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RE: Turtles and Crocodylians are not Reptiles - no? What are they?

Robert Takata wrote:
> Crocs are the extant closest relatives of
> birds - and probably turtles are the extant closest relatives of
> archosarus (birds + crocs).
Most (all?) of the morphology-based phylogenies I've seen that place the 
turtles (Testudinata) in the Diapsida have them pegged as either 
lepidosauromorphs, or as non-saurian neodiapsids (i.e., outside the 
archosauromorph-lepidosauromorph clade).  Or else, the turtles are retained in 
the Anapsida, as non-diapsid sauropsids.

As for the molecular phylogenies... they do tend to align turtles with the 
archosaurs, as either non-archosaur archosauromorphs, or as true-blue 
archosaurs (sister taxon to crocodilians, if I recall correctly).   

Mike Keesey wrote:

>> Personally, I prefer the name Sauropsida for the group that includes all 
>> extant reptiles, as well as birds, but not mammals (which are synapsids, not 
>> sauropsids). If Reptilia is the same as Sauropsida, then mammals did not 
>> evolve from from 'reptiles', which is a departure from the historical usage 
>> of the term 'reptile'.
> There is almost no way for them to be the same--_Sauropsida_ is a
> branch-based group (specifically a total group) and _Reptilia_ sensu
> Gauthier is a node-based group (specifically a crown group). They
> might have the same known membership, but that doesn't mean they're
> the same thing--the latter is a subset of the former.

As you say, the two clades (Reptilia and Sauropsida) are defined differently; 
although whether Reptilia is a subset of Sauropsida (or vice versa) depends 
upon the respective definitions.  For example, Modesto and Anderson's (2004) 
definition of Reptilia (such that it is the most inclusive clade containing 
lizards and crocs, but not mammals), and Laurin and Reisz's definition of 
Sauropsida (the last common ancestor of mesosaurs, testudines and diapsids, and 
all its descendents) would make Sauropsida a subset of Reptilia by definition, 
but effectively the same in content.  But Gauthier &c's (1988) definition of 
Reptilia, which limits it to the crown group, would put Reptilia inside 
Sauropsida, whatever the position of turtles turns out to be.

> I prefer abandoning (or at least deformalizing) the term "Reptilia",

Yep, so do I.  Reptilia has too much historical/typological baggage.

> but it does leave the crown group without a good name, beyond "crown
> sauropsids".

That was Gauthier &c's rationale behind their 1988 definition of Reptilia: to 
confine this name to crown non-synapsid amniotes.  But as Modesto and Anderson 
(2004) point out, the phylogenetic position of the turtle clade (Testudinata) 
is so uncertain that it makes Reptilia very unstable in terms of content.  For 
example, if the anapsid condition of turtles is primitive rather than secondary 
(and there are still quite a few people who are holding out on this score), 
then Reptilia would be a far more inclusive clade than than if turtles are 
lepidosauromorphs or archosauromorphs, in which case Reptilia is limited only 
to the Sauria (Archosauromorpha+Lepidosauromorpha).  With Modesto and 
Anderson's stem-based definition, Reptilia includes all non-synapsid amniotes.  
Contrary to what I said in a previous message, this is probably the best policy 
if Reptilia is going to be retained as a clade.  Modesto and Anderson's 
definition means that crocs, birds, squamates, turtles, 'parareptiles',
 basal diapsids, and all manner of sea reptiles all get scooped up into 
Reptilia, but mammals never can.


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