A 240-million-year-old fossil may form a "missing link" in the history of turtle evolution. It's called Pappochelys rosinae, or "grandfather turtle," and it fills a gap in the fossil record during the time turtles' ancestors were first evolving that feature that makes them so easily identified today: their shell.

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Pappochelys was a small reptile that looked more like an iguana than a turtle — but the skeleton, uncovered in a quarry in southern Germany, shows unmistakable signs of turtledom. Its broad ribs have a T-shaped cross-section, but have yet to fuse into a true shell, and there are also clear beginnings of a bony plate protecting its underside. Both its age and features fit neatly between an earlier (260 million years old), less turtle-like Eunotosaurus and more recent (220 million years old) Odontochelys.

The thick, spreading ribs (yellow) form the rudiments of a shell, while the red "gastralia" would in later species form the lower part of that shell. Rainer Schoch

"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," explained one of the researchers, Rainer Schoch, in a Smithsonian news release that also described Pappochelys as a "missing link." The paper describing Pappochelys, by Schoch and Hans-Dieter Sues, appeared Wednesday in the journal Nature.