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