Re: Tetanurae

[Tim Williams <twilliams_alpha@hotmail.com>]

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 
usage.

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 
_Massospondylus_).

> 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 
> confusion.

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.

Cheers

Tim

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