SVP 22 Pappochelys the basal placodont – not the basal turtle

Schoch and Sues (2015)
describe (without naming the previously published) Pappochelys, a basal placodont, actually far from turtles.

From the abstract
“The origin and early diversification of turtles have long been major contentious issues in the study of vertebrate evolution. This is due to conflicting character evidence from molecules and morphology, as well as a lack of transitional fossils from the critical time interval. The stem-turtle Odontochelys, from the early Late Triassic (Carnian) of Guizhou (China), has a partially formed shell and many turtle-like features in its postcranial skeleton. Unlike Proganochelys, from the Late Triassic (Norian) of Germany and Thailand, it retains marginal teeth and lacks a carapace. Odontochelys is separated by a considerable temporal gap from Eunotosaurus*, from the late Middle Permian (Capitanian) of South Africa, which has been plausibly hypothesized as the earliest stem turtle. A new taxon** from the late Middle Triassic (Ladinian) of Baden-Württemberg (Germany) represents a structural and chronological intermediate between Eunotosaurus and Odontochelys. The three taxa share the possession of anteroposteriorly broad trunk ribs that are T-shaped in cross-section and bear sculpturing, elongate dorsal vertebrae, and modified limb girdles. Unlike Odontochelys, the new stem-turtle has a cuirass of robust paired gastralia in place of a plastron***. It provides evidence that the plastron partially formed through serial fusion of gastralia. The skull of the new stem-turtle has small upper and ventrally open lower temporal fenestrae, supporting the hypothesis of diapsid affinities of turtles****. Both the upper and lower jaws bear teeth. Phylogenetic analysis found Pan-Testudines (including the new taxon) as the sister-taxon of Sauropterygia*****. Together these two clades form the sister-taxon of Lepidosauriformes******. The new stem-turtle lends additional support to an earlier hypothesis arguing for an aquatic origin for the turtle body plan.”

*not related to turtles, but convergent
**this is Pappochelys and the large reptile tree indicates it is not related to turtles nor to Eunotosaurus, but convergent with both.
***actual turtle sister taxa do not have gastralia, a plastron does not arise from gastralia.
****which is NOT supported by the large reptile tree
*****attracted by several turtle-like placodonts with convergent shells
****** neither turtles nor sauropterygians are lepidosauriform sisters in the large reptile tree. Neither are turtles archosaurs.

this phylogeny is all total rubbish when you add a sufficient number of pertinent taxa as discussed earlier. Taxon exclusion remains a problem here.

Pappochelys is a wonderful new addition any taxon list.

The large reptile tree
was updated recently (607 taxa) with the addition of another diadectid that tells us more about procolophonid and bolosaurid relations to diadectids, not too far from the clade of pareiasaurs that gave rise to turtles. 

Sues H-D and Schoch RR 2015.
A Middle Triassic stem-turtle form Germany and the evolution of the turtle body plan. Journal of Vertebrate Paleontology abstracts.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.