By Victoria Gill
Science reporter, BBC News
A Japanese team studied the development of turtle embryos to find out why their ribs grow outward and fuse together to form a tough, external carapace.
Reporting in the journal Science, the researchers compared turtle embryos with those of chicks and mice.
They found that, as turtles developed, part of their body wall folded in on itself forcing the ribs outward.
Read more at: http://news.bbc.co.uk/2/hi/science/nature/8142664.stm