Lyson et al. (2013) reported, “The goal here is not an exhaustive description of Eunotosaurus but rather one focused on shell related features and novel morphologies not apparent in previous descriptions. A more comprehensive treatment will be provided in a later publication.”
To their point, the expanded dorsal ribs of Eunotosaurus and Odontochelys are remarkably similar and T-shaped in cross-section. Unfortunately, the large reptile tree, which relies on no suprageneric taxa and includes a much larger number of pertinent taxa, found Eunotosaurus nested with Acleistorhinus (in Fig. 1 separated by 3 nodes in Lyson et al) and then Milleretta (in Fig. 1 separated by an additional node in Lyson et al).
Lyson et al. (2013) based their analysis on deBraga and Rieppel (1997), which relies heavily on suprageneric taxa, which always brings problems (like nesting gliding kuehneosaurs as sisters to swimming sauropterygians in figure 1). To their credit, Lyson et al. (2013) nested pareiasaurs and turtles as Milleretta descendants. Lyson et al. (2013) listed diadectids as outgroups outside the Reptilia along with Seymouriadae. As reported before, diadectids are basal reptiles, not amphibians.
We looked at Lyson et al. (2010) earlier here, in which they nested Eunotosaurus with turtles. Unfortunately in the large reptile tree moving Eunotosaurus to the base of turtles adds 144 steps.
Comparing turtle skulls to turtle ancestor candidates (Fig. 3) graphically demonstrates the differences that put Eunotosaurus as the odd man out.
We don’t know what the ribs of Stephanospondylus or Milleretta (RC70) look like. Thankfully there is no such thing as modular evolution. Rather phylogenetic bracketing hints that these two likely had broad ribs too, but then… pareiasaurs do not have such broad ribs. And, Odontochelys does not have transverse processes, but Proganochelys does. So…we’ll have to wait for that data.
DeBraga M and Rieppel O 1997. Reptile phylogeny and the affinities of turtles. Zoological Journal of the Linnean Society 120, 281–354.
Lyson T, Bever GS, Scheyer TM, Hsiang AY and Gauthier JA 2013. Evolutionary Origin of the Turtle Shell.Cell Biology 23(12): p1113–1119.