RE: What're Rebbachisaurids? (was - New Nemegtosaurus paper)

[Tim Williams <twilliams_alpha@hotmail.com>]

Rodlox R <rodlox@hotmail.com> wrote:

> out of curiosity, what're the Rebbachisaurids like?

Rebbachisaurids are a family of diplodocoids, perhaps most similar to 
dicraeosaurids, and so far known only from the Cretaceous of Africa and 
South America.  They were rather conservative (at least postcranially) and 
appear to be basal to dicraeosaurids and diplodocids; for example, they lack 
the "whiplash" tail and bifid neural spines seen in these groups.  One of 
the more characteristic features of rebbachisaurids is the broad 
"paddle-shaped" scapular blade.  The family Rebbachisauridae currently 
includes _Rebbachisaurus_, _Limaysaurus_, _Rayososaurus_, _Nigersaurus_, and 
some unnamed specimens.  _Nigersaurus_ is distinctive for its rather 
ornithischian-like dental battery, with over 600 slender teeth packed into 
the jaws.

The dentary assigned to _A. wichmannianus_ by Huene may also be from a 
rebbachisaurid, and _A. wichmannianus_ has been suggested by some authors 
(e.g., Sereno et al., 1999) to be a chimera based on the presence of a 
rather rebbachisaurid-like lower jaw combined with a titanosaur-like 
braincase.  However, the undoubted titanosaur _Bonitasaura_, which is based 
on a partially articulated skeleton, has a dentary similar to that of _A. 
wichmannianus_ (Apesteguia, 2004).  So _A. wichmannianus_ may not be a 
composite species, but 100% titanosaur.

> (were they armor-plated like the Titanosaurs?)

Not that we know of.

> why didn't Brachiosaurids, Dicraeosaurids, and Diplodocoids diversify?

They had a lousy financial adviser.

Seriously, diplodocoids did diversify in the Cretaceous, in the form of the 
rebbachisaurids - although they were overshadowed by the more abundant 
titanosaurs.  It appears that diplodocids and camarasaurids went extinct at 
the end of the Jurassic, and dicraeosaurids, brachiosaurids and some others 
(e.g., _Aragosaurus_) lingered into the earlier Cretaceous.  It is possible 
that the success of rebbachisaurids and titanosaurs had something to with 
the change in vegetation, and the shift from the gymnosperm-dominated 
"Mesophytic" flora to the angiosperm-dominated "Cenophytic" flora - but this 
is speculative.  Certainly, titanosaurs were the dominant herbivorous 
saurischians in the Late Cretaceous.

Cheers

Tim