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Re: Ameghiniana Round Up, Pt. II

In a message dated 97-08-18 18:20:35 EDT, th81@umail.umd.edu (Thomas R.
Holtz, Jr.) writes:

<< _Opisthocoelicaudia_ is a derived titanosaurid, with implications in the
 next paper. >>

I think the cladistics is wrong here. _O._ is no more a derived titanosaurid
than segnosaurs are theropods. It's much more likely some kind of derived
euhelopodid or derived basal diplodocid that has picked up a few random
titanosaur-like characters here and there that are throwing off the analyses.
The caudal-vertebral articulations and dorsal vertebrae are _completely_
unlike those of titanosaurids. It's incredibly improbable that _O._ just
happens to be the one titanosaurid in which the dorsal vertebrae and caudal
vertebrae simultaneously converge to a euhelopodid/diplodocid form,
particularly in view of its biogeographical closeness to all the other known
euhelopodids. Incidentally, the euhelopodids are also conspicuous by their
absence from the analysis. What did they do, pile them into Diplodocidae or

The papers seem most valuable for their analyses of the titanosaurians, which
have never adequately been done before. Cladistic analysis shines most
brightly when it's disentangling lineages already known to be closely

I'm pleased to see that, with the exception of genera such as _O._, the
phylogeny of the major groups resembles the phylogeny I sketched out in my
1992 article on brontosaurians for _Dino-Frontline_ #2. I think I had the
titanosaurs coming out of the cetiosaurids, but since these are also basal
brachiosaurs, it amounts to pretty much the same thing.