Mystery Of Turtle Shell Origin Solved

A team of Japanese researchers has solved the long-standing mystery of how the turtle shell originated.

Asian Scientist (Jul. 11, 2013) – A team of Japanese researchers has solved the long-standing mystery of how the turtle shell originated.

By observing the development of different animal species and confirming their results with fossil analysis and genomic data, the researchers show that the shell on the turtle’s back is derived solely from its ancestors’ ribcage. This contradicts what many had suspected: that the turtle shell is derived from a combination of internal and external bone structures.

The skeleton of vertebrates has historically evolved from two different structures, called the endo- and exoskeleton. In the human skeleton, the backbone and bones of the limbs evolved from the endoskeleton, whereas most of the skull elements derive from the exoskeleton. Fish scales and the alligator’s bony skin nodules are other examples of exoskeletons.

The origin of the shell on the turtle’s back, or carapace, was unclear until now because it comprises parts of obvious endoskeletal origin and others that look more like the exoskeleton of alligators and fish. The internal part of the turtle carapace was shown to originate from ribs and vertebrae and is connected to the internal skeleton of the animal. However, the outer part was thought to have derived from exoskeletal bones, though there was no direct evidence for this theory.

In their study, published in Nature Communications, the team carefully observed developing embryos of Chinese soft-shell turtles, chickens and alligators. In their analysis, they compared the development of the turtle carapace, the chick’s ribs and the alligator’s bony skin nodules. They found that the major part of the turtle’s carapace is made from hypertrophied ribs and vertebrae and therefore derives solely from endoskeletal tissue.

This finding was confirmed by fossils of the ancient turtle Odontochelys and the ancient reptile Sinosaurosphargis that both exhibit shells of endoskeletal origin. Odontochelys has a rigid shell instead of a flexible ribcage while Sinosaurosphargis possesses an endoskeletal shell similar to the turtle’s, though it is found under, and separate from, a layer of exoskeletal bones.

Taken together, these results show that the turtle carapace evolved independently from the exoskeleton. This is also consistent with the recent phylogenetic analyses based on genomic data that have placed turtles in the same group as birds, crocodiles and marine reptiles like Sinosaurophargis, contradicting other studies based solely on fossil record.

“Recently, genomic analyses had given us evidence that turtles evolved from reptiles closely related to alligators and dinosaurs, not from primitive reptiles as once thought. Our findings match the evolutionary history revealed by the genomic analyses, and we are about to unravel the mystery of when and how the turtle shell evolved,” said Dr. Tatsuya Hirasawa, who led the research.

“Our aim is to one day understand it as well as we understand the evolution of birds from dinosaurs.”

The article can be found at: Hirasawa et al. (2013) The Endoskeletal Origin Of The Turtle Carapace.

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Source: RIKEN; Photo: Roberto Verzo/Flickr.
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