Turtles are Not Primitive Reptiles: Genome Analysis Reveals Evolutionary Origin of Shells

Turtles are known for their hard shells, but how did they evolve this unique adaptation? Researchers have wondered the same thing. Now, a new study reveals that turtles are not the primitive reptiles we once thought they were; instead, they're part of a sister group of birds and crocodiles.

With their graceful movements and large shells, turtles are often described as evolutionary monsters. They possess an unusual body plan that is considered to be one of the most intriguing structures in the animal kingdom. Like crocodilians, these creatures were around during the age of the dinosaurs and survived the mass extinction event which killed off many other species.

In order to learn a little bit more about these creatures, researchers used next-generation DNA sequencers in order to decode the genome of the green sea turtle and the Chinese soft-shell turtle. They then examined the expression of genetic information in the developing turtle.

It turns out that, surprisingly, turtles are not primitive reptiles as previously thought. Instead, they are related to the group that is made up of birds and crocodilians and also includes extinct dinosaurs. It's likely that the turtles split from this group about 250 million years ago during one of the largest extinction events on the planet.

"Turtles are interesting because they offer an exceptional case to understand the big evolutionary changes that occurred in vertebrate history," said Naoki Irie from the RIKEN Center for Developmental Biology in a news release.  "The work not only provides insight into how turtles evolved, but also gives hints as to how the vertebrate developmental programs can be changed to produce major evolutionary novelties."

In fact, the study revealed that despite their unique anatomy, turtles follow the basic embryonic pattern during development. This means that instead of directly developing a turtle-specific body shape, they instead establish a vertebrates' basic body plan. Then, they enter a turtle-specific phase where they develop their shell.

The research could allow scientists to better understand the evolutionary history of these animals, and could also allow them to examine exactly how other species are related to turtles.

The findings were published in the journal Nature Genetics.