Re: Defining Characteristics of the Titanosaurids

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

Mickey Mortimer wrote:

> Because Titanosaurus is thought by many to be indeterminate (Wilson and
> Upchurch, 2003),

Wilson and Upchurch (2003) also note that the sauropod material from the 
_Titanosaurus_ type horizon ("Sauropod Bed", Lameta Formation) may belong to 
a single species.  The morphology is certainly consistent with the presence 
of one species (and perhaps one individual), and this species (which would 
take the name _T. indicus_) could perhaps be diagnosed on the basis of limb 
proportions.  However, given the fact that most of the material was not 
associated, and info on the relative position of each bone was not recorded, 
Wilson and Upchurch (2003) took the view that it was best not to assume that 
all the material belonged to a single taxon.

Apparently the Sauropod Bed has been rediscovered, and further titanosaur 
material remains to be described from the site (Chatterjee, 1996).  If *all* 
the titanosaur material agrees in morphology, and is consistent with the 
presence of a single taxon, then _T. indicus_ may prove to be a valid 
species after all.  The taxonomy of _T. indicus_ (and the Lameta titanosaurs 
in general) is not helped by the fact that Huene and Matley (1933) made a 
frightful mess of sorting the Lameta material into individual species.

> Procoelous proximal caudal centra are one synapomorphy of titanosaurs.

Derived titanosaurs have the procoelous condition extending into the middle 
third of the tail.  (By "procoelous" I mean deeply concave front surface and 
deeply concave back surface, such that the centra meet at cup-and-ball 
articulations).  Certain basal titanosaurs and non-titanosaurs have 
procoelous proximal centra, but this grades into the 
amphicoelous/amphilatyan condition by the mid-tail region.

A number of derived titanosaurs have non-procoelous vertebrae in the 
proximal two-thirds of the tail, including amphicoelous, opisthocoelous and 
biconvex vertebrae, e.g., type specimens for _Opisthocoelicaudia_ and 
_Rinconsaurus_, and material referred to _Titanosaurus_, _Aeolosaurus_, and 
_Macrurosaurus_.  (Also, many derived titanosaurs have a biconvex vertebra 
to begin the caudal seies.)  But in all cases the vertebrae in the first 
two-thirds of the tail maintains the cup-and-ball articulations between 
individual vertebrae, AFAIK.  Individual species may differ in whether the 
front or back surface (or both) of each caudal vertebra is deeply concave.

> Some titanosaurs (Agustinia, Ampelosaurus, Lirainosaurus, Magyarosaurus,
> Malawisaurus, Mendozasaurus, Neuquensaurus, Saltasaurus) were armored, 
> while
> others (Alamosaurus, Epachthosaurus, Isisaurus, Opisthocoelicaudia,
> Rapetosaurus) were not.

I've seen _Agustinia_ mentioned as a possible titanosaur, but I think more 
material is needed to verify this.  Its armor is certainly bizarre.

The Lameta Formation in India has also yielded scutes ("Lametasaurus") that 
may belong to a titanosaur; but they cannot belong to _T. indicus_, which 
comes from a different stratigraphic level within the Formation.



Tim

_________________________________________________________________
Limited-time offer: Fast, reliable MSN 9 Dial-up Internet access FREE for 2 
months! 
http://join.msn.com/?page=dept/dialup&pgmarket=en-us&ST=1/go/onm00200361ave/direct/01/