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RE: What group has the most work that needs to be done?



Ken Carpenter (Ken.Carpenter@dmns.org) wrote:

<Granted pachies had to evolve from another dinosaur group. But to unite them
with ceratopsians because they have an expansion of the rear cranial table is
very weak. The most primitive of ceratopsian frill is structurally different
from that seen in pachies. The skeleton of pachies don't figure much in many
cladograms, but pachies are more than their skulls. I would argue that there
are more similarities between the pachy skeleton and other ornithopods (such as
Thescelosaurus). Yes, I know that the argument could be made that these
similaries are plesiomorphic, but that is a perspective issue and may not hold
true relative to Heterodontosaurus-Lesothosaurus (OK, I admit cheating, haviing
looked at Mike Triebold's unpublished pachy skeleton).>

  I agree that expansion of the caudal cranial table is not a good criterion,
since it would seem to be convergent in ceratopsian evolution at least, with
that seen in pachycephalosaurs. However, the similarities have been extended to
more than caudal cranial expansion, including some occipital features variously
singled out, jaw features, and the presence of a set of rostral conical "fangs"
and a subnarial gap. Simiarly, pelvic and femoral features also seem to ally
them, as seems to be clear with *Stenopelix*, *Psittacosaurus*,
*Heterodontosaurus* and *Homalocephale*. A lot of the problems may derive from
most basal pachycephalosaurs being poorly representated postcranially, or
incompletely so, and that basal ceratopsians are already highly modified in the
cranium from any other ornithischian. Facial shortening, rostral development,
dorsal premaxillary expansion, jugal widening and ventrolateral tuberosity
expression all appear to vary, implying that Ceratopsia is not very well
supported cranially, either, but their consistency among taxa implies
gradational and functional variation.

  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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