RE: thanks and a note about _Alxasaurus_

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

Jack Conrad wrote:

<<I definitely agree that oviraptoroid pubes are likely to be
secondarily propubic. But is this 2ndary propuby from a vertical pubis
or a opisthopubic condition?>>

[snipped]

<<Based on the assumption (for now) that Therizinosauroidea is
monophyletic and that _Alxa._ is the most primitive member of said
taxon (as per Russel et. al), would it be unprudent to suggest that
the opisthopuby in Therizinosauridae (family exclusive of basal
_Alxa._) and the 2ndary propuby of oviraptorosaurs is could be derived
from a propubic ancestor? (legitimate question, not loaded or trying
to lead)>>

and Tom Holtz replied:

<Well, of course, they ULTIMATELY are from a propubic ancestor, since
that is the ancestral condition for all theropods (indeed, all
tetrapods!).
 
 The situation for the pubic condition runs thusly:
 
 Most recent analysis find the following relationships:
 
 (Oviraptorosaurs + Therizinosauroids) + (Dromaeosaurs + Birds)
 
 Therizinosauroids, dromaeosaurs, and basal birds are opisthopubic,
oviraptorosaurs are propubic. There are two equally parsimonious
explanations for this distribution: EITHER opisthopuby evolved in the
common ancestor of all these, and oviraptorosaurs reverted to propuby;
OR the common ancestor of all these was propubic and opisthopuby
evolved separately in theriz.s and dromaeosaurs+birds. Each requires
two evolutionary steps. Additional, more complicated scenarios are
possible, but require additional evolutionary steps for which there is
no evidence at present.>

  Okay, I ran this two-step analysis, and got the following results,
in THREE trees (abbreviations = o- Oviraptorosauria, th-
Therizinosauroidea (incl. *Alxasaurus*), d- Dromaeosauridae, a- A ves;
op- opisthopubic condition, pro- propubic condition):

o  th d  a       o  th d  a       o  th d  a
 \ /   \/__op     \ /   \/         \   \ \/
  \`op /           \`pro/           \   \/__op
   \  /             \  /             \  /
    \/               \/               \/
     \__pro           \__op            \__pro
      \_______________/________________/
                 /
basal condition_/

  The second tree is only different in there being a basal
opisthopubic nature to the group presented, and Oviraptorosaurs
reversed the condition from a basal configuration. The first required
convergence, and the third and second a single reversal event. The
third requires Therizinosauroidea to be closer to Dromaeosauridae+Aves
than to Oviraptorosauria. Tell me if I screwed up her, I did this on
paper. The trees may be scrambled for those without monospaced fonts
on their mail-readers. Sorry; format into monospaced to rearrange.

<<It will be two figures. One with opisthopuby and one with a vertical
pubis (similar to what has been suggested for _Archaeopteryx_).>>

<(And is found in _Rahonavis_, at least.)>

  Not to mention *Unenlagia*.

==
- Often, it is the man who is brought
  down the path to the end who does
  not see his own steps. -

Jaime A. Headden

Qilong, the website, at:
_________________________________________________________
DO YOU YAHOO!?
Get your free @yahoo.com address at http://mail.yahoo.com