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RE: Sauropods



Matthew Bonnan wrote:

> Whether we like it or not (and many folks don't), 

One high-profile pro-_Brontosaurus_ paleontologist (yes, him) would prefer
to see _Brontosaurus excelsus_ resurrected as a separate genus from
_Apatosaurus ajax_.  If you can't lick 'em split 'em.  However, the two
species may be so similar as to be not only congeneric but conspecific.
(This is based on McIntosh's work in the 1990's - the most recent study on
the status of _Apatosaurus_ species, AFAIK)

> Brontosaurus was sunk
> as a name when it was discovered that Apatosaurus was the same animal 
> as Brontosaurus but was named earlier on scrappier material.  

This could be the basis for a loophole; the type specimen for _A. ajax_ is
immature and very incomplete, meaning there is a possibility that the
species (and therefore the genus _Apatosaurus_) is a _nomen dubium_.
_Brontosaurus_ (which is based on a better-quality adult specimen) would
then become the next available name for this taxon.

(I'm not advocating that _A. ajax_ *should* be declared a _nomen dubium_,
and that the above nomeclatural shenanigans are at all justified; it is,
however, something to keep in mind.)

> I think this would cause more confusion than help right now, 
> because so many articles have used Apatosaurus -- we'd have to go back
> through a hundred years of literature or more and sort out what who
> meant by  what and what specimens they were referring to.  What a 
> nightmare!

You bet.

> This is where the confusion and interest comes in with titanosaurs.
> We just  don't know.  Our record of titanosaurs is still relatively 
> poor to have a  good idea of what is going on with them as a group.  

Thankfully, the titanosaur record is getting better every year and I think
we're starting to get a handle on what a "titanosaur" actually is.
Unfortunately, the taxonomic situation regarding titanosaurs is not helped
by the status of the genus _Titanosaurus_, which as it stands is in a
thorough mess: the Lameta type material (_T. indicus_) may or may not be
diagnostic; may or not belong to the same species as _Antarctosaurus
septentrionalis_; and may or may not belong in the same genus as _T.
colberti_.)

Nevertheless, many recent phylogenetic studies do find strong support
for a clade of Cretaceous sauropods that embraces _Saltasaurus_,
_Neuquensaurus_, _Rocasaurus_, _Rapetosaurus_, _Lirainosaurus_,
_Opisthocoelicaudia_, _Alamosaurus_ (though the hypodigm is unsettled),
_"Titanosaurus" colberti_, and perhaps a few others (?_Ampelosaurus_) that
combine to form a monophyletic clade (Titanosauridae or Saltasauridae).
This clade probably also includes _Nemegtosaurus_ (and _Quaesitosaurus_, if
the two genera are not the same).

Further, these same phylogenetic studies find support for a more inclusive
clade embracing the above taxa plus an array of less derived titanosaur-like
taxa: _Phuwiangosaurus_, _Tangvayosaurus_, _Sonorasaurus_, _Cedarosaurus_,
_Pleurocoelus_, _Chubutisaurus_, _Andesaurus_, _Malawisaurus_,
_Argentinosaurus_, _Aeolosaurus_ (?incl. _Gondwanatitan_), _Paralititan_ and
others.  These taxa usually come out as a paraphyletic series between
_Brachiosaurus_ (or Brachiosauridae sensu stricto) and the "true"
titanosaurids (or saltasaurids, above).  Some are closer to brachiosaurids
(and may prove to be true brachiosaurids, like _Pleurocoelus_) and others
are closer to the bona fide titanosaurids/saltasaurids (and may belong to
this clade, like _Aeolosaurus_).

There is good data supporting the inclusion of the brachiosaurids and
titanosaurids (and the numerous taxa in between) in a clade
(Titanosauriformes) that excludes camarasaurids, diplodocoids and
'cetiosaurids'.  Some problematic Jurassic taxa (_Volkheimeria_,
_Lapparentosaurus_, _Janenschia_) which show titanosauriform features might
one day end up in this clade.  Ditto for _Austrosaurus_ and _Mongolosaurus_
from the lower Cretaceous.


> Maybe diplodocoids, brachiosaurs, and camarsaurs DID survive in good 
> numbers into the Cretaceous  and they ARE what we call titanosaurs.  

Quite possibly.  But by the Late Cretaceous the titanosaurids seem to be the
dominant sauropod group across the globe - Europe, India, Madagascar, North
America, South America, and possibly continental Africa and Asia; (Australia
and Antarctica, who knows?).   

> Titanosaurs seem to be a hodge-podge of neosauropod characters and 
> perhaps this is because they actually represent the two major 
> neosauropods groups in their "new" Cretaceous forms. 

As I noted above, current phylogenies (aided and abetted by new specimens),
supports the interpretation that the vast majority of Cretaceous titanosaurs
are members of a monophyletic radiation (Titanosauriformes, or its less
inclusive subgroups) that *excludes* camarasaurs and diplodocoids - though
some people prefer to regard _Opisthocoelicaudia_ as a LK camarasaur and the
nemegtosaurids as LK diplodocoids.

> Hope some of this helps or stimulates other conversation about
> sauropods. =)

I'm doing my best!  :-)




Tim