Re: Dinosaurian Class

[chris brochu <cbrochu@fmppr.fmnh.org>]

>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 position of turtles and certain extinct groups (captorhinids,
procolophonids, pareiasaurs) is currently under debate.  Most people would
make this assemblage the monophyletic sister group to Diapsida, and hence
Reptilia would include Diapsida as well as these other things.  But a few -
including Olivier Rieppel, here at the Field Museum - have shown that
considerable support can be found for a placement of turtles within
Diapsida.

Now here's where it gets tricky.  What does this do to Diapsida?  It all
depends on where within that group turtles go, if they go there - and I am
skeptical.  Diapsida, IIRC, is a stem-based group including squamates,
archosaurs, and several other creatures, like Petrolacosaurus.  But
squamates and archosaurs shared a more recent common ancestor with each
other than either did with Petrolacosaurus, and the crown-group including
archosaurs and squamates is called Sauria.  Most of Olivier's analyses put
turtles closer to the squamates, meaning that turtles are members of
Diapsida, but that Reptilia (as defined above) would replace Sauria.


chris



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Christopher Brochu

Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL  60605  USA

phone:  312-922-9410, ext. 469
fax:  312-922-9566

cbrochu@fmppr.fmnh.org