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Re: Titanosaur-like sauropods



Tom Holtz writes, in response to a recent query (and if I've already
sent this to the list, sorry for the repeat post!):

>> Now, I understand that a new family of (mainly South American Upper
>> Cretaceous) sauropods, all formerly classified as titanosaurids, has
>> been proposed, the Andesauridae.  All of these sauropods were
>> formerly classed as titanosaurids. What is the justification for the
>> new family; i.e. what special characteristics set the Andesauridae
>> apart from the Titanosauridae?  Could they be a subfamily of
>> Titanosauridae?
>
> Andesaurids lack the strongly procoelous caudal centra of true
> Titanosauridae.  They may be the sister taxon to Titanosauridae, or
> even a paraphyletic assemblage of basal members of Titanosauria
> (basically, Titanosauridae + "andesaurids").

My understanding is that andesaurids didn't _lack_ procoelous caudals, just
didn't have as many. The first caudal was biconvex, then the next few caudals
were procoelous, gradually becoming the more usual amphicoelous farther down
the tail. Unfortunately, my Spanish is very limited indeed, so if I've
misread the _Andesaurus_ and _Argentinosaurus_ papers, please correct me on
this.

It is beginning to look as if most if not all of the Early Cretaceous
so-called "titanosaurids" are actually andesaurids.

>> A related (maybe) question: where does Opithocoelicaudia fit in?
>
> According to Paul Upchurch's phylogeny, Opisthocoelicaudia is the
> sister group to Titanosauria within Titanosauridae.

I've read the paper and I find this one pretty hard to swallow. Titanosaurids
and andesaurids did not have bifid dorsal neural spines, but
_Opisthocoelicaudia_ did. And titanosaurids and andesaurids had at least some
procoelous caudals, but in _Opisthocoelicaudia_ the caudals were--as the name
of the genus reminds us--opisthocoelous, the exactly opposite condition. The
characters that Upchurch uses to unite _O._ with "titanosauroids" seem small
potatoes next to the aforementioned. There seems to be no good reason not to
consider _O._ some kind of derived euhelopodid--a sauropod group which at
least did have bifid neural spines. (The opisthocoelous caudals are, I think,
unique in Brontosauria.)