Re: Triassic Sauropods

["Tim Williams" <twilliams_alpha@hotmail.com>]

Dinogeorge@aol.com wrote:

> My guess is that this and a lot of other minor things are long-branch
> convergent in Opisthocoelicaudia and titanosaurids. There is just NO WAY
> Opisthocoelicaudia is a titanosaurid, as in some cladistic classifications;

I tend to disagree.  The similarities in the limb elements and pelvis are 
striking.  True, _Opisthocoelicaudia_ has deeply opisthocoelous caudals 
whereas derived titanosaurs (its closest relatives, according to most 
analyses) show deeply procoelous caudals.  Impossible?  How about 
saltatorial evolution.  With what we know about homeotic genes and pathways, 
I'd be willing to bet on it.  Now, I'll just pop into my time machine 
(setting: 70 MYA, Mongolia) and grab a DNA sample from the nearest 
_Opisthocoelicaudia_.

> the dorsal and caudal verts couldn't be more different. Indeed, I'm fairly
> sure that Opisthocoelicaudia is a junior synonym of Nemegtosaurus, as has
> been proposed before; the Nemegtosaurus head probably does go with the
> Opisthocoelicaudia body. Just as the skull of Nemegtosaurus is >roughly<
> transitional between macronarian skull and diplodocimorph skull,

Is the skull of _Nemegtosaurus_ more diplodocid-like or brachiosaurid-like?  
This appears to lie in the eye of the beholder.  The known skulls of Late 
Cretaceous sauropods tend to be crushed and/or fragmentary.  The skull of 
_Nemegtosaurus_, like the skull of _Antarctosaurus_, has traditionally been 
reconstructed with _Diplodocus_ in mind.  However, several researchers 
(Salgado, Calvo among them) have presented a strong case for re-building the 
skull more along the lines of _Brachiosaurus_ than diplodocids.

Not surprisingly, this has come up before...
e.g. http://www.cmnh.org/fun/dinosaur-archive/1997Aug/msg00360.html


> the body of Opisthocoelicaudia >roughly< transitional between the 
> macronarian
> body and the diplodocimorph body.

Perhaps in overall form, but the devil's in the detail.  
_Opisthocoelicaudia_ shows a *lot* of titanosaurian characters, and I don't 
think they can be discarded just be invoking the specter of "convergence".  
("Convergence is an insidious and treacherous trap, baited and waiting for 
the unsuspecting worker... etc etc)


Tim

------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com