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Re: Archosauromorph classification & Thecodonts

Ken Kinman wrote...

>     But as far as "trash-cans" go, Dinosauria and a lot of other groups
>have had problematic groups thrown into them which were later removed.  If
>we discarded every group that has been used as a trash-can at one time or
>another (making them temporarily polyphyletic), we wouldn't have many taxa

     Dinosauria, as its currently understood, is monophyletic and useful to
refer to; Thecodontia is paraphyletic and is not.  If you look at any given
cladogram, count up the possible number paraphyletic groups you could name.
I happen to be looking at Sereno's (1991) cladogram right now (p. 43); I
count ten monophyletic groups (based on the number of nodes) and got up to
about three times as many paraphyletic grouping before I got tired of
counting.  I doubt anybody could argue that most of the groupings you could
make are useful for anything at all.  Paraphyletic groupings need to be
chosen carefully and conservatively to have some utility.  Now that the
interelationships of "thecodonts" are geting worked out, Thecodontia as a
paraphyletic lump of all non-dinosaur and non-crocodilain archosaurs has no
practical value at all.

LN Jeff