Turtles: Still not diapsids

Updated 02-25-2016 with a new pareiasaur skulls in dorsal view image (figure 2) based on updated data on Bunostegos 02-24-16.

This just in from today’s Dinosaur Mailing List.

Schoch and Sues
previously published on turtle origins and Pappochelys here, from which figure 1 was lifted.

Highlights from Schoch and Sues 2016:
New fossils from Germany reveal that turtles had ancestors with a diapsid skull.

That would be Pappochelys, which nests as a basal sauropterygian (an archosauromorph diapsid), far from turtles, and Odontochelys, a basal soft-shell turtle, not a basal turtle, despite the reappearance of teeth.

The turtle plastron formed in part from thickened gastralia.
Actual turtle sisters, like Scutosaurus and the pareiasaurs, did not have gastralia, so the plastron is not formed of gastralia.

The closed anapsid skull of turtles evolved late in the stem group.
Not so according to a larger gamut study, the large reptile tree. No turtle ancestors among the Pareiasauria had diapsid skull openings. And the LRT tests 658 taxa from a wide gamut of reptiles, so there is ample opportunity for all taxa to nest where they find it most parsimonious to nest.

There may also be confusion regarding the presence of a supratemporal in turtles (it is big bone, the remains of the former horn, but it is traditionally and widely misidentified, even today). There is also confusion regarding diapsids, which are diphyletic with lizards not related to archosaurs except through basalmost Viséan amniotes.

The traditional parareptile hypothesis of turtle origin is rejected. 
The Parareptilia is far from monophyletic so it should be rejected as a clade. Unfortunately, the authors are not considering the horned turtle Meiolania as the basalmost turtle and toothy horny Elginia as its proximal sister. Those two nest at the origin of turtles.

From the Shoch and Sues 2016 abstract
“The origin of turtles has been a persistent unresolved problem involving unsettled questions in embryology, morphology, and paleontology. New fossil taxa from the early Late Triassic of China (Odontochelys) and the Late Middle Triassic of Germany (Pappochelys) now add to the understanding of (i) the evolutionary origin of the
turtle shell, (ii) the ancestral structural pattern of the turtle skull, and (iii) the phylogenetic position of Testudines. As has long been postulated on the basis of molecular data, turtles evolved from diapsid reptiles and are more closely related to extant diapsids than
to parareptiles, which had been suggested as stem group by some paleontologists. The turtle cranium with its secondarily closed temporal region represents a derived rather than a primitive condition and the plastron partially evolved through the fusion of gastralia.”

Unfortunately
More parsimonious sisters appear among derived pareiasaurs. Pappochelys nests with basal placodonts, which independently developed turtle-like shells twice while a related taxon, Sinosaurophargis developed a third convergent shell. Schoch and Sues used too few taxa, too few pertinent taxa, and too many suprageneric taxa in an attempt at covering a wide gamut of reptiles. The large reptile tree (LRT) uses enough taxa, enough pertinent taxa and no suprageneric taxa to find the origin of every reptile on the inclusion list (658 at last count).

Figure 1. Schoch and Sues cladogram (black) with branches not recovered by the LRT (red) and LRT relationships (colored arrows). As you can see, using suprageneric taxa really does put one in a hopeful cloud, but species and specimen-based taxa will see you to your goals.

Figure 1. Schoch and Sues cladogram (black) with branches not recovered by the LRT (red) and LRT relationships (colored arrows). As you can see, using suprageneric taxa really does put one in a hopeful cloud, but species and specimen-based taxa will see you to your goals.

You can see a YouTube video
on the origin of turtles here, which was blogged earlier here.

Figure 3. Dorsal views of bolosaur, diadectid, pareiasaur, turtle and lanthanosuchian skulls. The disappearance of the turtle orbit in lateral view occurs only in hard shell turtles.

Figure 2. Dorsal views of bolosaur, diadectid, pareiasaur, turtle and lanthanosuchian skulls. The disappearance of the turtle orbit in lateral view occurs only in hard shell turtles.

References
Schoch RR and Sues H-D 2016. The diapsid origin of turtles. Zoology (advance online publication) doi:10.1016/j.zool.2016.01.004
http: // www.sciencedirect.com/science/article/pii/S0944200616300046?np=y

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