Soft shell turtle mystery resolved

Brinkman, Rabi and Zhao 2017 report: 
“The earliest pan-trionychids had already fully developed the “classic” softshell turtle morphology and it has been impossible to resolve whether they are stem members of the family or are within the crown. It remains unclear whether the more heavily ossified shell of the cyclanorbines or the highly reduced trionychine morphotype is the ancestral condition for softshell turtles. A new pan-trionychid from the Early Cretaceous of Zhejiang, China, Perochelys hengshanensis sp. nov., allows a revision of softshell-turtle phylogeny. Results indicate that the primitive morphology for soft-shell turtles is a poorly ossified shell like that of crown-trionychines and that shell re-ossification in cyclanorbines (including re-acquisition of peripheral elements) is secondary.”
The “reacquisition” presupposes
that those elements were lost earlier. That may not be true based on the topology of the large reptile tree (LRT 1040 taxa).
This confirms,
at least in part, earlier studies here that showed that Odontochelys was ancestral only to soft shell turtles. It also has a poorly ossified shell and was derived from a sister to Sclerosaurus. The LRT recovers a topology in which turtles are diphyletic and both derived from different yet closely related pareiasaurs, something traditional studies have yet to catch up to.
Brinkman D, Rabi M and Zhao L 2017. Lower Cretaceous fossils from China shed light on the ancestral body plan of crown softshell turtles (Trionychidae, Cryptodira).Scientific Reports 7, Article number: 6719 (2017)

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