EAVP 2021: Early protostegid turtle more like freshwater turtles

Rabit et al. 2021 studied a new Cretaceous hardshell turtle
and came up with contradictory conclusions.

From the abstract:
“Whether advanced marine adaptations like that of extant sea turtles (Chelonioidea) evolved once or twice in turtles remains unresolved owing to the contested relationships of Protostegidae, a Cretaceous extinct pelagic clade.”

This was resolved in February 2021 when the giant protostegid, Archelon, was added to the large reptile tree (LRT, 1890+ taxa, subset Fig. 1) and nested with fresh water snapping turtles, like Macrochelys, a taxon omitted from prior sea turtle cladograms.

Figure 1. Subset of the LRT focusing on turtle origins.
Figure 1. Subset of the LRT focusing on turtle origins.

The Rabit et al. 2021 abstract continues:
“Fossils of protostegids are globally rare and the absence of species showing a transitional stage between littoral and pelagic adaptation precludes rigorously testing whether this clade is related to extant sea turtles or represents an earlier, convergent marine radiation.”

You don’t have to wait for that transitional fossil to be discovered. Add taxa. Run an analysis. This is exactly why cladograms are useful.

“We report a new protostegid turtle from the Early Cretaceous Aptian Apón Formation of Venezuela based on a single, three dimensionally preserved, near-complete skull. This still unnamed taxon represents one of the oldest protostegids and is characterized by a narrow interorbital space, dorsolaterally oriented and relatively small-sized orbits, anteriorly sloping skull roof, relatively deep lower and upper temporal emarginations, and reduced vomer and cavum tympani. These traits are unlike those of other protostegid or chelonioid sea turtles but approximate the condition seen in freshwater turtles;”

Always wonderful to see new taxa — but wait a minute. The authors say this is “one of the oldest protostegids,” but they also say, “These traits are unlike those of other protostegid or chelonioid sea turtles.” What’s going on here?

“We hypothesize them [= these traits] as plesiomorphic. Parsimony analysis recovers this species as a basal protostegid on the stem-lineage of crown-sea turtles, indicating a single pelagization event during turtle evolution.”

Not a ‘single pelagization event’ according to the LRT. There were at least three entries of turtles into the sea. (Fig. 1). Freshwater turtles are mentioned (above), but snapping turtles are not mentioned in this abstract. Did they miss something? or deliberately holding back? Softshell turtles had their own marine entry, Ocepechelon.

“However, further (less derived) transitional forms are needed to rigorously test the global relationships of Protostegidae. The Venezuelan taxon nevertheless fills a considerable morphological gap in the early evolution of the group, perhaps corresponding to a littoral [=shore dwelling] phase. It represents only the third described protostegid from the Early Cretaceous southwestern Atlantic.”

I wish they hadn’t said,These traits are unlike those of other protostegid or chelonioid sea turtles” and then concluded they had “the third described protostegid“. Confusing and contradictory.

BTW, the “considerable morphological gap” disappears in the LRT with taxon inclusion. Be careful with unwarranted hyperbole. Looking forward to seeing this specimen and its cladogram published.

More details here.

Figure 1. Classic photos of Archelon in ventral and dorsal views.
Figure 2. Classic photos of Archelon in ventral and dorsal views.

References
Rabi M et al. 2021. Plesiomorphic morphology in an early protostegid turtle from the Cretaceous of Venezuela. EAVP 2021 abstracts.

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