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Re: Archosaur Origins...was:MESENOSAURUS ERRATA.



On Tue, 28 Aug 2001, Jaime A. Headden wrote:

> It is also best to define crowns as the content of their inclusion,
> i.e. a node, and not as a stem-group. Some taxa work best as a stem, based on 
> basal fossil
> membership (Aves, Crocodylia, etc.)

These are both node-based clades, though.

It really makes more sense to define groups traditionally based on extant
animals as crown groups than as stem groups. This makes for much better
stability in the literature, and, indeed, is how most of the taxa being
discussed in this thread are used by most systematists. There are a few
instances where this has not worked out, such as the crown group
definition of _Aves_, but it seems to work pretty well for _Squamata_,
_Crocodylia_, _Testudinata_, _Mammalia_, etc.

One thing I'd like to ask of George: if we are to use _Mammalia_ instead
of _Synapsida_ (or _Theropsida_), _Crocodylia_ instead of _Pseudosuchia_
(or _Crocodylotarsi_), _Aves_ instead of _Ornithosuchia_ (or
_Ornithotarsi_ or _Avemetatarsalia_), etc., then what DO we use for the
crown groups? (Or, in the case of _Aves_, for Clade(_Archaeopteryx_ +
Passer_)?)

(I do agree with his usage of _Amphibia_, since there is a perfectly
acceptable name (_Lissamphibia_) for the crown clade.)

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