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230-million-year-old turtle fossil deepens mystery of reptile's origins

Two-metre-long specimen could help researchers pin down how and when turtles developed traits such as their shell.

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Illustration of E. sinensis opening its beak on the sea bed. In the background two fish and another E.sinesis swim among seaweed

An artist’s impression of Eorhynchochelys sinensis, a turtle ancestor that lived about 230 million years ago. Credit: Yu Chen

A fossilized turtle discovered in southwestern China fills an evolutionary hole in how the reptiles developed features such as a beak and shell, researchers report1 on 22 August in Nature. Although the specimen can help scientists to pin down when modern turtles developed such characteristics, it’s also muddied the waters when it comes to illuminating the group’s origins.

The roughly 2-metre-long animal, dubbed Eorhynchochelys sinensis, lived about 230 million years ago. Its skull is similar to those of modern turtles, whereas the rest of the animal’s skeleton is more like that of a predecessor that lived 10 million years before.

This new species fits almost perfectly in the evolutionary picture that researchers conceived of years before regarding how turtles acquired their signature features, says Rainer Schoch, an amphibian and reptile palaeontologist at the Stuttgart State Museum of Natural History in Germany. “We’re really happy to see this.”

Filling in the gaps

Turtles haven’t changed much over the past 210 million years. They all have a top shell formed from the fusion of their spine and ribs, a bottom shell that protects their belly, a sharp beak and a mouth without any teeth. But the group lacks a feature common to most modern reptiles — two pairs of holes in their skull, behind their eyes, where jaw muscles attached.

The absence of those holes has contributed to a decades-long debate on the exact position of turtles on the reptile family tree. And this has compounded researchers’ struggle to work out when and how turtle characteristics first evolved.

A specimen discovered in 2008, called Odontochelys semitestacea, offered the first clues2. The roughly 220-million-year-old animal possessed teeth and a bottom shell, and its wide ribs hinted at the beginnings of a top shell. But it lacked a beak and the pairs of holes in its skull.

Then, in 2015, scientists found Pappochelys rosinae, a 240-million-year-old specimen that was missing a top shell, but showed the first signs of a bottom shell3. Unlike modern turtles, P. rosinae had two pairs of openings in its skull, indicating for the first time that turtles were closely related to other modern reptiles.

Now, the discovery of Eorhynchochelys fills in the gap between these two species. The fossil turtle possesses a single pair of holes behind its eyes, suggesting a gradual transition from Pappochelys to modern turtles.

Fossil of a complete E. sinensis skeleton, as preserved.

Eorhynchochelys, the latest fossilized-turtle discovery, was more than 2 metres long.Credit: Xiao-Chun Wu

But the presence of a beak on the Eorhynchochelys skeleton is what really intrigued study co-author Xiao-Chun Wu, a palaeontologist at the Canadian Museum of Nature in Ottawa. It’s a trait that researchers hadn’t seen in early turtle fossils until now, and that seems to have disappeared in some species and reappeared in others millions of years later. This suggests that the evolution of a beak in modern turtles was not a straight path, Wu says.

Family ties

But even though Eorhynchochelys helps to demonstrate the acquisition of turtle traits, Schoch says, it’s not so informative about their place on the evolutionary tree.

Most genetic studies over the past 20 years have positioned crocodilians, dinosaurs and modern birds as the turtles’ closest evolutionary relatives. But some studies looking at DNA or RNA, as well as analyses of turtle anatomy, have pointed to lizards and snakes as the group’s closest relatives.

After including Eorhynchochelys’s physical characteristics in an analysis with those of other fossilized reptiles, however, Wu and his colleagues say that turtles aren’t as closely related to any of those groups as other research suggests. They’re more of an offshoot from earlier ancestors, Wu says.

Schoch is sceptical of this claim, however, saying that researchers don’t know enough about the anatomy of early reptilian ancestors to know for sure where turtles fall.

We just need to find out more about those early ancestors, Schoch says. “That is now the big problem and the next step that will have to be taken.”

doi: 10.1038/d41586-018-06012-0
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References

  1. 1.

    Li, C., Fraser, N. C., Rieppel, O. & Wu, X.-C. Nature https://doi.org/10.1038/s41586-018-0419-1 (2018).

  2. 2.

    Li, C., Wu. X.-C., Rieppel, O., Wang, L.-T. & Zhao, L.-J. Nature 456, 497-501 (2008).

  3. 3.

    Schoch, R. R. & Sues, H.-D. Nature 523, 584-587 (2015).

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