Re: Dinosaurian Class

[Jonathon Woolf <jwoolf@erinet.com>]

Bill Adlam wrote:

> Jaime wrote:
>
> >  It's: Amniota including Diapsida (crocs, dinos, birds, lizards) and
> >Synapsida (pelycosaurs, us) with the outgroup as Anapsida (your
> >favorite box turtle, for instance).
> >+--+--Anapsida
> >   +--+--AMNIOTA
> >      +--+--Diapsida
> >         +--Synapsida
>
> This classification has now fallen out of favour.  It is more likely that
> Chelonians are Diapsids which secondarily reverted to the anapsid state
> (their skulls are clearly heavily modified).  Other anapsids are a
> paraphyletic mix of early Amniotes.
>
> I do not know the difference between Reptilia (new definition) and Diapsida.

The most recent reference I have ("Diversification of the Amniotes," Jacques
Gauthier, 1994) states it like this:

Amniota = Synapsida + Sauropsida

Synapsida = mammals and everything closer to modern mammals than to modern
reptiles

Sauropsida = Mesosauridae + Reptilia

Reptilia = Anapsida + Diapsida

The article isn't clear on how Anapsida is defined.  The text says "turtles and
everything closer to turtles than to saurians," where "Sauria" includes
lepidosaurs + archosaurs + a few minor groups.  The cladograms say "turtles and
everything closer to turtles than to diapsids," with rather more critters
included in Diapsida than just the saurians.

There was a study a year or two back that suggested chelonians are not anapsids
but heavily modified diapsids, which would make a nice hash of much of the
Sauropsida, but that classification hasn't been generally accepted.

-- JSW