Re: titanosaurs

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

DRosent (DRosent288@aol.com) wrote:

<Query on Nemegtosaurus and Opisthocoelicaudia:
 
1.  Opithocoelicaudia and Nemegtosaurus have recently been reclassified as
aberrant titanosaurids.  But the neural spines on the neck vertibrae are
bifurcate, whereas those on titanosaurids are single.  The caudal
vertibrae are opisthocoelus, whereas on titanosaurids they are procoelus.
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.  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.>

  The idea of these two being synonymous has been taken both seriously and
as a joke in the past. Only one phylogenetic analysis has included both in
a cladistic matrix, whereas both have been assessed seperately based on
the thrust of the analysis. The problem? *Opisthocoelicaudia*, in short;
it is an aberrant titanosaur. Note, there seems to be a generic
"Titanosauridae" above for titanosaurs, but this just be my
interpretation. *Opisthocoelicaudia* is certainly unique in its caudal
vertebral structure, but as few sauropods period have such a one, much
less sauropods with it's _other_ features, this is not evidence of it
being a non-titanosaur or even non-titanosaurid. Many titanosaur features
are found in it, in fact: non-forking haemal spines, neural arches of
caudal vertebrae set forward on the centrum, neural arches of dorsal
vertebrae short in height and the spine comprises less than 1/2 the height
of the vertebrae, as well it is caudally inclined, six sacral vertebrae,
pubis longer than ischium, femur with distinct lateral tubercle and femur
with deeper fibular condyle than tibial, resulting in a wide-gauge stance,
manus without much articular area for extra phalanges, and metacarpal arch
bound into a tight C-shape, ulna with distinct olecranon jutting far out
from the humeral posterior margin in side view, and with a distinct medial
crista separated from the proximal margin by a cleft or fossa, ulna
distinctly shorter than the humerus by less than 75%, a squared off distal
scapula and a gross scapulocoracoid shape identical to some titanosaurs.
In fact, the only cladistic analysis, not truth in and of itself (the
*Rapetosaurus* analysis of Curry-Rogers and Forster, 2001), finds this
taxon particularly close to the Saltasauridae, where its unique features
(opisthocoelous caudals, forked anterior dorsal and last few cervical
vertebrae) only seem to serve to distinguish it, as unique, while others
(including fusion of the haemal arches to the vertebrae) occur in other
varied taxa, regularly in *Phuwiangosaurus*.

  Curry-Rogers and Forster went on to detail and reflect on the work of
*Nemegtosaurus* and found that both *Nemegtosaurus* and *Quaesitosaurus*
are closer to _bona fide_ titanosaur *Rapetosaurus* more advanced than
*Malawisaurus* and *Tangvayosaurus* but less so than Saltasauridae;
Upchurch's work used what can be considered simple plesiomorphies and
unique shared features of the jaw structure with parasphenoid rostrum
(also influenced by jaw muscles) to relate nemegtosaurs to diplodocoids,
but as it might be clear from above, and from Upchurch's work, these
features depend on the structure of the jaw, a likely convergent structure
among similar-feeding animals. *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).

  The archives are rife with discussions of the affinities of these
sauropods.

  Cheers,

=====
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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