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Re: Dino status



----- Original Message -----
From: "Eric Hall" <Hall966@msn.com>
Sent: Saturday, March 06, 2004 5:58 AM

> All the talk of feathers and warm-bloodedness plus the incredible
variation of dinosaur species has left me wondering: are dinosaurs still
classed as reptiles or are they now considered a class all their own?  What
is the general opinion on this matter?

The general opinion agrees with you: "class" is a very subjective term. So
subjective in fact that everyone is better off if it's dropped, along with
all the other ranks like kingdom, phylum, order, family and genus... some
are even arguing to abandon species.

In addition, there is a growing sentiment that paraphyletic groups (which
include an ancestor and some but not all of its descendants) are more
subjective than monophyletic ones (which include an ancestor and all its
descendants). Such groups are, for example, Pisces (fish), Amphibia (from
*Ichthyostega* to frogs and salamanders), Reptilia (Amniota without birds
and mammals), Sauria/Lacertilia (lizards without snakes and sometimes
amphisbaenians), Thecodontia (Archosauria without crocs, dinosaurs and
pterosaurs if they are archosaurs), Pelycosauria (Theropsida/Synapsida
without Therapsida and Mammalia), and so on.

Now the question is what to do with the names of such paraphyletic groups.
Some call for the abandonment of most or all such names. Others think most
of them should be redefined to designate monophyletic groups. A few examples
are Reptilia = the most recent common ancestor of turtles, lizards and
crocodiles, and all its descendants -- which just so happen to include
dinosaurs including birds, but to exclude e. g. *Dimetrodon*; Amphibia =
everything that shares a more recent common ancestor with frogs, salamanders
and caecilians than with amniotes -- which happens to exclude *Ichthyostega*
and many more Paleozoic "amphibians"; Sauria = the most recent common
ancestor of crocodiles (!) and lizards -- which happens to include snakes
and dinosaurs including birds.

Personally I think the redefinition of Amphibia is a good move, but those of
Reptilia and Sauria change the contents of the respective groups so far that
the names should rather be abandoned. This topic is likely to turn into a
great big discussion during the First International Phylogenetic
Nomenclature Meeting http://www.phylocode.org this July.

Hope this helps! :-)