Re: Family Nemegtosauridae

[<TWILLIAMS@canr1.cag.uconn.edu>]

> 
> 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.

If I've got a handle on titanosaur evolution, the primitive condition 
is to have amphicoelous/platyan centra throughout the tail. 
_Malawisaurus_ and _Janenschia_ both show procoelous anterior 
caudals.  (I know _Janenschia_ could be an amalgam, but the 
strongly procoelous anterior caudals look titanosaurid).  

In more derived titanosaurids like _Alamosaurus_ and _Saltasaurus_ 
procoely extends through the mid-caudals too.  According to recent 
cladistic analyses, _Opisthocoelicaudia_ belongs among these derived 
titanosaurids.  This means either that procoelous caudals evolved 
independently among certain titanosaurid genera (not so unlikely in 
my view, since some basal titanosaurs show anterior caudals that 
are only mildly procoelous), or that _Opisthocoelicaudia_'s ancestors 
were procoelous, then reverted to the amphicoelous/platyan condition, 
then became opisthocoelous.  I still think this transformation is a 
little improbable.


Tim