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RE: titanosaurs

DRosent (DRosent288@aol.com) wrote:

> Opithocoelicaudia and Nemegtosaurus have recently been reclassified
> as aberrant titanosaurids.  

The Titanosauria is so replete with peculiar sauropods that I'm not sure
what qualifies as 'aberrant'.  As Jaime noted, _Opisthocoelicaudia_ seems to
come out as close (or even within) to a derived subset of titanosaurs, the
Saltasauridae.  The status and position of _Titanosaurus_ itself (and
therefore the validity of Titanosauridae) is controversial; but
_Titanosaurus_, saltasaurids and a number of other titanosaurian genera
(including _Opisthocoelicaudia_) appear to belong to a clade of derived
titanosaurs that have strongly balled-and-cupped caudal vertebrae.

> The caudal vertibrae are 
> opisthocoelus, whereas on titanosaurids they are procoelus.

The opisthocoelous caudals are certainly unusual; but other derived
titanosaurs beside _Opisthocoelicaudia_ have non-procoelous caudals in the
first half of the tail.  BTW, as Virginia Tidwell (among others) has pointed
out, the term 'procoelous' covers several different morphologies.  Strong
(or deep) procoely (=cupped anterior articular surface + balled posterior
articular surface) is the morphology expressed in most derived titanosaur
caudal series.  _Mamenchisaurus_ (a non-titanosaur) also has strongly
procoelous proximal caudals.

A combination of strongly amphicoelous and strongly procoelous caudals are
evident in certain derived titanosaurs.  Strongly opisthocoelous caudals
have also been reported for at least one South American titanosaur specimen;
though this morphology does not extend for most of the caudal series in this
undescribed taxon.

> Assuming that Nemegtosaurus and Opisthocoelicaudia are the same animal,
> wouldn't it make sense to erect a new family for them.  I agree that it
> was wrong to shoehorn them into Camarasauridae, but it seems now that
> paleontologists are making the same mistake trying to shoehorn them into
> the Titanosauridae.  

I don't think so; _Opisthocoelicaudia_ sits quite comfortably in the
Titanosauria, close to the saltasaurids.  Don't be thrown off by the
strongly opisthocoelous caudals; in most other respects, the anatomy of
_Opisthocoelicaudia_ is that of a derived titanosaur (as Jaime discussed).
The unique caudal morphology of _O_ is an autapomorphy of the genus, and
even though the idea seems strange, this morphology appears to have evolved
from the procoelous condition.  Stranger things have happened.  

Recent papers have drawn attention to the range of caudal morphologies found
in various titanosaur species.  The notion that a sauropod must have at
least the first half of the tail composed of strongly procoelous vertebrae
in order to qualify as a titanosaurid is no longer true.

> It has also been suggested that they are derived Euhelopodids, but they 
> don't seem to fit there either; the chevron bones inthe caudal 
> vertibrae are simple, not forked.>

"Forked" or skid-like" chevrons actually have a wide distribution among
sauropods, and are not limited to diplodocoids and

Jaime Headden wrote:
> *Rapetosaurus* is a very similar skulled animal, and shows that 
>*Nemegtosaurus* was a un-*Opisthocoelicaudia*-like
> animal with a titanosaurian nature, but more basal (*Opisthocoelicaudia*
> clades as a basal "saltasauroid", just outside the Saltasauridae).

This analysis has a few caveats, among them the data matrix entries for
_Titanosaurus_ and _Antarctosaurus_, which left me a little bewildered.