From the Lyson et al. 2016 highlights:
“Recently discovered stem turtles [in this case Eunotosaurus] indicate the shell did not evolve for protection. Adaptation related to digging was the initial impetus in the origin of the shell. Digging adaptations facilitated the movement of turtles into aquatic environments. Fossoriality likely helped stem turtles survive the Permian/Triassic extinction.”
From the Lyson et al. summary:
“Developmental and fossil data indicate that one of the first steps toward the shelled body plan was broadening of the ribs (approximately 50 my before the completed shell.”
Lyson et al. have been beating this dead horse [or, dead turtle] for several years. Eunotosaurus is not a stem turtle, but they keep trotting it out. Currently it is the last of its kind and is most closely related to Acleistorhinus.
The real stem turtles
(Fig. 1) are small pareiasaurs, Elginia and Sclerosaurus according to the LRT, which tested all currently proposed candidates, including Eunotosaurus. It is important to have the right phylogeny or you’re sunk in a morass of fiction and faith that will never make sense.
Lyson et al. are unaware
that living turtles had dual origins among small pareiasaurs. Among domed hard-shell turtles, tabular and supratemporal horns were the first distinct traits, along with a reduced size. We don’t have post-crania for Elginia, but that’s where and when the high-domed shell first appeared if not slightly earlier or slightly later. Sister taxa, including the late surviving Meiolania, are already full shelled and with a long armored tail.
Among low-domed soft-shell turtles, once again tabular and supratemporal horns were the first steps. A pattern of ossified scutes preceded broader ribs in Sclelrosaurus.
Protection from Permian predators
was likely the raison d’être for the origin of turtle shells among phylogenetically miniaturized pareiasaurs. Large pareiasaurs were slow and otherwise defenseless so they depended on their bulk. Smaller forms survived with greater armor, both over the torso as a carapace that also covered the limbs and with greater armor over the head and tail with horns, clubs and spikes. Fossoriality and aquatic submergence are additional strategies for defense. To that end, it is easier for small tetrapods to hide whether underground or underwater. Moreover, a low metabolism helped basal turtles survive without food for long periods.
But what about Eunotosaurus?
Unfortunately the sisters of Eunotosaurus are known chiefly from skulls. The closest taxa with post-crania are the caseasaurs, including Oedaleops, and Milleretta, which has slightly expanded ribs.
At the same time
Some Diadectes specimens and Stephanospondylus were developing greatly expanded ribs beneath their pectoral girdles. I don’t know if Lyson et al. 2016 discuss these taxa, but in the past they have not done so.
Lyson et al. (8 co-authors) 2016. Fossorial Origin of the Turtle Shell.
Current Biology (advance online publication)