The Mysterious Turtle Shell

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The bony shells have amused the scientist from a long time about the elegant structures formations. The shells are usually meant for protection and shelter. The turtles have remained unchanged since the age of the dinosaur, lacking any evidence of evolution.

Working with colleagues in China and Canada, Olivier Rieppel, chairman of The Field Museum's department of geology, has analyzed the Chinese turtle fossil, finding evidence to support the notion that turtle shells are bony extensions of their backbones and ribs that expanded and grew together to form a hard protective covering.

The fossilized turtle ancestor, dubbed Odontochelys semitestacea (half-shelled turtle with teeth), likely lived in the water rather than on land. Prior to discovery of Odontochelys, the oldest known turtle specimen was Proganochelys, which was found in Germany. Because Proganochelys has a fully-formed shell, it provides little information about how shells were formed. Odontochelys is older than Proganochelys and is helpful because it has only a partial shell, according to Rieppel.

"This is the first turtle with an incomplete shell," Rieppel said. "The shell is an evolutionary innovation. It's difficult to explain how it evolved without an intermediate example," he added.

A few researchers believe that turtle shells started as bony skin plates, called osteoderms, which eventually fused to form a hard shell. Rieppel said that embryo studies show that the turtle backbones expand outward and the ribs broaden to meet and form a shell. While paleontologists take such studies into account, they aren't sufficient to prove how anatomy evolved over time, and evidence can be read in different ways.

The limbs of Proganochelys, for example, show signs of bony plates in the skin, but according to Rieppel, Odontochelys has no osteoderms and it has a partial shell extending from its backbone. It also shows a widening of ribs. Although Odontochelys has only a partial shell protecting its back, it does have a fully formed plastron complete protection of its underside, like the modern day turtle.

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