Pappochelys: Can taxon deletion force a relationship with turtles?

Earlier we looked at a new diapsid, Pappochelys (Schoch and Sues 2015, Fig. 1), touted as a basalmost turtle nesting along with Odontochelys.

Unfortunately
Pappochelys nested in the large reptile tree with basalmost placodonts, like Palatodonta and Majiashanosaurus. That nests it far from turtles and their kin.

Today
we’ll see if we can get Pappochelys to nest with turtles by taxon deletion (step-by-step removal of all putative sister taxa).

Figure 6. Pappochelys compared to placodont sister taxa and compared to the Schock and Sues reconstruction, which appears to have several scale bar errors and underestimated the number of dorsal vertebrae. Click to enlarge. So few ribs and vertebrae are known for Pappochelys that their order, size and number could vary from that shown here. Note the ribs of Paraplacodus are also expanded. The number of dorsal vertebrae is unknown and probably more than nine based on sister taxa.

Figure 1. Pappochelys compared to placodont sister taxa and compared to the Schock and Sues reconstruction, which appears to have several scale bar errors and underestimated the number of dorsal vertebrae. Click to enlarge. So few ribs and vertebrae are known for Pappochelys that their order, size and number could vary from that shown here. Note the ribs of Paraplacodus are also expanded. The number of dorsal vertebrae is unknown and probably more than nine based on sister taxa.

 

If we remove
200+ of the closest known sisters to Pappochelys, extending those deletions to half of the large reptile tree, where do you think Pappochelys will nest?

With turtles?
No.

For some reason,
perhaps due to its diapsid temporal region, the basal placodont, Pappochelys nests with long-necked tritosaur, Tanystropheus, when 200+ of its otherwise closest known sisters are deleted. That’s a classic ‘by default’ nesting.

Take away all of the tritosaurs
and Pappochelys nests at the base of the Sphenodontia/lepidosauria.

Take away all of the lepidosauria
and Pappochelys nests at the base of the lepidosauriformes, taxa with a diapsid temporal morphology, but not related to convergent diapsids related to Petrolacosaurus and kin.

Take away all of the lepidosauriformes
and Pappochelys nests with Barasaurus and other owenettids.

Take away all of the owenettids 
and Pappochelys finally nests between two turtles, Proganochelys and Odontochelys.

So Pappochelys resists nesting with turtles, given several grand opportunities to do so. It is morphologically that different. Pappochelys would rather nest with a long list of other taxa than nest with turtles.

References
Schoch RR and Sues H-D 2015. A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature (advance online publication) > doi:10.1038/nature14472 online

 

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