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Mike Keesey wrote:
Stability of content is not necessarily a good thing. After all, no taxon
can have stable content unless we absolutely know every member.
In this situation, there is a difference between "stable" and "immutable".
I think a given clade can still be "stable" while new taxa are added and old
taxa are removed. What's important in preserving "taxonomic stability" is
to prevent the taxonomic content from diverging too far from historical
For example, Yates (2007) re-defined the branch-based clade Sauropoda such
that _Plateosaurus_ was replaced by _Melanorosaurus_ as the external
qualifier. Sereno (2005) had the same idea when he proposed _Mussaurus_ and
_Jingshanosaurus_ as external qualifiers for Sauropoda. In both cases, big
changes in the relationships among basal sauropodomorphs (traditional
"prosauropods") prompted the definition of Sauropoda to be altered, or else
Sauropoda would expand to include a great many basal sauropodomorphs (like
Besides, I *understand* _Tetanurae_ to be a branch-based clade with
neotetanurans internal and _Ceratosaurus_ external. I don't understand it
to be some preset composition list. Changing the definition causes
In the case of Sereno's Tetanurae, the change to the definition is an
emendation - just one extra external qualifier. I think less confusion is
caused by adding one more external qualifier (_Carnotaurus_) than having
abelisauroids end up inside Tetanurae. Same with the changing definitions
of Sauropoda, mentioned above. This is an 'in-the-eye-of-the-beholder'
thing, I know, so I'm not going to argue it too forcefully.
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