Einstein Science Reporting for Kids
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American Association for the Advancement of Science

How the turtle got its shell

A comparison of the musculoskeletal system between humans and turtles.
(Click for a high-res image.)

In their earliest stages as embryos, turtles, chickens, mice, and even humans all look pretty much the same big head, tiny arms, long spine that looks like a tail. By the time they hatch, however, turtles have taken a major detour and developed shells on their backs.

Chicken, mice and humans have our differences, of course, but the turtle's shell really sets its owner apart from other vertebrates.

A Japanese research team now has an idea of how the turtle gets its shell.

In a study appearing in the 10 July issue of the journal Science, Hiroshi Nagashima of the RIKEN Center for Developmental Biology in Kobe, Japan and colleagues compared chicken, mouse and Chinese soft-shelled turtle embryos.

The researchers figured out that as the turtle embryo develops, a portion of its body wall folds in on itself. This allows some of the growing bones and muscles to connect in ways that they don't in most other vertebrates.

The shell on the turtle's back, the carapace, is actually formed from the growing ribs, which fuse together. As the carapace forms, the animal's scapulae (which are the shoulder blades, in humans) develop inside the shell. This is really unusual. In most other vertebrates, including us, the scapulae lie outside of the ribs.