Re: 2 become 1 (spinosaurian & titanosaurian)

["Jaime A. Headden" <qilongia@yahoo.com>]

Øyvind M. Padron (gorgosaur@hotmail.com) wrote:

<What are the actual possibilities for Nemegtosaurus to be the head of 
Opisthocoelicaudia and
Angaturama to be the snout of Irritator? Both cases seems pretty obvious, so 
there must be some
other kind of evidence keeping them apart as separate genera?>

  As the situation with *Irritator* is quickly becoming resolved (*Angaturama* 
is a nomen dubium
because the snout probably comes from *Irritator* and not a sunk taxon until 
you can _prove_ the
synonymy), I'll just jump on to the other two, which I've spent some time 
looking at.

  For one, *Opisthocoelicaudia*, but it's anatomy, appears to be an advanced 
titanosaur, though
the caudal morphology has me stumped, as there appears to be succession in 
titanosaur caudals to
become progressively caudally ball-like on the centrum, becoming most derived 
in the
titanosaurids. In all other features, the skeleton is diagnostically 
titanosaurid. The problem
here is *Nemegtosaurus*. The skull resembles that most particularly of 
*Rapetosaurus*, and in that
paper (Curry-Rogers & Forster, 2001) *Opisthocoelicaudia* and *Nemegtosaurus* 
occupy two different
parts of the tree, not together, and the latter nests with *Rapetosaurus*. The 
postcranial anatomy
of *Rapetosaurus* does not in any specific way resemble that of 
*Opisthocoelicaudia*, and there is
such a distinctiveness its incredible. It seems improbably therefore that the 
two may actually be
congeneric. Though certainly, the matrix could be wrong. So I looked at the 
skeletons themselves
(well, through photos and published figures) and found that there are so many 
differences between
*Opisthocoelicaudia* and *Rapetosaurus* that if the skull of *Nemegtosaurus* 
really does indicate
a "rapetosaur" type (i.e., *Rapetosaurus* is a nemegtosaurid) then there is no 
way that the two
Mongolian taxa are the same animal. There is some fuzziness in this, but the 
data appears to be
pointing to two different titanosauroids in the same horizon (like, that hasn't 
happened before --
and look at the Morrison!)


=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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