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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)
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.