[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: thanks and a note about _Alxasaurus_



At 11:23 AM 1/29/99 -0600, Jack Conrad wrote:
>As
>to the opisthopubic condition versus a propubic _Alxasaurus_, I realize that
>the pubic peduncle on the ilium of _may_ indicate opisthopuby.  However, it
>has also been noted that _Alxasaurus_ resembles oviraptoroids in some
>regards in some regards.

In fact, the ilium of therizinosauroids in general share several derived
features with oviraptorosaurs, as noted by Russell & Dong (1993) and
subsequent authors.

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.

>Now I'm not saying that I am right, I'm only trying to explain why I
>restored the animal as I have.  I will say, however, that I am working on a
>new illustration of the skeleton with a less derived skull and perhaps a
>more vertical pubis.  I may, however, redo the existing illustration a
>little "better" and present both on the site together.  

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_.

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.

_Alxasaurus_ *seems* nothing of the sort.  *If* you were to find a propubic
condition in this dinosaur, or even a partial pubis, this statement might be
fair.  At present there is no reason to assume other than opisthopuby.

Furthermore, the idea that _Alxasaurus_ seemed to be propubic was cited as
evidence that _Alxasaurus_ was not closely related to the classic
"segnosaurs".  This statement is also misleading, as we have no pubes of
_Alxasaurus_ at present, and nothing in the ilium suggests it was propubic.
Yes, it could have had an _Ingenia_-like pubis, but there is no positive
support for this supposition.

Reconstruction of phylogeny should be based on positive observations, not on
the missing parts.

Looking forward to the revised figure.

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