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Turtles' shells made from shoulder blades and ribs, study says

A folding process takes place in the egg, leading to the bony exterior that is an integral part of the reptile's anatomy, scientists say.

July 11, 2009|Shara Yurkiewicz

The turtle's shell is unique, but the evolution of the structure has been a mystery. Now Japanese scientists have figured out that the shell develops from an unusual folding process inside the egg that pushes the turtle's shoulder blades inside its rib cage and directs the ribs to grow around them.

The study, published Friday in the journal Science, was conducted by scientists at the RIKEN Center for Developmental Biology in Kobe, Japan. To reach their conclusions, they examined development of embryos of the Chinese soft-shelled turtle, chicken and mouse, and compared the anatomy of their muscles and bones.

They observed a folding process that happened only in the turtle, which produced a disk-shaped thickening of the skin on the back that mapped out the position of the future shell.

The ribs then grew outward into this disk, expanding in a fan-shaped pattern and trapping the shoulder blade inside.

"This folding of the body wall is the first and major modification in the embryonic development of the turtle, which leads to changes in subsequent development. It is the first step to build a turtle," said Olivier Rieppel, a paleontologist and curator at the Field Museum in Chicago, who wrote a commentary accompanying the paper.

"This is a new explanation about the turtle's body plan," said study coauthor Shigeru Kuratani of the RIKEN Center. "The turtle is just an extreme case of origami-like phenomena in evolution."

The result is that the turtle has a skeleton different from any other vertebrate. In other animals, the shoulder blade is outside the rib cage. The turtle's shoulder blade is inside the rib cage, and the ribs themselves are part of the shell.

This means that a turtle's shell is more than just an outer covering -- it is an integral part of the animal's body.

"In cartoons, you sometimes see a turtle take off its shell like a coat. A turtle cannot do this. It is not simply a shelled reptile. It has modified its basic body plan a great deal," Kuratani said.

The new findings challenge the traditional view that the shell forms from small bony plates within the turtle's skin that spread out and fuse. Other reptiles, such as crocodiles and some lizards, have this type of exoskeleton, formed directly in the skin.

The researchers also found that one stage in the modern turtle's embryonic development resembles a 220-million-year-old fossil discovered in China last year that has an incomplete shell.

Turtles are thought to have evolved 200 million years ago, but no one knows from what. The 2008 find in China is the oldest turtle fossil on record.

"The study is a wonderful example of good old-fashioned comparative anatomy," said Ann Burke, an associate professor of biology at Wesleyan University. "It demonstrates that even though we've come far with molecular techniques, there's no substitution for knowing and understanding what's going on in the embryo."


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