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Re: Tetanurae

Tim Williams wrote-

I disagree. I think adding an abelisauroid as an external specifier is a great idea. Sereno added _Carnotaurus_" to ensure the stability of the taxonomic content of the Tetanurae". The revised definition succeeds on this point, and is much better in this respect than having only _Ceratosaurus_ as the external specifier.

Although Gauthier (1986) was the guy who erected Tetanurae (and resurrected Ceratosauria), abelisaurids (collectively or individually) had not yet been exposed to a cladistic analysis. _Indosaurus_ and _Indosuchus_ might have been classified as tetanurans by Gauthier, but he also treated tyrannosaurids as carnosaurs. These were all a priori assumptions made by Gauthier, and not actually tested.

Paul's (1988) classification of theropods was not cladistic. But if, for example, a cladistic analysis does recover abelisauroids as the sister taxon to carcharodontosaurids (as has come up), then Tetanurae would certainly contract (and would probably be equivalent in content to Neotetanurae!). So much for "taxonomic stability". But I would regard an abelisaurid+carcharodontosaurid clade as a remote (but exciting) possibility.

I suppose the difference is that for stem-based clades, I value the idea of the clade more than the content of the clade. "This clade specifies this idea, and let the taxa fall where they may" etc..
Is a (Ceratosaurus(Abelisaurus,Ornithomimus)) topology really likely enough to validate using an abelisaurid external specifier for Tetanurae? I don't believe it's occurred in any published analysis.

Mickey Mortimer