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.

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.

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

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