How Did the Turtle Get its Shell?

Fables say that "the slow and steady turtle wins the race" – and historically, we've thought the turtle needs its shell for protection from faster-moving predators. However, recent fossil findings from China, published in Nature, indicate that one of the turtle's ancestors may have only had the bottom half of the shell.

That's right, a shell that covers the belly and leaves the back exposed – but why?

Fossil Features

The fossils of Odontochelys semitestacea (which means "half-shelled turtle with teeth") belong to a transitional animal dating back to about 220 million years ago. Fossil evidence shows that this animal only had a plastron, which is the lower part of the shell that covers the animal's belly. No hard upper portion was present.

The fossils also showed that these animals lived in a marine environment – so perhaps the point of this shell was to protect the animals from attacks from below.

Embryo Evolvement

Even more recently, a Japanese team published findings in Science which compared turtle embryos with those of chicks and mice. The purpose of the study was to determine why turtles' ribs grow outward, fuse together, and form the turtle's tough shell.

This major difference between species takes place during embryonic development when the upper part of the body wall folds in on itself. The fold prevents the ribs from growing inward (as in most mammals), and keeps the shoulder blades encased in the rib cage.

Some scientists argue that such a skeletal construction would restrict movement of the shoulder too much. Others say it would overrule the prevailing view that a turtle's shell had been made up of osteoderms – the skin that makes up reptiles like crocodiles.

There are various programs worldwide for studying turtles, which can provide evolutionary insight due to their age.

Resources:

http://www.sciam.com/article.cfm?id=new-fossil-origin-of-the-turtle-discovered

http://news.bbc.co.uk/2/hi/science/nature/8142664.stm

http://www.abc.net.au/science/articles/2009/07/10/2622282.htm