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Re: Family Nemegtosauridae

At 11:31 AM 4/7/99 -0500, TWILLIAMS wrote:
>This reminds me of something which is bugging me.  One thing 
>which both Upchurch (1995) and Sereno and Wilson (1998) agree on is 
>that _Opisthocoelicaudia_ is a titanosaurid.  But 
>_Opisthocoelicaudia_ has opisthocoelous caudals, whereas other 
>titanosaurids have PROcoelous caudals (at least for the anterior 
>part of the tail).  How did _Opistho.._ get opisthocoelous 
>caudals from an ancestor with the exact opposite (procoelous) 
>arrangement?  For this to happen the caudals must have passed 
>through an amphicoelous/amphiplatyan phase, but this seems improbable 
>to me.

If titanosaurs were just caudals (like the type of _Titanosaurus_) your
point would be pretty strong.  As it is, though, we actually know a WHOLE
lot about titanosaur postcrania, and there is more to this group than just
procoelous caudals.  It is on the basis of these other specializations (of
the vertebral column, forelimb, manus, pelvis, etc.) that _Opistho._ lies
among the titanosaurs.  (Oh, yeah, add in Salgado et al. to the folks who
found it was a titanosaur).

In fact, basal titanosaurs actually have amphicoelous/amphiplatyan caudals.
Further, I don't see why the transformation of caudal centra articulations
would be so improbable, in comparison to the rest of the anatomy of the animals.

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