[Taxacom] Turtles are diapsids, not anapsids

[Ken Kinman]

Dear All,
      The just published phylogenomics article in Proceedings of the 
National Academy of Sciences (Shedlock et al., 2007) confirms that turtles 
(Order Testudiniformes) are indeed diapsid reptiles (and thus only 
secondarily anapsid).  So where do we place turtles in the diapsid clade?  
They clearly split off after the sphenodontiforms and squamatiforms (of my 
Lepidosauria Clade 5), but there are still at least three main hypotheses to 
consider:

     Turtles could: (1) belong to Clade 6 (within or sister group to 
Sauropterygiformes); (2) possibly a sister group to Rhynchosauriformes in 
Clade 8; or (3) a member of the exgroup (Subclass Archosauria), the most 
likely sister group therein being the aetosaurs.  Only the third grouping 
contains living taxa, so the answer will most likely need to come from 
morphological data (which could mean waiting for even more additional fossil 
discoveries).

     My intuition is that the answer is hypothesis 2 or 3, so I am placing 
turtles as incertae sedis next to Rhynchosauriformes, although they could 
slide either up or down a couple of clades.  What is now clear, however, is 
that they do NOT belong in Subclass Cotylosauria with any of the true 
anapsids!!!  Here is the relevant part of the reptile classification which I 
presented here in April 2004, but with turtles inserted near rhynchosaurs 
(although they could certainly slide closer to, or even within, 
Archosauria):


    _1_ LEPIDOSAURIA% (basal diapsids)
          1  Araeosceliformes
          2  Plesion Coelurosauravus
          3  Plesion Apsisaurus
          4  Younginiformes
          5  Eolacertiliformes
          B  Sphenodontiformes
          C  Squamatiformes
          6  Ichthyosauriformes
          B  Sauropterygiformes
          ?  Thalattosauriformes
          7  Choristoderiformes
          8  Trilophosauriformes
          B  Rhynchosauriformes
          ?  Testudiniformes (turtles)
          9  Protorosauriformes
         10  Proterosuchiformes%
         _a_  {{ARCHOSAURIA}}

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