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Pygostyle Development in Oviraptorosauria



  Dan (dbensen) asked if there were pygostyles known
in the two more basal known oviraptorosaurian taxa
which can be definitively ascribed to the
Oviraptorosauria; and Justin (Thescelosaurus) rightly
pointed out the lack of caudal verts in the one and an
incomplete series in the other. Mike (tkeese1) asked
about the completeness of tail verts and series in
oviraptorosaurs.

  Because Justin handled most of Dan's questions, I'll
grab the rest and Mike's, and it goes this way in fact
-- and the rest is theory:

phylogeny (as published)   stratigraphy (as published)

           ^   Elmi        Maas    |    Hell Creek.
            \ /            ---------    Nemegt
           Chiro           Camp    |    Judith R.
             |  ^ Co Ing           |    Barun Goyot
             |  \  \/      ---------    Djadokhta +
             |   Ovi-*     Sant    |    Ukhaa Tolgod
             \    |  ^     -----------------------
              \   / /      Coni    |
               \ / new     -----------------------
      Caena     |          Turo    |    Bissekty
        |       |          -----------------------
        \       /          Ceno    |
         \     /           -----------------------
          \ *caud/jaw?     Alb     |    Dino Cove
Micro      \|/_*sacrm      Apt     |    Cloverly
  |         /                      |    Santana
  \        /               -----------------------
   \      /                Barr    |
    \    /                 Haut    |
     \  /                  Vala    |
      \/                   Berr    |
       |                   ========================
       \  *cerv            Tith    |    Morrison
        \/                 ------------------------
         |

  Not to get overly complex with this (which I just
did), the pygostyle occurs fairly late in Ovi
evolution, where previously fairly developed tails
indicate possibly longer forms. While the tail of
*Microvenator* is fairly incomplete, there's enough of
it to compare to the new taxon, and this suggests the
tail was fairly longer -- possibly intermediate
between late ovis and the new taxon in length. Or
longer. If *Caudipteryx* (not included above) proves
to be an oviraptorosaur, then we have an interesting
problem: either late stage ovis elongated the tail
from a previously shortened form, a secondarily
elongated tail!, or are not directly descendant to the
short tailed form. What joy.

  Should the first hypothesis prove true, then,
applying the biological principle of "what's lost is
gone," unless the embryos retained the ability to
ossify caudals while passing through the brevicauda
stage, then the second one applies, then the
short-tailed forms achieved their status (twice?)
independantly. Either hypothesis is plausible, and
given the embryo known, provable if someone could
count the caudals.

  Hmm. I'd like to have that specimen in hand right
now ... ah, dreams ... :)

  However, Brush is correct is pointing out the
terminology of "pygostyle" does not apply to the new
oviraptorosaur, in that such a structure is a fusion
of vertebrae or a single ossified unit forming a basis
for retrices, and the new specimen, as in
*Caudipteryx*, does not appear to posses fusion, but
rather strong appression, of the elements in question.

  Or we could always wait until it's described and see
what Clark et al. are going to pull out of their
sleeves....


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

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

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