From ScienceDaily:
There are a couple of key features that make a turtle a turtle: its shell, for one, but also its toothless beak. A newly-discovered fossil turtle that lived 228 million years ago is shedding light on how modern turtles developed these traits. It had a beak, but while its body was Frisbee-shaped, its wide ribs hadn’t grown to form a shell like we see in turtles today.
“This creature was over six feet long, it had a strange disc-like body and a long tail, and the anterior part of its jaws developed into this strange beak,” says Olivier Rieppel, a paleontologist at Chicago’s Field Museum and one of the authors of a new paper in Nature. “It probably lived in shallow water and dug in the mud for food.”
Classic turtle, apart from the shell free lifestyle.
Eorhynchochelys isn’t the only kind of early turtle that scientists 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,” says Rieppel. “Now with Eorhynchochelys, how turtles evolved has become a lot clearer.”
The fact that Eorhynchochelys 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. Paper. (open access) – Chun Li, Nicholas C. Fraser, Olivier Rieppel, Xiao-Chun Wu. A Triassic stem turtle with an edentulous beak. Nature, 2018; 560 (7719): 476 DOI: 10.1038/s41586-018-0419-1 More.
There is probably more to the story. From Nature:
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. Jeremy Rehm, “230-million-year-old turtle fossil deepens mystery of reptile’s origins” at Nature
The find fills in some gaps but creates other, bigger ones. As the Nature article goes on to explain, turtles have traditionally been thought to be closest on the Tree of Life to crocodilians, dinosaurs and modern birds but some genetic studies have suggested that they are closer to lizards and snakes. However…
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. Jeremy Rehm, “230-million-year-old turtle fossil deepens mystery of reptile’s origins” at Nature
So we know more about their history but it looks less like the history we had come to expect. Of course, that’s when you know you are getting somewhere with a serious investigation.
See also: More turtle shell puzzles
Turtles: Shells evolved for digging, not protection?
and
Not a Big Bang of turtles TOO? As well as birds and mammals