Re: Sereno's Prosauropoda

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

At 04:17 PM 7/7/99 -0500, Tim Williams wrote:

>Does this mean the Melanorosauridae could be the basal prosauropod family?

Err, tough call.  Monophyly of "Melanorosauridae" has not been well
established.  If Sereno (and others working on prosauropods, whose
unpublished work shows a similar position) are correct and _Riojasaurus_ is
a basal taxon, then most of the characters that have been used to unite
melanorosaurids are in fact simply primitive characters.

>Sereno regards the Prosauropoda as monophyletic i.e. the sister-group to the 
>Sauropoda, and not a paraphyletic series of outgroups.  This could mean that 
>the melanorosaurids are closest to the origin of the sauropods after all - 
>not as the most "advanced" prosauropod group, but as the most "primitive" 
>prosauropod group.

Indeed (and technically not "closest" but "most similar in form").  However,
I have to admit that I have yet to see an analysis where prosauropod
monophyly was tested rather than assumed a priori (that is, a datamatrix
which includes basal members of non-sauropodomorph dinosaur lineages, as
well as individiually coded "prosauropod" and primitive sauropod taxa).  I
would not be terribly surprised, for example, in seeing _Thecodontosaurus_
as the sister taxon to all other sauropodomorphs, and an unresolved
trichotomy between _Riojasaurus_, Plateosauria (sensu Sereno), and
Sauropoda.  Other topologies would not surprise me as well.

However, word on the sauropodomorph grapevine is that Plateosauria
(plateosaurids and massospondylids) is very likely a monophyletic group
exclusive of Sauropoda, regardless of the position of other classic
prosauropods (_Thecodontosaurus_, _Riojasaurus_, _Ammosaurus_,
_Anchisaurus_, _Melanorosaurus_, etc.).

                        Thomas R. Holtz, Jr.
                        Vertebrate Paleontologist
Deptartment of Geology                  Director, Earth, Life & Time Program
University of Maryland                  College Park Scholars
College Park, MD  20742       
Webpage: http://www.geol.umd.edu        Phone:301-405-4084
Email:tholtz@geol.umd.edu               Fax:  301-314-9661