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_Odontochelys semitestacea_ might get its shell back



Fig. 1 contains a nice photo -- you can blow it up to 400 % in the pdf without it getting pixelated; this is very atypical for Nature -- and an interpretative drawing of that photo. Well, I would say what both the photo and the drawing show are not "broadened dorsal ribs" as seen in the infamous *Eunotosaurus*, but ribs fused to costal plates. In other words, the carapace is there: there's one dermal bone plate above each dorsal rib (except the last pair) -- these plates are just not sutured to each other, unlike in any other turtle known so far. To test this, we'd need more specimens: an ontogenetic series or a specimen that we could dare make a histological analysis of.

That could explain why the neural plates (those above the vertebrae) are apparently sutured to each other or at least make the back immobile: it's difficult to imagine why that would evolve before the costal plates had made the ribcage immobile.

It appears that the shoulder girdle is in front of the ribcage (rather than inside -- it's inside the carapace, but not inside the ribcage!), which is generally the case in turtles according to a talk at this year's SVP meeting and in fact generally the case in amniotes. The left cleithrum ("dorsal process of the epiplastron" -- but see below) is preserved reaching above the vertebral column, though of course the fossil is crushed, so this may not mean anything.

Fig. 2a appears to contradict the claim in the text (and the supplementary information) that, like in lepidosaurs, astragalus and calcaneum are fused and the perforating foramen between them is absent, but the resolution is _just_ too bad to be sure.

The supp. inf. contains a phylogenetic analysis: *Odontochelys* was added to the matrix of Joyce (2007) and the result analyzed. That wouldn't have been necessary. The matrix contains an all-zero ancestor, and *O.* is indistinguishable from it -- all scores are either 0 or ?.

Then there's another phylogenetic analysis (*O.* added to the matrix of Rieppel & Reisz (1999) and the result analyzed). It contains an assumption on the ontogenesis of the "broadened ribs" that has, see above, not been tested and may not matter at all in the first place -- the paper itself says that the neural plates are separate bones in some extant turtles (and in *O.*) but ossify as outgrowths of the vertebrae in others, in other words, ontogeny can evolve.

The supp. inf. also says that the evidence for the presence of a cleithrum in *Kayentachelys* (and therefore in other turtles with a "dorsal process of the epiplastron") is equivocal, citing the following book chapter, which I have yet to get.

Olivier Rieppel (2008): The relationships of turtles within amniotes, 345 -- 353 in J. Wyneken, M. H. Godfrey & V. Bels, eds.: Biology of Turtles, CRC Press.

The cleithrum is a bone in the shoulder girdle that is absent in all known crown-group diapsids.

Incidentally, *O.* is Carnian in age.