Video: How the turtle got its shell

Embryo of a Chinese soft-shelled turtle (Pelodiscus sinensis) (Image: Shigeru Kuratani and HiroshiNagashima) The turtle rib cage and shoulder blade as compared to a human (Image: Shigeru Kuratani and Hiroshi Nagashima) Detail of the turtle rib cage and shoulder blade as compared to a other vertebrates (Image: Shigeru Kuratani and Hiroshi Nagashima)


The way the body wall of the growing embryo folds inwards helps to explain how the reptiles achieve their unique body shape.

The origin of the turtle’s body plan has been a mystery. Unlike most vertebrates, its ribs are short and wide, like paddles. They do not wrap around the body like humans, but rather fuse with vertebrae and superficial bony plates in the skin of its back called osteoderms to form a carapace. Previously, it was unclear whether the flattened ribs or osteoderms formed first in the turtle’s evolutionary history.

Last November, researchers in China uncovered a 220-million-year-old fossil that had a complete plastron – the portion of the shell on the turtle’s underside – but an incomplete carapace. Its ribs were short, wide, and flat like modern turtles, but the osteoderms were absent. Some took this as evidence that the ribs drove the evolution of the shell, rather than the skin.

Now Hiroshi Nagashima and colleagues at the RIKEN Center for Developmental Biology in Kobe, Japan, have fresh evidence that supports the “ribs first” interpretation, by helping to explain a mystery surrounding the turtle’s oddly positioned shoulder blade.

Outside in

The turtle shoulder blade is unique among tetrapods – vertebrates with four limbs – because it sits inside the reptile’s ribcage, which runs beneath its shell. To understand how this came about, Nagashima’s team compared Chinese soft-shelled turtle embryos with mouse and chicken embryos.

They found that early on in development, the body wall folds down and in towards the centre of the turtle’s body. The fold defines the edge of the future carapace and draws the shoulder blade down beneath the ribcage (see image, top right).

“We have successfully explained why the scapula of the turtle is found inside the ribcage,” says Shigeru Kuratani, director of the lab where Nagashima made the discovery. “It’s a simple folding, but it doesn’t happen in other animals,” he explains.

Turning turtle

The upshot of this folding is that it keeps the ribs from growing right around the animal’s belly. Hence rib growth is restricted to the upper part of the shell, where the bone fuses with other body parts to form the hard part of the shell.

The embryonic stage at which the folding occurs suggests that it happened very early in the evolutionary history of turtle ancestors. “It’s this fundamental morphological event that characterises the turtle body plan,” says Olivier Rieppel, a curator of Evolutionary Biology at the Field Museum in Chicago, who was not involved with the study.

Journal reference: Science (DOI: 10.1126/science.1173826)