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THE mystery of the origin of the turtle’s shell is a step closer to being solved.

Unlike most vertebrates, turtles have ribs that are short and wide, and do not wrap around the body. Instead they fuse with vertebrae and bony plates in the skin of its back to form a shell.

The big question is whether the flattened ribs or the bony plates evolved first. Last year, a fossil was uncovered with a complete shell on its belly but an incomplete shell on its back. Its ribs were short and wide as in modern turtles, but the bony plates were absent – hinting that the ribs rather than the skin drove the evolution of the shell.

Now Hiroshi Nagashima and colleagues at the RIKEN Center for Developmental Biology in Kobe, Japan, have backed up this idea. Looking at the way muscle and bone develop in turtle, mouse and chicken embryos, they found that there is initially a common pattern – also likely shared with their last common ancestor. But early in the turtle’s development, a portion of its body wall folds in on itself, forcing the ribs to fuse and thicken rather than wrapping round the body (Science, ).