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  6. Discovery of first fossil turtle that lacked shell but had a toothless beak offers new insights into turtle evolution

Discovery of first fossil turtle that lacked shell but had a toothless beak offers new insights into turtle evolution

Yu Chen.

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Illustration of Eorhynchochelys sinensis in it marine habitat. With strong limbs equipped with large claws, it probably lived in shallow water along the seashore and dug in the mud for food. 

OTTAWA, Aug 23, 2018 - An international group of paleontologists, including Dr. Xiao-chun Wu with the Canadian Museum of Nature, has described a 228-million-year-old turtle that lacked a shell but had the first toothless turtle beak. The fossil provides new insights into the complex evolution story of turtles, whose origins have been an unsolved problem in palaeontology for decades. The study is published today in the scientific journal Nature.

A number of key features define a modern turtle: not only its shell but also its toothless beak, a short trunk (with 10 trunk vertebrae), rigid pelvis, and the lack of openings on the skull behind the eyes. The newly-identified fossil sheds light on how modern turtles developed these traits. While its body was Frisbee-shaped, its wide ribs hadn’t grown to form a shell like we see in turtles today.

“In examining this remarkably well-preserved fossil, we see that the evolution of turtles came about by a complex series of events, rather than a more straightforward step-by-step accumulation of unique traits,” explains Dr. Xiao-chun Wu co-author of the paper along with Dr Li Chun, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China; Dr. David Norman with the National Museums Scotland; and Dr. Oliver Rieppel with the Field Museum in Chicago.

The new species has been named Eorhynchochelys sinensis. Eorhynchochelys (“Ay-oh-rink-oh-keel-is”) means “dawn beak turtle”—essentially, first turtle with a beak—while sinensis, meaning “from China,” refers to the country where it was found by Zheng-Yuan Sun of the Sanya Museum and the study’s lead author Chun Li in 2015 in Guizhou Province.

Glenn Poirier © Canadian Museum of Nature

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The fossil of Eorhynchochelys was extremely well-preserved, giving the team detailed information about its skeleton.

The fossil was over 2.5 metres (eight feet) long and preserved in marine black-shaly marlstone. It had an unusual disc-like body and a long tail, with the anterior part of its jaws developing into the beak. With strong limbs equipped with large claws, it probably lived in shallow water along the seashore and dug in the mud for food.

Eorhynchochelys joins a few other early fossil turtles that scientists, including the authors of this paper, have discovered and described. One from 220 million years ago (Odontochelys semitestacea) has a partial shell but no beak. Until now, it’s been unclear how they all fit into the reptile family tree. How turtles evolved has become a lot clearer now with the presence of Eorhynchochelys. The fact that Eorhynchochelys developed a beak before Odontochelys (a more advanced form), but lacked a shell is evidence of mosaic evolution—the idea that traits can evolve independently from each other and at a different rate, and that not every ancestral species has the same combination of these traits.   

Modern turtles have both shells and beaks, but the evolutionary path to get there was not a straight line. Instead, some turtle relatives got partial shells while others got beaks, and eventually, the genetic mutations that create these traits occurred in the same animal. In addition, the trunk shortened over time, even though Eorhynchochelys still had 12 vertebrae, two more than modern relatives and Pappochelys (a more primitive form from Germany, much older than Eorhynchochelys at 240 million years old).  

For years, scientists were uncertain whether turtle ancestors were part of the same reptile group as modern lizards, snakes, crocodiles, and birds (diapsids, which early in their evolution had two pairs of holes on the roof and sides of their skulls), or if they were anapsids that lack these openings. Two small openings on the skull roof of the older Pappochelys and the closure of the openings in Eorhynchochelys solved this mystery in turtle evolution. The skull of Eorhynchochelys still had a pair of holes on the skull sides, which shows that it is an intermediate between early members of turtles (with a diapsid skull) and later members of turtles (with an anapsid skull).

Fortunately, the Eorhynchochelys fossil was extremely well-preserved, giving the team detailed information about its skeleton. The study of the fossil provides clearer evidence about how and when turtles developed shells, a rigid pelvis and closed skull-roof openings—gradually losing their ‘status’ as diapsids, which will change how scientists think about this branch of animals.

The fossil is now in the collections of the Sanya Museum of Marine Paleontology in Hainan Province, China. This study was contributed to by the Institute of Vertebrate Paleontology and Paleoanthropology, the Chinese Academy of Sciences, National Museums Scotland, the Field Museum, and the Canadian Museum of Nature.

NOTE TO MEDIA: Photos of the complete skeleton and the skull, as well as an artist illustration of Eorhynchochelys in its water habitat are available.

Information for media:
Dan Smythe
Media Relations
Canadian Museum of Nature
613.566.4781; 613.698.9253 (cell)
dsmythe@nature.ca