RE: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis

["Jaime A. Headden" <qilongia@yahoo.com>]

Mike Milbocker (mmilbocker@psdllc.com) wrote:

<I'm a bit curious why you offered Carcharodontosaurus as a
counter-example, the images of which are insufficient by your own
estimation. The reference to the anterior thickness was an extrapolation
from T-Rex, and I should have stated as such. You are correct that
multiple views are essential to be quantitative. I have no really good
examples of Carcharodontosaurus in profile, but if you agree this picture
  www.grinpach.cl/ Carcharodontosaurus1.html

  or Sereno's illustration

  http://www.projectexploration.org/carcharodontosaurus.htm
 
  are accurate, then the anterior mass, helped my the more massive pubic
peduncle suggests the hypothesis is valid for this case.>

  In my post, http://www.cmnh.org/dinoarch/2004Apr/msg00585.html, I
mentioned that mean length and distribution of the heavier, "would be"
wider and deeper postacetabular ala is greater than the preacetabular.
This was all. I noted that by means of comparison, this would imply it
would be caudally heavy, using *Allosaurus* and *Sinraptor* to compare
three-dimensionally known ilia, close relatives of *Carcharodontosaurus.*
Unlike other relatives, *C. saharicus* (refered material) has a deeper,
longer postacetabular ala; at the time this was used to attempt to refute
the typical "heavier anterior blade," especially given the relatively more
massive ischiadic peduncle than in relatives. This was used to illustrate
that not all theropod ilia are so easily described as cranially heavy just
from "looking" at them.

<I agree Alvarezsaurus is closest to a counter-example, but the fossil
evidence is inadequate for reasons discussed earlier. If I understand
correctly the words above, you are suggesting the majority of theropod
ilia are caudally heavy.>

  No, rather I am suggesting that the ilia are not so easily described as
being "cranial" or "caudal" heavy based on just looking at them. As was
mentioned for ankylosaurs, ilia must always be considered in three
dimensions, via mass distribution laterally and medially, as well as
craniocaudally. Similarly, the lateral acetabular shelf, a medial
acetabular shelf, and the relative distribution of cortical versus
cancellous internal bone and their effect on mass distribution further
complicates the idea that a simple observation can determine mass. I would
think CT scanning of reference ilia (an expensive project, one would
think) is the only way to properly test the idea that cranial or caudal
heavy ilia, as well as the potential mechanical loading effects of ilia,
as being neccessary referents.

<For Caudipteryx, I can unequivocally say the postacetabular ala is not
more massive than the preacetabular ala per unit length - I'll make a cast
and send you one if you wish.>

  A cast of which specimen? The five known specimens, all with ilia, and
only three with complete ilia, are all preserved flattened. Any three
dimensional effects would be artificial. I would wait for a Yanzigou or
other lower Yixian Formation site to preserve such a specimen that could
be modelled three-dimensionally.

<Wouldn't the absence of an ischiadic peduncula in Alvarezsaurus, using
your words "skew the effects" to greater anterior mass distribution making
it according to the hypothesis more Saurischian? I agree the peduncula do
affect the mass distribution, and why not?>

  *Alvarezsaurus* does, in fact, preserve an ischiadic peduncle ... the
"neck" of this to the diaphysis, or contact surface with the ischium, is
simply missing, only the surface contact is present, and this is preserved
as a broadening of the ilium at the antitrochanter. Without getting the
specimen and modelling it's mass, I couldn't guess at its possible
volumetric effects.

<In this example, you will notice the ilium tapers ventrally as you
traverse caudally. Indeed, the postacetabular ala is large compared to the
preacetabular ala which places the peducula in the cranial half. The
penducula more than offset, in weight, the expansion of the dorsal margin
of the ilium. Given the caudal shortening of the height of the ilium, I
find the balance point cranial to the mid-point.>

  How so? Do you have a mathematical or volumetric model of the ilium to
acheive this? In this case, I would really like to see the data, if this
is freely available at this point.

<I guess I'm confused here as to where you are taking your reference
point.>

  *Lesothosaurus* would seem to displace a good deal of it's mass
cranially, whereas *Thecodontosaurus* and *Saturnalia,* unlike
*Herrerasaurus,* have much of what would seem to be a massive ilium,
displaced caudally, as do other basal sauropodomorphans. These are not
used to counter-example, but provide the line is probably not so
"ornithischian/saurischian" thin, but related to work in the 70's and 80's
to the distinction between dolichoiliac condition, and brachyiliac
condition, where the postacetabular length reflects distribution of mass
in the ilium cranially or caudally, and the altiiliac condition, seen in
segnosaurs and sauropods, reflects to mass distributed dorsally, as the
acetabulum remains evenly cranial and caudal despite elongation of the
preacetabular ala.

  It's not that I disagree with you, as I understand that mass
distribution in ilia is an interesting and possibly locomotory issue that
is of interest to myself, and you took the time to describe your method
clearly. It's that I feel the distinction is not so clean, but the grey
area is more important to me than the taxa at each end of the spectrum.
These taxa tend to be basal, and mix it up, as a result of what Novas and
Sereno have described as part of the experimental evolution of exclusive
bipedal locomotion in basal dinosaurs. Adam Yates and especially Max
Langer are working on the evolution of the hip in basal sauropodomorphans,
and John Hutchinson has done so for the evolution of theropods, and these
are areas that need elaboration.

=====
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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