New Nemegtosaurus paper

[Tim Williams <twilliams_alpha@hotmail.com>]

Wilson, J.A. (2005).  Redescription of the Mongolian sauropod _Nemegtosaurus 
mongoliensis_ Nowinski (Dinosauria: Saurischia) and comments on Late 
Cretaceous sauropod diversity.  Journal of Systematic Palaeontology 3: 
283-318.

Abstract: "The isolated skulls of _Nemegtosaurus mongoliensis_ and 
_Quaesitosaurus orientalis from the Nemegt Basin of Mongolia are among the 
most complete sauropod cranial remains known from the Late Cretaceous, yet 
their evolutionary relationships to other neosauropods have remained
uncertain.  Redescription of the skull of _Nemegtosaurus_ identifies key 
features that link it and its closely related counterpart _Quaesitosaurus_ 
to titanosaur sauropods. These include a posterolaterally orientated 
quadrate fossa, `rocker'-like palatobasal contact, pterygoid with reduced 
quadrate flange and a novel basisphenoid?quadrate contact.  Other features 
are exclusive to
_Nemegtosaurus_ and _Quaesitosaurus_, such as the presence of a symphyseal 
eminence on the external aspect of the premaxillae, a highly vascularised 
tooth bearing portion of the maxilla, an enclosed `maxillary canal', orbital 
ornamentation on the postorbital, prefrontal and frontal, exclusion of the 
squamosal from the supratemporal fenestra and dentary teeth smaller in 
diameter than premaxillary and maxillary teeth.
"Re-examination of Late Cretaceous sauropod distributions in the light of 
this well-supported phylogenetic hypothesis has important implications for 
their diversity at the end of the Mesozoic in Asia and elsewhere.  
Cretaceous Asian sauropod faunas consist solely of titanosauriforms, which 
probably migrated there from other landmasses during the Late Jurassic, 
during which time neosauropods were absent from Asia.  Globally, 
narrow-crowned titanosaurs and rebbachisaurids radiated during the 
Cretaceous, but only titanosaurs survived into the latest Cretaceous.  These 
late-surviving sauropods flourished on most continental landmasses until the 
end of the Maastrichtian."

Wilson (2005) re-defines the Nemegtosauridae as the stem-based clade 
including all titanosaurs more closely related to _Nemegtosaurus_ than to 
_Saltasaurus_.  This group includes (at least): _Nemegtosaurus_, 
_Quaesitosaurus_, _Rapetosaurus_, possibly _Mongolosaurus_ (which is 
regarded as a valid genus) and maybe even _Phuwiangosaurus_ (although the 
postcranium indicates that it was a more basal titanosaur).  
_Quaesitosaurus_  is not considered a junior synonym of _Nemegtosaurus_, 
though the two are sister taxa.  _Antarctosaurus wichmannianus_ may also be 
a nemegtosaurid; but Wilson is not certain that all the material referred by 
Huene to _A. wichmannianus_ comes from a single individual or species.

Like many previous authors, Wilson also raises the possibility that 
_Nemegtosaurus_ and _Opisthocoelicaudia_ may be the same; but overlapping 
material is required before this can be considered.  Considering that 
Nemegtosauridae and Opisthocoelicaudiinae have both received phylogenetic 
definitions, such a synonymy would raise interesting nomenclatural issues...

Wilson (2004) also notes that titanosaurs and rebbachisaurids appear to be 
the only two sauropod clades that flourished in the Cretaceous.  Although 
other groups (brachiosaurids, dicraeosaurids, basal diplodocoids) survived 
into the Cretaceous "they did not appear to diversify".

Cheers

Tim