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Re: Archosauromorph classification & Thecodonts



Ken Kinman wrote:

     Actually I have been asking around for opinions on whether I should
include choristoderes as well, and if so, should they be placed as (1)
sister group to rhynchosaurs;  or (2) as sister group to rhynchosaurs plus
all the rest of the thecodonts.

This came up recently: http://www.cmnh.org/fun/dinosaur-archive/2000Jun/msg00136.html

It's no
different than Class Reptilia being all amniotes minus their mammal and bird
descendants, but being paraphyletic I guess you would call that a
"trash-can" too.

Yep, I would regard "Reptilia" as a trash-can. At least under the definition (as opposed to "diagnosis") under which birds (Aves) and mammals (Mammalia) are excluded because they are "warm-blooded". Crocodiles are more closely related to birds than to turtles, yet the crocs are lumped in with the turtles. Mammals get their own class, yet pelycosaurs have to stay behind in the Reptilia because they weren't clever enough to evolve an endothermic metabolism. Etc.



    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.

Yes, but the difference is that the "Thecodontia" seems to be specifically designed to *accommodate* problematic taxa. Any archosaur that wasn't a dinosaur/crocodilian/pterosaur was stuck in the Thecodontia. Non-dinosaurs are occasionally assigned to the Saurischia and Ornithischia, but these are removed when their true affinities are realised. With thecodonts like _Lagosuchus_ and _Sphenosuchus_, you seem intent on keeping them among the thecodonts *despite* the fact that their closest relatives have been identified (and they're *not* thecodonts).



Tim ________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com