The fossilised remains of an ancient reptile that lived 220 million years ago may have solved the puzzle of how the turtle got its shell and, in the process, cleared up one of the most enduring mysteries of animal evolution.

It is the oldest known turtle-like fossil and its shell appears to be only half-formed, covering its belly but leaving its back unprotected. Scientists believe it shows the evolutionary transition from the shell-less state of the earliest turtle ancestor to the fully formed shell of all living turtles.

Turtle shells are among the most intriguing structures in the animal kingdom, and zoologists have long argued about how this protective device and unusual body plan could have evolved from previously existing anatomical structures.

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The latest discovery, made by palaeontologists excavating in China's fossil-rich province of Guizhou, shows that the shell of the turtle evolved from bony growths that expanded from the spine and ribs, rather than the merging of bony plates found in the skin of some reptiles.

"Since the 1800s, there have been many hypotheses about the origin of the turtle shell," said Xiao-chun Wu, a palaeontologist with the Canadian Museum of Nature in Ottawa, Ontario, who was part of the research team. "Now we have these fossils of the earliest known turtle. They support the theory that the shell would have formed from below as extensions of the backbone and ribs, rather than as bony plates from the skin as others have theorised."

Turtles have a unique body plan that has barely changed since the demise of the dinosaurs 65 million years ago, and its distinctive shell characteristically defines its place in the animal kingdom.

The discovery of the earliest turtle fossils was made in 2005 but the creature's full scientific description and name – Odontochelys semitestacea – appear for the first time in the latest issue of the journal Nature, by a team led by Chun Li of the Chinese Academy of Sciences in Beijing.

"This is the first turtle with an incomplete shell," said Olivier Rieppel of the Field Museum in Chicago, who was also on the team. "The shell is an evolutionary innovation. It's difficult to explain how it evolved without an intermediate example."

Turtle shells are divided into two parts. The lower plastron covers the underside and protects the swimming turtle from predators attacking from below, and the upper carapace protects it from above. The 220-million-year-old fossil found in China has a fully-formed plastron indicating that the creature was free-swimming and needed to be protected from below, but lacks the back carapace.

"Reptiles living on the land have their bellies close to the ground with little exposure to danger," said Dr Rieppel, explaining why the presence of the plastron shows that the ancient turtle must have been a

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water dweller. Instead of a fully formed upper shell, the fossil has flattened ribs and a broadened backbone which would have given it partial protection. Its skull also contains teeth, which in later descendants are replaced by the horny beak possessed by modern-day turtles.

Some reptiles, such as crocodiles, have bony plates in their skin called osteoderms, also found in extinct relatives such as the dinosaurs. This characteristic was seen as a possible explanation for the turtle shell if they fused into a single structure. But studies showed that during embryonic development the turtle backbone expands outward and the ribs broaden to meet and form the shell. This suggested an alternative evolutionary pathway which is now supported by the fossil found in China.

"This animal tells people to forget about turtle ancestors covered with osteoderms," Dr Rieppel said. A much more likely explanation is that the shell evolved from outgrowths of the rib and spine which eventually fused to form a single shell, as they do during embryonic development.

Animal magic Darwin's theory

Scientists call them major transitions in evolution, but Rudyard Kipling made them into Just So stories to explain how, for instance, the elephant got its trunk. Some changes are explained with the help of fossil intermediates that describe how one pre-existing anatomical structure can evolve into another, such as a front leg into a feathered wing. Others have left no fossil traces.

How the frog got its legs

Amphibians such as frogs were the first terrestrial creatures with backbones. Their legs evolved from the fins of fish and this vertebrate transition from a fully aquatic life to a terrestrial existence is seen in the fossil record with the "fishibians", extinct creatures that are intermediate between fish and amphibians. This transition from water to land occurred about 400 million years ago.

How the wasp got his stripes

Certain visible features are signs to warn off potential predators. One of the best examples is the yellow and black stripes of wasps and stinging bees. The sting protects it from being eaten by birds, but it is better to advertise clearly to stop something from even trying you for breakfast. Many animals use bright stripes and zigzags as warnings.

How the elephant got his trunk

According to Kipling, the Elephant Child got its trunk on the banks of the Limpopo river by getting too near a crocodile, which caught the youngster by the nose, stretching it. The reality is that the trunk is not just a long nose, but an extra limb. It could have evolved in response to the increasing size of the ancestral elephants, making grazing by mouth difficult. Another theory is that ancestral elephants lived an aquatic existence, and used their trunk as a snorkel.

How birds got feathers

Another major transition in evolution is the appearance of birds with feathered wings capable of powered flight. Archaeopteryx, a fossil discovered in the mid-19th century, clearly shows the transition from reptile to feathered bird. It lived about 150 million years ago and its feathered wings and breastbone are avian. Experts believe feathers evolved to keep the dinosaurs warm, and later became useful for gliding and powered flight.

How the peacock got his tail

A special kind of natural selection has been at work on the peacock's magnificent tail. It is called sexual selection, a result of females choosing males with the biggest tails as mates.

How the giraffe got his long neck

In the 19th century, Jean-Baptiste Lamarck, a French scholar, suggested that the giraffe stretched to reach the highest leaves, and this caused changes passed to subsequent generations. Charles Darwin's theory of evolution exposed the fallacy of this, showing that a trait such as this would come about as a result of natural selection.