Exposé: Do you know how the turtle got its shell?

Long, long ago, in a time so far in the past it preceded the dinosaurs and the continents, lived a tiny creature named "grandfather turtle". It had many of the qualities of the turtles we know and love today: a boxy body, plodding legs, a long neck topped by a small, round head.

It was only missing one thing: a shell.

Thanks to the newly discovered fossil of that tiny creature, scientists say they have solved the story of how the turtle got its shell. But this is no baseless fable. It's science.

The not-so "Just So" story, published in the journal Nature recently, tracks the evolution of the turtle body plan through millions of years of history. By examining fossils that spanned millennia and continents, researchers were able to figure out how the modern turtle's unique shell evolved from what was just a brief expanse of belly bones about 240 million years ago.

The origin of the turtle shell has long bewildered scientists. Though they had fossils of turtle predecessors from the beginning and the end of the Triassic period, there was little evidence of what happened to ancient turtles during the intervening years. The bones of the 260 million-year-old Eunotosaurus, a reptilian creature found in South Africa, had wide, flat ribs and a sprawling, turtle-like figure, but it was far from the armour-encased animal we know today.

The next time a turtle ancestor popped up in the fossil record, the Odontochelys about 220 million years before present, it had a fully developed belly plate called a "plastron" that would eventually expand to enclose the turtle's whole body, protecting it from attacks from above and below.

But there was nothing in the yawning 40 million-year void between the two ancient species to explain where that plastron came from.

"Hopefully we'll find more," Robert Reisz, a palaeontologist at the University of Toronto, told National Geographic after the Odontochelys was first found in 2008. "We're closing the gap, but there is still a big morphological gap between this turtle and its non-turtle ancestors." Enter Pappochelys, the hero of our story, ready and willing to fill that gap.

Pappochelys, whose name means "grandfather turtle," lived about 240 million years ago in a warm sub-tropical lake, Hans-Dieter Sues, a co-author of the Nature study and curator at the Smithsonian Museum of Natural History in DC.

Credit: Nature Journal 2015