RE: New sauropod refs: Rinconsaurus & Amygdalodon

["Tim Williams" <twilliams_alpha@hotmail.com>]

Ben Creisler wrote:

> A cladistic phylogenetic analysis placed
> Rinconsaurus in the family Titanosauridae. Within
> Titanosauridae (Rinconsaurus + Aeolosaurus) is considered
> a sister group of the clade (Opisthocoelicaudia +
> (Alamosaurus + (Neuquensaurus + Saltasaurus))).

Wilson and Upchurch (2003) define Saltasauridae as all descendents of the 
most recent common ancestor of _Opisthocoelicaudia_ and _Saltasaurus_.  
Wilson and Upchurch (2003)  also regard _Titanosaurus indicus_ as a nomen 
dubium, since the type material is non-diagnostic.  However, apparently the 
site that yielded the original tail vertebrae of _Titanosaurus_ ("Sauropod 
Bed", Bara Simla hill), as well as further material described by Huene and 
Matley, 1933), has been re-discovered, and has yielded more titanosaur 
bones.  There may be hope yet for the genus _Titanosaurus_.

In addition to the saltasaurids, there would appear to be a second "family" 
of Cretaceous titanosaurs.  This includes _Aeolosaurus_, _Gondwanatitan_ and 
_Rinconsaurus_, and perhaps _Titanosaurus_ and _Laplatasaurus_, once the 
hypodigms of these latter genera are sorted out.  Among other features, 
saltasaurids (and _Pellegrinisaurus_, and to some extent _Isisaurus_) have 
dorsoventrally compressed caudals, whereas "titanosaurids" have caudals that 
are more laterally compressed.


> Rinconsaurus caudamirus has preserved short articulated
> posterior caudal series with amphicoelous, opisthocoelous
> and biconvex centra.

Interestingly, an articulated caudal series referred (provisionally) to 
_Aeolosaurus rionegrinus_ also includes an amphicoelous caudal at the end of 
the middle third of the tail.  (This caudal is strongly amphicoelous, with 
deeply concave front-and-back surfaces, unlike the weak 
amphicoely/amphiplaty of basal titanosaurs.)  It has been suggested (by 
LeLoueff, and much earlier by Nopcsa) that such amphicoelous caudals are 
just the result of non-coossification of the posterior ball in a procoelous 
caudal, with the ball somehow becoming coossified to the next caudal.  
However, I think there is good evidence that this feature is taxonomically 
significant.

> From the systematic point of view, this discovery is important
> because Titanosauridae were traditionally defined, among
> other characters, by strongly developed procoelia in their
> caudal vertebrae.

Extension of strong procoely into the MIDDLE of the caudal series still 
appears to be diagnostic for a clade of higher titanosaurs.  Nevertheless, 
it is obvious that some taxa deviated from this condition - with 
_Opisthocoelicaudia_ representing an extreme case.



Tim

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