The mystery of how the turtle got its shell has finally been solved by scientists studying a 260-million-year-old fossil.
All other animals with body armour, including lizards and armadillos, are known to form it from bony scales on the surface of their skin.
Turtles are the only ones that fuse ribs and vertebrae to make a shell made up of around 50 bones on the outside of their bodies, it has been found.
Until now palaeontologists have been scratching their heads as to how their shell is formed, and have struggled with an up to 55-million-year gap in the fossil record.
A team from Yale University have discovered the secret by studying the fossils of a bizarre South African reptile known as Eunotosaurus, the earliest known turtle ancestor, which lived 260 million years ago.
"It helps to bridge the morphological gap between turtles with their funky body plan and the more generalised lizard body plan," Dr Tyler Lyson, lead author of the study, told the Times .
"Eunotosaurus has some things that are turtley, but then it also has other features that are more like a generalised amniote [a group of vertebrates that includes reptiles, mammals and birds]."
Until recently, the oldest known fossil turtles, dating back about 215 million years, had fully developed shells, making it hard to see the sequence of evolutionary events that produced them.
The study, published in the journal Current Biology, found that Eunotosaurus has nine broadened ribs as well as elongated vertebrae, similar to those seen in turtles.
By examining marks left by fibres, the scientists have also deduced that it lacked intercostal muscles which are used to move the rib cage and aid breathing as turtles do.
"The reason, I think, that more animals don’t form a shell via the broadening and eventually suturing together of the ribs is that the ribs of mammals and lizards are used to help ventilate the lungs,” Dr Lyson said.
“If you incorporate your ribs into a protective shell, then you have to find a new way to breathe.”
Turtles have done just that, with the help of a muscular sling, he said.
The discovery of the fossil of a 220 million-year-old Odontochelys semitestacea, in China in 2008, provided the first firm clues about shell formation as is had only a partial shell on its back and broadened ribs. It quashed the theory that the shell formed from the fusion of many bony scales.
Dr Lyson's team found that the Eunotosaurus, the "proto-turtle”, had begun to display broadened ribs and no longer had intercostal muscles, revealing a sequence of evolutionary events that have led to the modern turtle.
The team said these events resemble the steps seen as turtle embryos develop their shells.
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