Schoch et al. 2019 rehash an old trope.
suffering from overlooked taxon exclusion and convergence revealed by a wider gamut phylogenetic analysis, the large reptile tree (LRT, 1542 taxa).
From their abstract:
“Unlike any other tetrapod, turtles form their dorsal bony shell (carapace) not from osteoderms, but by contribution of the ribs and vertebrae that expand into the dermis to form plate-like shell components. Although this was known from embryological studies in extant turtles, important steps in this evolutionary sequence have recently been highlighted by the Triassic taxa Pappochelys, Eorhynchochelys and Odontochelys, and the Permian Eunotosaurus. The discovery of Pappochelys shed light on the origin of the ventral bony shell (plastron), which formed from enlarged gastralia. A major question is whether the turtle shell evolved in the context of a terrestrial or aquatic environment. Whereas Odontochelys was controversially interpreted as aquatic, a terrestrial origin of turtles was proposed based on evidence of fossorial adaptations in Eunotosaurus. We report palaeohistological data for Pappochelys, a taxon that exemplifies earlier evolutionary stages in the formation of the bony shell than Odontochelys. Bone histological evidence reveals (1) evolutionary changes in bone microstructure in ribs and gastralia approaching the turtle condition and (2) evidence for a predominantly amphibious or fossorial mode of life in Pappochelys, which support the hypothesis that crucial steps in the evolution of the shell occurred in a terrestrial rather than fully aquatic environment.”
We’ve explored such possibilities
earlier here, here and here. A wider gamut (1542 taxa) competing cladogram of vertebrate interrelationships recovers Pappochelys close to the ancestry of placodonts, and Eunotosaurus + Eorhynochelys close to Acleistorhinus. Neither are close to turtles. Odontochelys is close to the origin of soft shell turtles. The small horned pareiasaur, Sclerosaurus, is closer. Proganochelys is close to the origin of hard shell turtles. Meiolania and the small horned pareiasaur, Elginia, are closer. Convergence and taxon exclusion seem to be the trouble here.
Back to the Schoch et al. question:
“A major question is whether the turtle shell evolved in the context of a terrestrial or aquatic environment.”
Both. See dual origin of turtles from pareiasaurs and notes above. Schoch et al. are not working within the correct phylogenetic context. When they get the same results while including the turtle ancestors recovered by the wide gamut LRT, let me know. For now, apparently, they appear to be content to play in their little corner ignoring data that has been online for several years.
Online cladogram here:
Schoch RR, Klein N, Scheyer TM and Sues H-D 2019. Microanatomy of the stem-turtle Pappochelys rosinae indicates a predominantly fossorial mode of life and clarifies early steps in the evolution of the shell. Nature Scientific Reports 9:10430 online here