Turtles derive their shells from its ancestors' ribcage and not from internal and external bone structures as believed previously, according to a new study from RIKEN Center for Developmental Biology. The study was based on analysis of fossils and genetic data.

The origins of turtle shell have been a mystery for a long time. A recent study had shown that rudimentary forms of the shell on turtles began appearing about 40 million years earlier than previously believed.

The endo and the exoskeletons are two bone systems from which skeletons of vertebrates have evolved. Humans' spines and bones are an example of endoskeleton whereas the skull has origins in the exoskeleton. Scales on fish and nodules on alligators are other examples of exoskeleton.

Previous research had failed to find how the shell on the turtle's back, or carapace, originated as it had elements of both endoskeleton and exoskeleton. It was believed that the outer shell was derived from the exoskeleton while the internal part of the carapace came from endoskeleton and was connected to internal bones. But, there was no direct evidence linking the structures outside the ribcage with skeletons originating from the exoskeleton.

For the study, Dr. Tatsuya Hirasawa and his team looked at how embryos of Chinese soft-shell turtles, chicken and alligators develop. They compared the alligators' nodules, chicks' ribcage and turtles' shells in its earliest stages.

The study found that a major part of the carapace is actually developed from the endoskeleton as the turtle shell showed large parts of hypertrophied ribs and vertebrae.

Researchers later confirmed their findings by analyzing fossil records of turtle, Odontochelys and reptiles called Sinosaurosphargis had shells of endoskeleton origins.

The study also supports the recent categorization of turtles along with birds, crocodiles and marine reptiles like Sinosaurophargis based on genetic analysis.

"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, lead author of the study, according to a press release. 

The study, "The endoskeletal origin of the turtle carapace," is published in the journal Nature Communications, 2013.