[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Tetanurae

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.



Get the latest Windows Live Messenger 8.1 Beta version. Join now. http://ideas.live.com