228-Million-Year-Old Fossil Reveals Complex Early History of Turtles
Paleontologists in China have discovered a 228-million-year-old extinct species of turtle, known for its weird disc-like body without a shell and its toothless beak.
Named Eorhynchochelys sinensis, the newly-discovered turtle lived approximately 228 million years ago (Triassic period) in what is now southwestern China.
“This creature was over 6 feet long (1.8 m), it had a strange disc-like body and a long tail, and the anterior part of its jaws developed into this strange beak. It probably lived in shallow water and dug in the mud for food,” said Dr. Olivier Rieppel, a paleontologist at Field Museum.
“Eorhynchochelys sinensis isn’t the only kind of early turtle that paleontologists have discovered — there is another early turtle with a partial shell but no beak. Until now, it’s been unclear how they all fit into the reptile family tree.”
“The origin of turtles has been an unsolved problem in paleontology for many decades. Now with Eorhynchochelys sinensis, how turtles evolved has become a lot clearer.”
The fact that Eorhynchochelys sinensis developed a beak before other early turtles but didn’t have 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 path evolution took to get there wasn’t 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.
“This impressively large fossil is a very exciting discovery giving us another piece in the puzzle of turtle evolution. It shows that early turtle evolution was not a straightforward, step-by-step accumulation of unique traits but was a much more complex series of events that we are only just beginning to unravel,” said Dr. Nick Fraser, from the National Museums Scotland.
Fine details in Eorhynchochelys sinensis’ skull solved another turtle evolution mystery.
For years, paleontologists weren’t sure if turtle ancestors were part of the same reptile group as modern lizards and snakes — diapsids, which early in their evolution had two holes on the sides of their skulls — or if they were anapsids that lack these openings.
The skull of Eorhynchochelys sinensis shows signs that the species was a diapsid.
“With Eorhynchochelys sinensis’ diapsid skull, we know that turtles are not related to the early anapsid reptiles, but are instead related to evolutionarily more advanced diapsid reptiles. This is cemented, the debate is over,” Dr. Rieppel said.
Eorhynchochelys sinensis is described in a paper published online this week in the journal Nature.
Chun Li et al. 2018. A Triassic stem turtle with an edentulous beak. Nature 560 (7719): 476-479; doi: 10.1038/s41586-018-0419-1