Dermochelys coriacea (Vandelli 1761, originally Testudo; Blainville 1816) lacks a bony shell (Fig. 1), replaced by thick, oily, leathery skin. The mouth and esophagus are filled with backward pointing spikes arising from toothess jaws (Fig. 1). Here (Fig. 2) a juvenile skull demonstrates the elevation and lengthening of the postorbital region in the adult.
Note the lack of temporal emargination
in the skull of Dermochelys (Fig. 2). That turns out to not be such a big deal in turtle evolution. The invagination occurred several times in turtles by convergence. This is something one finds out by phylogenetic analysis—if you don’t have an initial bias.
Alone no longer.
In the large reptile tree (LRT, 1200 taxa), the freshwater turtle Carettochelys (Fig. 3, 4; Ramsay 1886; 70 cm) nests with Dermochelys. Brinkman, Rabi and Zhao 2017 nested Carettochelys basal to soft shell turtles, unaware that soft shell turtles had a separate origin among small horned pareiasaurs. Like the soft-shell turtle, Trionyx, the soft nose tissue of Carettochelys extends slighly from the skull (Fig. 4) by convergence. Dermochelys (Fig. 1) does not have this proboscis.
The pig-nosed turtle
(genus: Carettochelys) is also unique. It is the only freshwater turtle with flippers. The carapace is not scaly, but leathery (hmm, where have we hear that before?) over bone (Fig. 5).
Despite their differences
Carettochelys and Dermochelys find no closer sisters in the LRT than with each other.
Why was this not discovered earlier?
Mistaking the supratemporal for the squamosal was only part of the problem. Fusion of the skull bones in turtles (as in birds) gives paleontologists trouble. The dual origin of turtles was not previously considered a possibility.
When workers expand their taxon list
they will recover what the LRT recovers. Until now (Fig. 6), unfortunately, that has not happened.
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).
Gaffney ES 1979. Comparative cranial morphology of recent and fossil turtles. Bulletin of the American Museum of Natural History 164(2):65-376.
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.