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Thyreophora and the Ever-Burgoening Question



  Which is: When a group is assumed to be monophyletic, is it
really?

George Olshevsky (dinogeore@aol.com) wrote:

<What I'm saying is that since so many stegosaurian characters
are autapomorphies, it is quite possible that a few characters
here and there could have evolved independently and
coincidentally to resemble characters in other, distantly
related, groups.>

  Indeed-arooney. *Echinodon* has fanglike mesial teeth, the
problem is that the most fang-like is on the _upper_ jaw, behind
the possible diastema; no fang-like teeth on the lower jaw, and
the crowns are distinctly phyllodont and resemble those of
*Scutellosaurus* and *Scelidosaurus* than they do
*Heterodontosaurus*; they are not in the least chisel-like.
Thus, it shares more similarities to basal thyreophores/basal
ankylosaurs/basal stegosaurs or wherever they might go, than
even cerapods.

<In other words, the apparently shared characters could have
appeared by long-branch convergence rather than via common
ancestry. If there is long-branch convergence, then the
resulting shared characters are not evidence of common
ancestry.>

  No, but it's not possible to test long-branch attraction
without having genetic markers and rates of mutation to clock
the splits in lineages. The considered groups here in even the
most radical interpretation George has offered as a possibility,
are separated by only one or two nodes, or even as a strict
polytomy, in which case they arise all from the same single
ancestor, and not a bunch in a serially radiating paradigm.

 --+--Pisanosaurus
   `--+--Lesothosaurus
      `--+--Ankylosaurus
         |--Stegosaurus
         `--Cerapoda

  or:

 --+--Pisanosaurus
   `--+--Lesothosaurus
      `--+--Ankylosaurus
         `--+--Stegosaurus
            `--Cerapoda

  instead of:

 --+--Pisanosaurus
   `--+--Lesothosaurus
      `--+--+--Ankylosaurus
         |  `--Stegosaurus
         `--Cerapoda

  If even George's most extreme restructuring, thus:

 --+--Ankylosaurus
   `--+--Lesothosaurus
      `--+--Stegosaurus
         `--Cerapoda
             |--+--Marginocephalia
             |  |--Heterodontosaurus
             |  `--Pisanosaurus
             `--Ornithopoda

  were true, it would require massive amounts of reversals
between basal dinosaurs to Pisanosaurus, and Ornithopoda, with a
basal non-genasaurian being interpreted as a possible relative
of heterodonts and marginocephalian dinosaurs. That the
literature is rife with historic reference to the
mutually-closest relationships between ankylosaurs and
stegosaurs (though, true, the earliest were based primarily on
the prescence of dermal armor), and continues to be expressed in
both intuitive and empirical study. Not that I'm a
go-with-the-flow kinda guy [I'm not, and I know George is not
:)], but I find the monophyletic [or holophyletic, if you
prefer] Thyreophora to be the best supported by empirical data
thus far, including parsimony analysis.

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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