is
not the statement of the hypothesis. A simple way to think about the
hypothesis is to imagine balancing the ilium on a rod perpendicular to the
axis of longest dimension, if the balance point is cranial to the midpoint
the ilium is Saurischian (cranially heavy). Remember also, in Car. the
anterior portion is thicker than the posterior portion, so the side profile
is misleading.

Regards,

Mike Milbocker

-----Original Message-----
From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
Jaime A. Headden
Sent: Tuesday, April 27, 2004 5:59 PM
To: dinosaur@usc.edu
Cc: Mickey_Mortimer111@msn.com
Subject: Re: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis


Mickey Mortimer (Mickey_Mortimer111@msn.com) wrote:

<Yes, well the palaeos drawing (by Jaime Headden) is inaccurate in that
regard.  Note the flared brevis fossa on the left process of my scan.
Jaime doesn't have the original description this was scanned from, perhaps
causing his error.>

  The error was not caused by not having the original paper. I had the
full anatomical analysis at hand, but at the time chose to interpret the
unusual ilium as if it were reversed by Bonaparte originally; the drawing
of the skeleton was performed more than 4 years ago, and has since become
severely outdated. Having a cuppedicus and brevis fossa of the same width,
a pubic peduncle appearing as a typical maniraptoran ischiadic peduncle,
and no apparent antitrochanter, does not help to determine orientation of
the ilia; similarly, at the time, the ilium of *Patagonykus,* another
basal alvarezsaur, has a robust, rectangular pubis peduncle, unlike either
*Alvarezsaurus* (sensu Bonaparte) or *Shuvuuia,* leading one to assume the
triangular peduncle was, in fact the ischiadic ... I still have
trepidation because of this. At this time, more complete ilia were unknown
prior to the description and use of ilia of *Shuvuuia,* and the longer
brevis fossa than cuppedicus fossa inherent in theropods would agree that
the original, sensu Bonaparte, orientation is correct and my skeleton is
outdated and the orientation in error. A more current skeletal drawing by
myself is in submission to the Dinosauricon's image gallery, and this
shows that the ilium would be, in fact, caudally heavy. An articulated
pubis and ischium in further specimens would be appreciated, but the upper
Campanian-Maastrichtian Rmo Colorado Formation from whence *Alvarezsaurus*
derives is unknown for it's preservation of fine details in small
terrestrial forms, preserving instead semi-marine and large terrestrial
forms much more easily.

  Another caudally heavy ilium in similar token was noted by Glut in 1997
as the referred ilium to *Carcharodontosaurus* (Stromer, 1931) in which
the postacetabular is much longer than the preacetabular ala.

  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)




__________________________________
Do you Yahoo!?
Win a $20,000 Career Makeover at Yahoo! HotJobs
http://hotjobs.sweepstakes.yahoo.com/careermakeover