Turtle body plans 2020: still not diapsids

Lyson and Bever 2020 once again propose
a diapsid origin for turtles that is not supported by the large reptile tree (LRT, 1719+ taxa, subset Fig. 1) where hardshell and soft-shell turtles arise in parallel from small horned pareiasaurus without temporal fenestrae — and all competing candidates for turtle ancestry are tested.

From the Lyson and Bever abstract:
“The origin of turtles and their uniquely shelled body plan is one of the longest standing problems in vertebrate biology.”

In the LRT this problem has been resolved for several years. Click here for an online paper on the dual origin of turtles from pareiasaurs. Click here for the dual origin of turtles to scale blogpost. Click here for the latest LRT cladogram.

“The unfulfilled need for a hypothesis that both explains the derived nature of turtle anatomy and resolves their unclear phylogenetic position among reptiles largely reflects the absence of a transitional fossil record.”

Not so. In the LRT several overlooked taxa well document a good transitional fossil record. Lyson and Bever omit, ignore and overlook these taxa in favor of several unrelated turtle mimics.

“Recent discoveries have dramatically improved this situation, providing an integrated, time-calibrated model of the morphological, developmental, and ecological transformations responsible for the modern turtle body plan. This evolutionary trajectory was initiated in the Permian (>260 million years ago) when a turtle ancestor with a diapsid skull evolved a novel mechanism for lung ventilation. This key innovation permitted the torso to become apomorphically stiff, most likely as an adaption for digging and a fossorial ecology. The construction of the modern turtle body plan then proceeded over the next 100 million years following a largely stepwise model of osteological innovation.”

Not so. Overlooked taxa known for decades (Elginia (Fig. 2), Sclerosaurus) have been traditionally excluded from turtle origin studies. Some recent discoveries (Eorhynochelys, Pappochelys) nest elsewhere, apart from turtles, as turtle mimics. The LRT tests all known candidates. Lyson and Bever do not. They are still excluding pertinent taxa. Adding more taxa shows that turtles and their ancestors have never been diapsids.

Figure 1. Carbonodraco enters the LRT alongside another recent addition, Kudnu, at the base of the pareiasaurs + turtles.

Figure 1. Carbonodraco enters the LRT alongside another recent addition, Kudnu, at the base of the pareiasaurs + turtles.

The most primitive hardshell turtles 
are not the oldest known hardshell turtles. Horned turtle skulls are widely and traditionally considered derived, not primitive. Elginia and Meiolania (Fig. 2) have never been tested together in analysis, and not by Lyson and Bever, despite their obvious similarities and homologies.

Turtle respiration was a big issue for Lyson and Bever.
Earlier we looked at pre-softshell turtle respiration in Sclerosaurus here.

Figure 2. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

Figure 2. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

Turtle mimics are out there.
Evidently only a wide gamut phylogenetic analysis, like the LRT, can lump and separate turtle ancestors from turtle mimics without bias and without traditional influences. Lyson and Bever mistakenly accepted several turtle mimics as turtle ancestors, then built a diapsid story around their cherry-picked taxa. Referees and editors also accepted this invalid scenario.

Add taxa
to find and separate real turtle ancestors from turtle mimics.


References
Lyson TR and Bever GS 2020. Origin and Evolution of the Turtle Body Plan Annual Review of Ecology, Evolution, and Systematics 51:- (Volume publication date November 2020) Review in Advance first posted online on July 31, 2020. (Changes may still occur before final publication.) https://doi.org/10.1146/annurev-ecolsys-110218-024746

researchgate.net/_The_dual_origin_of_turtles_from_pareiasaurs

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