Adding the Triassic turtle Proterochersis to the large reptile tree

No surprises here.
The Late Triassic German dome-shelled turtle, Proterochersis (Fraas 1913, Szczygiellski  and Sulej 2016; ZPAL V.39/48), was added to the large reptile tree. No surprise, it nested with the other Late Triassic German dome-shelled turtle, Proganochelys. I was worried that Proterochersis would cause loss of resolution because the specimen lacks a skull, cervicals, caudals and limbs. Thus, all scores were based on the dorsal verts, ribs and girdles. And that was enough.

Proganochelys and Proterochersis, two Traissic turtles.

Figure 1. Proganochelys and Proterochersis, two Traissic turtles.

Szczygiellski and Sulej 2016
recently looked at Proterochersis together with a new Triassic turtle, Murrhardtia.

Here’s a big question
Proganochelys has a tall set of clavicles (aka epiplastra) that contacted and braced both the plastron and carapace (Gaffney 1990). Several basal dome-shelled turtles have these. In the basal dome-shelled turtle, Meiolania, Gaffney (xxxx) reports, “In the plastron the epiplastra meet on the midline and bear a short median process, apparently not homologous to that in Proganochelys and Kayentachelys, that bifurcates dorsally and articulates with the scapula. The epiplastron is a paired, curved element, meeting on the midline at the front of the plastron and forming a dorsal process. None of the specimens show a midline suture.”

Szczygiellski and Sulej 2016 reported, “the sturdy build of Proganochelys quenstedti should … be considered its own apomorphy. The presence of strong dorsal epiplastral processes contacting the carapace may be one of the consequences: although the dorsal processes themselves are interpreted by Gaffney (1990) as remnants of ancestral amniote clavicles, their additional articulation with the carapace and strengthening might have stabilized the shell, and thus serve as a more rigid point of attachment for the limb musculature (which probably was required to support the heavy body). Large dorsal epiplastral processes are present in the slightly smaller Palaeochersis talampayensis (Sterli et al., 2007), but are weaker and do not articulate with the carapace in more basal Proterochersis spp. and Keuperotesta limendorsa gen. et sp. nov. In Odontochelys semitestacea they obviously do not contact the carapace, because no suitable point of attachment was available (Li et al., 2008), but they possibly played a similar role, temporarily supporting and strengthening the limb musculature (weakened by changes in rib position), and disappeared when the torso of the animal became fully stiffened and the pectoral girdle received its derived shape.”

References
Fraas E. 1913. Proterochersis, eine pleurodire Schilderöte aus dem Keuper. Jahreshefte des Vereins für Vaterlänzische Naturkunde in Württemberg 69: 13–30.
Szczygiellski T and Sulej T 2016. Revision of the Triassic European turtles Proterochersis and Murrhardtia (Reptilia, Testudinata, Proterochersidae), with the description of new taxa from Poland and Germany. Zoological Journal of the Linnean Society 177:395-427.
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the American Museum of Naturaly Histoyr 229: 1-165.

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