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How Did the Turtle Get its Shell?

Posted July 22, 2009 12:01 AM by SavvyExacta

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

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#1

Re: How Did the Turtle Get its Shell?

07/22/2009 4:01 AM

So do the Japanese paleontologists know where in the evolutionary chain does this particular species of turtle fit in ?

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#5
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Re: How Did the Turtle Get its Shell?

07/23/2009 12:33 AM

That's a case of special creation.

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#2

Re: How Did the Turtle Get its Shell?

07/22/2009 7:56 AM

Good stuff!

One can also estimate a turtle's age by counting the growth rings on the scutes, which comprise the shell's carapace.

Had a yard full of pet box turtles when I lived in warmer climes (where they also make good eating, sorry, but mine were PETS). Interesting, intelligent critters.

The most intelligent is believed to be the Pennsylvania wood turtle, now an endangered species due to collecting and habitat loss. I think that's one in your top photo, SavvyExacta.

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Re: How Did the Turtle Get its Shell?

07/22/2009 8:03 AM

Hi Sue, thanks for your comment. The turtle in the top photo was crossing my grandparents' yard in upstate N.Y. He/she was about 1/4 mile away from a stream, heading toward the woods and a field. I do have a photo of the underside of that turtle but I'll have to wait until I'm home to find and post it!

The second turtle lives in The Wild Center in Tupper Lake, N.Y. - in the Adirondacks. Neat place!

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#4

Re: How Did the Turtle Get its Shell?

07/22/2009 9:09 AM

Good entry. I couldn't imagine a turtle being a turtle without it's upper shell.

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Re: How Did the Turtle Get its Shell?

07/23/2009 10:59 AM
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