Lyson et al. (2010 – available online) put together their hypothesis on the origin of turtles. In their abstract, they wrote, “We reanalysed a recent dataset that allied turtles with the lizard–tuatara clade and found that the inclusion of the stem turtle Proganochelys quenstedti and the ‘parareptile’ Eunotosaurus africanus results in a single overriding morphological signal, with turtles outside Diapsida.”
Lyson et al. (2010) did not include Romeria primus, Orobates (Fig. 2) and Stephanospondylus, three taxa found to be closer to the origin of turtles than Eunotosaurus, a terminal taxon with only one known sister, Acleistorhinus. Unfortunately we have no post-crania for Romeria primus (other than slender manual digits) or Acleistorhinus. That lack of data makes it less obvious how they are related to other taxa, but still the large reptile tree nested them in that fully resolved tree. Stephanospondylus was also the sister to the pareiasaurs, a derived clade previously and correctly associated with turtles, but only at the bases of both clades.
Would be nice to find the common ancestor of both pareiasaurs and turtles, something a little less turtle-like than Stephanspondylus. For now, Orobates(in yellow, Fig. 2) is the best candidate, and prior to that, Romeria primus (in pink). Orobates and Stephanospondylus are Early Permian. The two turtles are Late Triassic. That gives 60-70 million years to evolve a carapace and plastron, plenty of time for transitional taxa to be discovered in.
Lyson et al. 2012 did find turtle genes closer to lizard genes, while others did not.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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