Re: Triassic Sauropods

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

George Olshevsky (dinogeorge@aol.com) wrote:

<There is just NO WAY Opisthocoelicaudia is a titanosaurid, as
in some cladistic classifications; 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.>

  Possibly. Premise #1: *Nemegtosaurus* and *Opisthocoelicaudia*
are synonymous. Refutation: Preservation does not find this
synonymy; we must conclude the premise plausible, not a basis
for diagnosis or analysis.

<Just as the skull of Nemegtosaurus is >roughly< transitional
between macronarian skull and diplodocimorph skull, so is the
body of Opisthocoelicaudia >roughly< transitional between the
macronarian body and the diplodocimorph body.>

  Premise #2-a: *Opisthocoelicaudia* is a form transitional
between the diplodocimorph and macronarian morphologies; Premise
#2-b: *Nemegtosaurus* is a form transitional between the
diplodocimorph and macronarian morphologies. Refutation:
Upchurch(1995, 1998) provides a non-Macronarian position (within
Diplodocimorpha has been suggested) for *Nemegtosaurus,* Wilson
& Sereno (1999) puts it into Macronaria, and Wilson (1999)
provides data to indicate that the femoral morphology, and
Wilson and Sereno (1999) provide the dorsal/caudal morphology
[both of these for *Opisthocoelicaudia*] is diagnostic of basal
titanosauriforms, and in fact titanosauroids. Refutation 2:
Skulls of true titanosaurids (the argentine forms for one thing)
and braincase material from India referred to *Titanosaurus* (I
don't like this kind of referral) are more similar to
*Nemegtosaurus* than to even *Diplodocus* or *Camarasaurus*, but
a trend can be realized stemming along from *Euhelopus* to
*Camarasaurus* to *Brachiosaurus* to the new argentine
titanosaur, and this final morphology is what *Nemegtosaurus*
fits (Calvo and Salgado, 1995, 1998 -- give me a little leeway
on the dates, I have these somewhere, I'm preparing to move).
The postcrania of *Opisthocoelicaudia* is classically
titanosaur, including phalanx reduction, caudal reduction,
expanded but singular lateral camerae (non-complexity to
pleurocoels), femoral morphology (wide-gauge hips), etc.

<Not surprisingly, Chinese euhelopodids are basal to both
Nemegtosaurus and Opisthocoelicaudia.>

  Upchurch finds this a monophyletic non-Neosauropoda group,
Wilson and Sereno find this a paraphyletic assameblage with
*Euhelopus* a true titanosauriform (somphospondylian), and other
"euhelopodids" splayed radially. No concensus.


=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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