Yes, underwater wings.
We’re talking today about the pig-nosed turtle, Carettochelys insculpta (Figs. 1-3), which became interesting when Brinkman, Rabi and Zhao 2017 nested it basal to soft-shell turtles. The large reptile tree (LRT, 1043 taxa, subset Fig. 3) does not replicate those results. Rather the LRT nests Carettochelys with Foxemys.
Carettochelys insculpta (Ramsay 1886; 70 cm) is the extant pig-nosed turtle. Unlike any other species of freshwater turtle, the feet are flippers, like the marine sea turtle Chelonia. The carapace is not scaly, but leathery. It remains domed and the solid plastron is strongly connectedd to the carapace. Brinkman, Rabi and Zhao 2017 nested Carettochelysbasal to soft shell turtles, but the large reptile tree nests it with Foxemys. Like Trionyx, the nose extends slighly from the skull.
Not sure why
Brinkman, Rabi and Zhao 2017 nest Carettochelys with soft shell turtles, but I suspect it has to do with taxon exclusion (a limited gamut of tested taxa) and an improper traditional inclusion.
The skull of Carettochelys
includes large and extensive postorbital fenestrae. The jugal is quite tiny. The squamosal (blue) and quadratojugal (beige) are fused, as in sister taxa. The supratemporal (orange) has been traditionally mislabeled as a squamosal.
As an experiment
I deleted all taxa other than turtles (Fig. 5) and decided that Proganochelys would be the outgroup to match the analyses of other workers. Even so, soft shell turtles do not nest with Carettochelys.
A subset of the LRT
(Fig. 6) shows the relationship of soft shell and hard (dome) shell turtles to pareiasaurs. Note: turtles are not monophyletic, unless you also include the pareiasaurs Bunostegos and Arganaceras, which I do here to document the clade of crown turtles. The LRT includes enough characters to separate soft shell turtles from others, despite a long list of similar traits. That should give one great confidence that the character list is sufficient at its present number.
Marine turtles with flippers (underwater wings)
include Dermochelys, the extant leatherback turtle (Fig. 7), convergent with Carettochelys. The LRT includes enough traits to separate these two similar yet distinct taxa.
Brinkman D, Rabi M and Zhao L-J 2017. Lower Cretaceous fossils from China shed light on the ancestral body plan of crown soft-shell turtles (Trionychidae, Cryptodira). Nature Scientific Reports 7(6719).
Ramsay EP 1886. On a new genus and species of fresh water tortoise from the Fly River, New Guinea. Proceedings of the Linnaean Society of New South Wales (2) 1: 158-162.