Re: Synapsids and Basal Reptiles (was: Re: Elvis, Sue, and a guy

[Pieter.Depuydt@rug.ac.be]

> Are Araeoscelids considered the most primitive of all amniotes, even more
> primitive than _Hylonomus_ and _Limnoscelis_, and that's why Synapsida is
> placed outside the Reptilia clade?

No, in fact Araeoscelis was traditionally considered a basal 
'Euryapsid' (an obsolete taxon which included nothosaurs, 
plesiosaurs, placodonts etc...) on the basis of the morphology of the 
temporal fenestrae. The current thinking has Araeoscelis together 
with creatures as Petrolacosaurus, Spinoaequalis,...  as basal 
diapsids.
Limnoscelis is related to Diadectes and Tseajaja, and is pretty close 
to Amniota (some authors place Diadectomorpha as sister-group to 
Amniota), although probably not amniote itself.
Hylonomus is the oldest amniote (when we leave out Westlothiana) and 
is probably already a member of the clade Reptilia, although a very 
basal one.
The most primitive synapsids are Archaeothyris (not to confuse with 
Araeoscelis) and fragmentary bits known as Clepsydrops and 
Protoclepsydrops.
All these early taxa are very similar to each other and very 
plesiomorphic; the study of their phylogeny is a hard nut to crack, 
and the thinking of early amniote relationships is in a constant flux 
of change.

(written without references at hand, please correct me if there are 
inaccuracies).

Pieter Depuydt