RE: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis

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

Mike Milbocker (mmilbocker@psdllc.com) wrote:

<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.>

  I am not sure how one determines the anterior ilium is thicker than the
caudal region without observing the three-dimensional qualities of it. In
the case of the referred ilium to *Carcharodontosaurus,* this is
impossible as the element has not been described in multiple views to
begin with.

  The cranial and caudal blades of the ilium in a theropod should be
observed as triangles, with the preacetabular ala bearing a medial
cuppedicus shelf, and the postacetabular ala bearing a medial brevis
shelf. The brevis shelf is invariably larger, wider and thicker than the
narrow cuppedicus shelf, reaching even bredth in *Alvarezsaurus* for
example. In some forms, such as *Caudipteryx,* other oviraptorosaurs, and
birds, the cuppedicus shelf is lost or reduced to a lamina, whereas the
brevis shelf is simply reduced and, at least in birds, lost. This would
make the postacetabular ala of a given length in *Caudipteryx* relatively
more massive than the preacetabular ala of the same length, and in
*Caudipteryx* as in several oviraptorids and *Nomingia,* both alae are of
nearly equal dimensions in lateral surface area and length; the presence
of a flange or expansion of the anterior blade is almost nothing compared
to the shelf on the posterior blade. A means of explaining this can be
described as a series of triangles:

  In the preacetabular ala, most theropods have alae whose vertical sides
can be described at a length 4, and the transverse surface ventrally,
corresponding to the cuppedicus shelf width, as a length 1, roughly an
isoceles triangle; the postacetabular ala, for the same height 4 on one
medial side, will have a height of around 4 on the other, but a width of
2-3, depending on the taxon, the height may change and become shallower
than in the anterior blade, as in dromaeosaurids, troodontids, and birds
(aka, Paraves). This provides a cross-sectional width relative to height
between the alae as being subequal, but in *Alvarezsaurus* because the
blades are evenly wide and the posterior blade deeper, as in *Shuvuuia,*
the postacetabular ala has greater mass than anteriorly. In
*Carcharodontosaurus,* as in other basla theropods, the postacetabular ala
is wider than the preacetabular ala, and in this case, also deeper with an
elongated ischiadic peduncle. This makes the above comments on being
"front-heavy" confusing. In *Axasaurus,* for example, there is NO
cuppedicus shelf, as in most segnosaurs, aside from a nascent lamina; the
postacetabular ala and anteriorly to the ischiadic peduncle and large
antitrochanter, are massively widened and rectangular in section, despite
a narrow brevis shelf. This condition is indepedant of inferred comments
on the lower pelvic elements as being "ornithischian." In segnosaurs, the
preacetabular ala is, as in sauropods and some ornithischians, a simple
curved, flattened blade with a thickened dorsal/anterior margin, whereas
nearly the entire caudal 1/3 is bone three times thicker; this is less so
in *Alxasaurus* as it is in *Segnosaurus,* (and is also seen in
*Chirostenotes* convergently but without the lateral curve of the
preacetabular ala), but provides a handy explanation in regards, and is
convergent as basal segnosaurs, such as *Beipiaosaurus* lack the
incrassate, hypershortened postacetabular ala.

  Mass distribution is also affected by the peduncula, as the ischiadic
peduncle also bears a large, massive antitrochanter that is largely
prismatic in shape, whereas the pubic peduncle is anywhere from an
elongate rectangular solid to a prism in shape, and can skew the effects
of "anterior vs. posterior" especially in alvarezsaurs without ischiadic
peduncula, and small (*Shuvuuia,* *Alvarezsaurus*) to large
(*Patagonykus*) pubic peduncula. In ornithischians, the massive rear half
of the ilium is enhanced by the antitrochanter, but the anterior half,
with the large pubic peduncle and the elongated blade, would seem to
afford a balance around the ischiadic peduncle, as enhanced in some taxa,
notably iguanodonts and stegosaurids, by the dorsolateral flange of the
ilium. However, caudally massive theropod pelvises such as in some
abelisaurids and *Coelophysis,* where the balance is similarly around the
ischiadic peduncle, would seem to afford an example of how such variation
cannot be so simply expressed as "ornithischian" or "saurischian."

  Another case example can be found in the herrerasaurid *Herrerasaurus,*
where the caudal half of the ilium is extremely massive and rectangular in
section, owing to the expansion of the dorsal margin of the ilium
laterally, and the preacetabular ala being a small block, a massive
ischiadic peduncle and antitrochanter, and a relatively mediocre,
rectangular solid pubic peduncle. By analyzing the ilium from Novas, 1994,
(Novas, F.E. 1994. New information on the systematics and postcranial
skeleton of *Herrerasaurus ischigualastensis* (Theropoda: Herrerasauridae)
from the Ischigualasto Formation (Upper Triassic) of Argentina. _Journal
of Vertebrate Paleontology_ 13(4):400-423.) one can in fact determine
without too much math that the balance point is, once again, further
caudal than the half-way point.

  This is not surprising given that herrerasaurids, as in non-dinosaurian
dinosauriforms like *Marasuchus,* have caudally-heavy ilia, and were
transitional in this respect prior to expanded caudal alae in
*Thecodontosaurus* (without requisite cranial expansion, another
counter-example), *Saturnalia* (both of whom are saurischian; Langer, M.C.
2003. The pelvic and hind limb anatomy of the stem-sauropodomorph
*Saturnalia tupiniquim* (Late Triassic, Brazil). _PaleoBios_ 23(2):1-40.),
whereas in basal ornithischians the ilia expanded in the other way, in the
so-called saurischian condition, where the basal ornithischian
*Lesothosaurus* shows a "front-heavy" ilium with large pubic peduncle,
short and unexpanded, primtive but narrow postacetabular ala, and slender
ischiadic peduncle without a massive antitrochanter (Sereno, P.C. 1991.
*Lesothosaurus,* "fabrosaurids", and the early evolution of Ornithischia.
_Journal of Vertebrate Paleontology_ 11(2):168-197.).

  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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