RE: thanks and a note about _Alxasaurus_

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

Jack Conrad sent out the following to me, but later wrote me to say he had
wanted to send it to the whole list.  I'm forwarding his message, with my
replies interpolated.

>From: Jack <jconrad@lib.drury.edu>
>At 11:23 AM 1/29/99 -0600, Jack Conrad wrote:
>
>> Both Makovicky & Sues (1998) and my own phylogeny paper in Gaia
>> (submitted
>> prior to having seen their _Microvenator_ paper) suggest that the
>> oviraptorosaur condition could be a secondary reversal from the
>> opisthopubic
>> condition under certain optimizations.
>
>Can you provide the full reference for your Gaia paper?  I've heard about
>it, but haven't seen it.  


Would love to, but it is still not published yet.  Grrr... (editorial slow
down due to other projects on the part of my co-editors).


>> It must be pointed out that NO pubis is known for _Alxasaurus_ and there
>> is
>> nothing in the anatomy which indicates it was other than opsithopubic.
>> Given that there is no positive data for propuby in this dinosaur, and
>> given
>> that all other therizinosauroids are opisthopubic, our initial
>> assumption
>> should be opisthopuby in _Alxasaurus_.
>
>Just don't buy the monophyly of Therizinosauroidea, but point well taken.


I still don't see your reasoning that Therizinosauroidea is non-monophyletic
(para- or polyphyletic), given that nothing known in the anatomy of _Alxa._
seems to link it more closely to anything but other therizinosauroids.


>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?


Very hard to say: have to base this on outgroup comparison.


>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)


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.


>> So, the following statement from the website is misleading:
>> 
>> ]] When known, the pubis of these "segnosaurs" is completely retroverted
>> as
>> in the
>> ]] Ornithischia, but Alxasaurus seems to possess a secondarily propubic
>> condition.
>
>Yeah, that is pretty bad.  Apologies to all.  It will be fixed today, I
>guess I was basing this on the idea that therizinosauroids were more
>advanced than basal oviraptoroids.  


Urgh.  Why "more advanced"?  Advanced in a different way, yes.  However, on
skull evidence, you could argue oviraptorosaurs are more advanced.  That's
why it is more important to think about sister group relationships and
character distribution than (relatively metaphysical) concepts of "advancement".


>> Reconstruction of phylogeny should be based on positive observations,
>> not on
>> the missing parts.
>
>Indeed.  I did have some rather good logic going at one point, but it got
>tainted along the way.  Thanks, Dr. Holtz, for pointing this out.  


No problem.


>> Looking forward to the revised figure.
>
>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.)


>Thanks again for setting me straight, Dr. Holtz.  This is the type of thing
>I need. 
>
>Jack 


Again, no problem, and good luck.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661