A team of paleontologists has discovered a new species of turtle-like reptile that lived in what is now Germany during the Middle Triassic period, approximately 240 million years ago. According to the team, this extinct creature, named Pappochelys, is a missing link in the evolutionary history of turtles.

“The mystery of how the turtle got its shell has been a long-standing question in evolutionary biology. In the case of Pappochelys, we see that its belly was protected by an array of rod-like bones, some of which are already fused to each other,” said team member Dr Hans-Dieter Sues of the Smithsonian’s National Museum of Natural History.

“Such a stage in the evolution of the turtle shell had long been predicted by embryological research on present-day turtles but had never been observed in fossils – until now.”

Dr Sues and his colleague, Dr Rainer Schoch of the State Museum of Natural History in Stuttgart, studied more than a dozen specimens of Pappochelys that were collected beginning in 2006.

Pappochelys was about 8 inches (20 cm) in length and lived in a tropical environment along the shores of a lake in what is now southern Germany. The reptile used its small, peg-like teeth to feed on insects and worms and had a long tail, possibly to help with swimming.

The scientists said that the discovery of this species confirms that the belly portion of the turtle shell, called the plastron, formed through the fusion of rib-like structures and parts of the shoulder girdle.

The physical traits of Pappochelys make it a clear intermediate between two of the earliest known turtles, Eunotosaurus and Odontochelys.

Eunotosaurus, considered the oldest precursor of turtles, dates back to 260 million years ago and lived in present-day South Africa. It had many characteristics only found in turtles, including broad ribs and a lack of intercostal muscles, which attach between the rib bones. Eunotosaurus also had a long slender tail.

By comparison, Odontochelys lived in present-day China 220 million years ago and had a fully developed plastron, a long tail and jaws with small teeth.

Pappochelys fits neatly between these two turtle precursors at 240 million years, sharing some characteristics with Eunotosaurus but having only a partially fused plastron.

Features in the skull of Pappochelys also provide critical evidence that turtles are most closely related to other modern reptiles, such as lizards and snakes. Previously, paleontologists believed that turtles may have descended from the earliest known reptiles.

Dr Sues and Dr Schoch have documented the discovery of Pappochelys in a paper published in the journal Nature.

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Rainer R. Schoch & Hans-Dieter Sues. A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature, published online June 24, 2015; doi: 10.1038/nature14472