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Re: ANKYLOSAURS, STEGOSAURS, AND PSITTACOSAURS



Steve Brusatte (dinoland@lycos.com) wrote:

<Any additional insights appreciated!  Feel free to jump in (especially
Jaime, who I know was interested in this very subject a few months ago).>

  I haven't much to say that I have not already. My perspective is not on
psittacosaurid relationships, but more on marginocephalian
inter-relationships and the relationship of the group as a whole. 

  That the only other reasonable topologies contra Marginocephalia have
been the Orbitoscuta (pachycephalosaurs plus ankylosaurs), and pachys with
ornithopods. These are based first on the supraorbital ossifications, and
second on the general form of the postcrania, and premax tooth count.
Otherwise, though continuously suggested as stem-marginocephalians,
heterodontosaurs have also been allied to ornithopods upon nearly one
feature, the contact between premaxilla and lachrymal. This has been
Sereno's contribution to the group. Psittacosaurus are themselves united
on their short snouts, which can be considered a character complex
involving reduced dentition count, contact between otherwise unaffiliated
snout bones, closure of the antorbital fenestra into a foramen or
completely absent, elevation of the external nares, and so forth. These
can be both diagnostic to the group and considered expected in a
short-snouted form, thus are mechanically widespread. I have a paper in
prep on this very issue.

  All other forms possess a distinct cranial form and similar pelvic form.
Including almost or compeletely absent prepubic process. There are really
few cranial features that can be shown to group one or two psittacosaur
taxa together, but for the majority, a grade can be shown to progress from
sattayarki to sinensis and meileyingensis, and the topology of characters
suggest that there may be wisdom in recognizing a few polyspecific taxa
commonly referred to as "genera". Resolution may offer the often appealing
idea of naming a new genus to every species, but this resolves nothing of
the relationships of the species, and all species must be treated equally
as sets of independant taxa.

  However, this is not my thing; more Pete's territory than mine, and this
is why I refrained from comment until he. Nonetheless, there is work in
press on this issue. Stay tuned.

  As for me, I am more interested in the sequence of transformation of an
essentially heterodont dentition to the homodont dentition of ceratopsians
proper and higher diplodont pachycephalosaurs. There are also ecological
implcations, and these are in prep as well....

  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.

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