Re: Titanosaurs and their relationship to Brachiosaurs and Diplodocoids.

[Denver Fowler <df9465@yahoo.co.uk>]

--- Mike Lima <taradosgon@yahoo.com> wrote:

> >From what I've read (online) Titanosaurs were once
> thought to be more closely related to Diplodocoids
> than Brachiosaurs. But now they are grouped under
> Macronaria with the Brachiosaurs. What led to the
> change? I remember asking (and having someone
> referencing a paper) about restorations in which
> Nemegtosaurus had a more Brachiosaurid skull.  My
> understanding is that Rapetosaurus however had
> dorsally located nares and that Titanosaurs in
> general
> had dentition more closely resembling that of a
> Diplodocid. 

latest maastrichtian titanosaurs have teeth
superficially similar to diplodocoids, but when you
look at them closely, they are really not that alike.
Part of the problem with earlier analyses is that
teeth were categorised into 'peg-like' and 'spatulate'
which is overly simplistic: nemegtosaurus et al teeth
certainly are more peglike than spatulate, but other
than in gross morphology they don't look like dippy
teeth at all.

Further finds of more intermediate titanosaurs has
helped resolve things somewhat. Really it was another
case where comparison to the classic Morrison Fm
sauropod faunas caused more confusion than help.

Another part of the problem was the historical
misidentifictaion of rebbachisaur teeth as
titanosaurid: again the 'peg-like' problem. through a
combination of Sereno & Wilson (2005, in press), and
the MS I am preparing, this should be a bit more
clear.

Denver.



Outside of this characteristics such as
> posteriorly oriented neural spines on the dorsal
> vertebrae, a "wide-gauge" gait, procoelus caudals,
> and
> crescent shaped sternal plates aren't useful because
> they differentiate Titanosaurs from both groups. And
> then characteristics like simple chevrons and
> unsplit
> neural spines aren't useful because Rebbachisaurids
> possess these traits as well (maybe I am wrong on
> that
> though)? Aside from the absence of a ungual claw
> (which I read was reduced in Brachiosaurids) I
> haven't
> read of anything that connects Titanosaurs and
> Brachiosaurids (but I acknowledge that this is
> probably due to poor sources). What finding led the
> revision of the classification of Titanosaurs?
> 
> Also, is the absence of a ungual claw a defining
> characteristic of Titanosaurs or not. I found a post
> from a couple years back (made by Mickey Mortimer I
> believe) in which all that defining characteristics
> were listed and I don't believe the absence of a
> claw
> was on it. Is this a more recent discovery? Or is it
> not a characteristic present in all Titanosaurs?
> 
> 
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