On Dicynodonts and Oviraptors

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

  I'm going to cut this one down quickly, and put
replies to several posts into the form of a short
essay, instead of reply seperately to each and annoy
Mickey by having fifteen posts in a day or so. :)

  Oviraptorosaurs (or more correctly, *Oviraptor*) was
first compared to dincynodonts by Osborn, in 1924, way
back in the day. He just didn't describe the
relationship. Cracraft (1970) went into further
detail, but forgot that other oviraptorosaurs had the
morphology of the jaw articulation and palate, and so
emphasized the avain relationship of the
caenagnathids.

  Romer (1966) and Carroll (1988) have both drawn the
eye to dinosaurian oviraptorosaurs as convergent to
dicynodonts, and what's more, described the
similarities in even more detail. But the extent to
which they did this leaves you gasping for more (if
you're like me) and so one has to research the
dicynodont literature to understand the full set of
similarities between the two.

  Most advanced dicynodonts lack teeth, with some
lineages, like the diictodontoids preserving maxillary
tooth rows; the basal venjukoviamorphs are cool ro the
extreme in have full complements of chopping and
grinding teeth, resembling hippos in their complement,
and show that dicynodonts evolved towards a condition
of edentulousness and features of the articular hinge
that allowed the jaws to slide fore-aft against each
other (propaliny). Hotton (1992) used *Emydops*, a
dicynodontoid, as an example of this remarkable jaw
structure, where the massive symphysis was the lathe
which sheared against the possibly keratin-covered
palate and anterior beak. Hence loss of anterior
teeth.

  Problematically, Smith (1992) and Barsbold (1977)
described cheifly the ab/adduction actions of the jaws
in oviraptorids, but did not remark more upon the
propalinal capabilities thereof, and thus lose some
weight for their arguments of dietary function. The
jaws, though capable of only two out of the three
possible planes of movement, were nonetheless more
advanced over the single (sagittal) plane described in
most other theropods (I personally think
ornithomimosaurs and troodonts need more examination).
The increase in the size of the mm. pterygoideus and
adductor mandibulae (externus and profundus) and their
more rostral orientation relative to other theropods
explains the rostral shift in the ectopterygoid, seen
also in dicynodonts, and in the extraordinarily huge
adductor chambers; similarly, long-arm levers in the
jaw were increased to give more power to the pulling
of the muscles and give greater control over
orientation of the jaw in any given phase of the
adduction/abduction cycle. These relative similarities
between the dicynodonts and oviraptorosaurs have been
touched on only briefly in the literature, and require
much more examination. Guess what I'm doing.

  Likewise, relative lightening of the skull in
oviraptorosaurs gives them more room for their
musculature, and reduces the skull to a simple kinetic
construction externally. Norell disagrees with me, but
I feel the palate needs much more closer attention
then as such afforded lately by Osmolska or Barsbold.
Maybe when I get a look at Elzanowski (in press), I'll
be able to get a clearer look; now I just need to get
out to the ROM and AMNH (MAE) to get into their
collections.

  Toby White specifically wondered about the huge size
of the orbit in oviraptorosaurs; this is related to
the lightening of the skull and the emphasis on jaw
occlusion and such, not to eyeball-size, and the eye
as such was only ~50% the orbital diameter. Such an
estimate is derived from study of sclerotic rings in
ornithomimosaurs and other dinosaurs, as well as
birds. Oviraptorosaurs were also mildly stereoscopic,
a big shift from the former apparent trend in
increased stereoscopism in coelurosaurs, suggesting
the eye was not only small relative to the skull, but
projected mainly laterally, and that oviraptorids (at
least) were not big on hunting. Contrariwise for
*Avimimus*, whose eyes may have been more stereoscopic
than tyrannosaurine and troodont theropods (see
Kurzanov, 1983, 1987).

  I've posted before on the cassowary analogy, so
mention this only here.


=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

Qilong---is temporarily out of service.
Check back soon.
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