A new fossil study reveals that the hard-shelled turtle that lived in China’s coastal waters 220 million years ago is the “oldest” turtle ever.

They have belly shells but their backs are not covered at all .

Last week, a team of scientists announced the discovery of the most ancient aquatic turtle, dating it to 164 million years ago . But it was only a short-lived title. The hard-shelled water turtle that Chun Li (Chinese Academy of Sciences in Beijing) and his colleagues researched and named Odontochelys semistestacea existed a long time ago.

Li and his colleagues studied the remaining remains, including two skulls and skeletons , excavated in Guizhou province, China in 2007. Their analysis was published in the November 27 issue of Nature. Analysis shows that modern turtles have descended from aquatic ancestors. The results also provide evidence for the hypothesis about the evolutionary path of turtle shells.

The shell forms from the abdomen

The research sample has many signs of primitive turtles. For example, researchers discovered that the Odontochelys turtle had a pointed, elongated snout. Most modern turtles have short snouts. In addition, the base of the mouth along with the upper and lower jaw all have teeth. According to researchers, this is a primitive feature, while today’s turtles have beaks on their snouts but no teeth.

In fact, turtles once had a partial shell (which only covered the abdomen). This sheds light on an intermediate step in turtle shell evolution that researchers have never seen before. Before the discovery of Odontochelys, the oldest known turtle was the tortoise Proganochelys, which lived about 210 million years ago . But turtles now have fully developed shells, which provide little evidence about the evolutionary path of turtle shells.

It was once thought that turtle shells evolved from bone plates in the skin, then expanded and joined together to form armor for turtles. The overall structure then incorporates the spine and ribs below. (Modern reptiles, such as crocodiles, have bony plates, as do some other dinosaurs.)

However, recent research samples of Odontochelys species show no traces of bone plates on the skin. Instead, they have large ribs and an abdominal shell that extends from the spine. These features point to another mode of turtle shell evolution, in which the ventral shell develops first. The ribs and spine then expand to form the upper carapace.

Ancient turtles lived underwater

The evolutionary step, according to research findings, matches the process of shell formation in modern turtle embryos in which the spine expands outward, and the ribs expand further to combine to form the shell.

The completed shell on the underside of the newly discovered fossil also shows that the turtles once lived underwater, where their abdomens were vulnerable to attacks by opponents. Olivier Rieppel – geographer at the Field Museum in Chicago – said: “Reptile living on the edge has a belly close to the ground so there is little danger.”

Along with the Nature article, the researchers point out another hypothesis about this partially protected turtle. They think that older turtles even had complete upper and lower shells. As for the newly discovered turtle species, they have a reduced upper shell, and the upper shell is not as hard as bone. And this is a form of adaptation for aquatic life.

But Rieppel disputes the view. He said that if this turtle really did have a complete shell, researchers would recognize changes in its ribs and spine. But in fact they did not detect any changes.

The research was funded by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences and the Canadian Museum of Nature.