Re: diplodocid radiation

[Dinogeorge@aol.com]

Adam Yates wrote:

> I had always thought that the Nemegtosaurus skull looked a lot less
> like diplodicids than had been made out, so I was very pleased when
> Upchurch's parsimony analysis found them to be outside of the
> diplodicid-dicraeosaurid clade. One aspect of Nemegtosaurus puzzles
> me, the dorsal surface of the rostrum is always dotted in lateral
> views (and indeed photos seem to show that this area has broken
> away, yet the skull is restored in dorsal view confidently. It shows
> a single diplodicid like narial opening, however this should not be
> determinable if the dorsal surface of the rostrum is missing.
> Indeed it could have had a more primitive (Euhelopodid like)
> snout. Until a good Titanosaurid skull is found we can't be sure
> that the entirely cranial synapomorphies of Upchurch's Diplodicoidea
> aren't infact apomorphies of the larger Titanosauroidea +
> Diplodicoidea clade. BTW is the Titanosauria the valid name for this
> larger clade?

The skull of _Nemegtosaurus_ retains enough of the dorsal surface that the
narial opening can be restored quite confidently with septum absent. Don't
take my word for it; check the photos and figures in Nowinski's 1971 paper.

_Nemegtosaurus_ is quite diplodocoid/dicraeosaurid in its skull anatomy, with
such features as orbits positioned posteriorly and above the infratemporal
fenestra, narial opening positioned dorsally between the orbits, pencil-like
teeth located solely at the front of the snout, antorbital fenestra reduced,
etc. I would agree, however, that it could well be at the far end of the
transitional form of skull between camarasauroid and diplodocoid, in which
case it becomes somewhat more likely that the skull of _Nemegtosaurus_ goes
with the body of _Opisthocoelicaudia_.

I can't understand why titanosaurians and diplodocoids continue to be linked
as sister groups in the sauropod cladogram. They're utterly different in
postcranial anatomy (the only way they can presently be compared), and the
small amount of skull material available for _Malawisaurus_ suggests a short,
high, camarasaur-like skull for that genus. The dentition is likewise
leaf-shaped, as in primitive sauropods. Titanosaurians probably represent a
distinct Gondwana sauropod radiation that diverged from the lineage leading
toward diplodocoids well before the appearance of bifid neural spines, that
is, even earlier than camarasaurids and euhelopodids.

_Antarctosaurus_, whose meager skull material is the basis for all
restorations of titanosaurians as diplodocoid sauropods, was likely not a
titanosaurian at all but a derived diplodocoid or dicraeosaurid, of which
there were probably plenty in the Cretaceous of South America.