here, here and here we looked at turtle origins — a controversial topic in mainstream paleontology resolved quickly and surely in the large reptile tree, which gives 639 taxa the opportunity to be ancestral to turtles.
Long story short
Toothy Elginia currently nests outside the turtles (only because we don’t have any post-crania) and toothless Meiolania nests as the basalmost turtle (Fig. 1) because it retains supratemporal horns and the elbows still extend laterally, not anteriorly. These taxa are derived from pareiasaurs, which are themselves sisters to diadectids, bolosaurs and proclophonids.
When the skulls of pertinent taxa
are seen in dorsal view (Fig. 2) it is easier to see the reduction of the horns in pre- and basal turtle skulls. One also gets the impression that when Proganochelys and Odontochelys arrived on the scene in the Late Triassic, they both represent a much earlier radiation of turtles, both horned and not horned. So there are many more basal turtles out there waiting for us to discover them.
The Odontochelys tooth problem
Odontochelys is a Late Triassic toothed turtle that originally was considered (Li et al. 2008) a very basal turtle. Not so according to phylogenetic analysis which nests it with soft shell turtles like Trionyx. The odd thing is this soft shell turtle appears to have regrown teeth. More basal and sister taxa do not have teeth (Fig. 3). Odontochelys is also unusual in having nares in the anterior lateral orientation, not completely anterior, as in Trionyx, as in virtually all other turtles, and not dorsal, as in Ocepecephalon, which is also very off for a turtle.
The supratemporal problem
This evolutionary sequence demonstrates that the large supratemporal bones of turtles (the supratemporal horns of pre-turtles and Meiolania) have been traditionally mislabeled. This may be part of the problem that workers have had in nesting turtles in prior studies.
The molecule problem
Some researchers have found that turtle DNA is most closely matched to that of living archosaurs: crocs and birds. Everyone knows morphology does not support that nesting. Someone somewhere will figure this out someday.
Li C, Wu X-C, Rieppel O, Wang L-T and Zhao L-J 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.