Lizard looker: Artist's reconstruction of how Odontochelys semitestacea, an ancestral turtle from the Triassic of China, might have looked in life.
Credit: Marlene Donnelly
PARIS: A stunningly intact 220-million-year-old turtle fossil with a shell on its front, but not its back, appears to have settled a long-simmering debate over reptile evolution: how did the turtle get its shell?
In a study published today in the British journal Nature, scientists report the discovery of a missing-link species – Odontochelys semitestacea ('toothed, half-shell turtle') – whose outer shell emerged directly from the ribs and backbone and not from the skin, as some have argued.
Back in time
The find also suggests that turtles originated in water rather than on land, and pushes back the group's first known appearance on Earth by some 10 million years.
Since the era of dinosaurs, which roamed the planet until 65 million years ago, turtles have looked pretty much the way they do today. In fact, another recent discovery of an ancient turtle suggested they have changed little for as many as 164 million years (see, Scottish turtle is missing link, Cosmos Online).
Turtles sport an armour-like upper shell, known as a carapace, connected to a softer lower part, called a plastron. But in the absence of hard evidence, scientists have argued since the early 1800s over exactly how this reptilian mobile home came into being.
One school of thought said the shell evolved from skin. According to this theory, small bony plates called osteoderms – like those found on crocodiles – broadened in size to form a kind of dermal plating that fused over time with the ribs.
Developmental clue
The competing theory said that the plastron formed first, followed by an outgrowth and widening of the ribs and backbone to form the hard-shell carapace into which turtles withdraw to escape predators.
A similar process unfolds in the transformation of modern-day turtle embryos into hatchlings.
"With Odontochelys, we now have clear fossil evidence of this process emerging in an adult," said Xiao-chun Wu, a palaeontologist at the Canadian Museum of Nature in Ottawa and co-author of the study.
