How the turtle got its shell

Researchers have long argued over how the turtle got its shell and a new fossil finally revealed the missing link. Ribs in most other animals protect internal organs and help ventilate the lungs to assist breathing.

Because the ribs of turtles have been modified to form the shell, they have also had to modify the way they breathe with specialized muscles.

The oldest known fossil turtle dates back about 210 million years, but it had an already fully formed shell, giving no clues to early shell evolution.

Now researchers have shown that Eunotosaurus africanus, a South African species that existed 260 million years ago, has large broadened ribs that researchers believe eventually formed a shell.

The researchers say the first clue came in 2008 when the 220 million-year-old fossil remains of an early turtle species, Odontochelys semitestacea, were discovered in China.

It had a fully developed plastron (the belly portion of a turtle's shell), but only a partial carapace made up of distinctively broadened ribs and vertebrae on its back.

Their detailed study of Eunotosaurus indicated it uniquely shared many features only found in turtles, such as no intercostal muscles that run in between the ribs, paired belly ribs and a specialized mode of rib development, which indicates that Eunotosaurus represents one of the first species to form the evolutionary branch of turtles.

“Eunotosaurus neatly fills an approximately 30-55-million year gap in the turtle fossil record,” said Tyler Lyson, a Peter Buck Postdoctoral Fellow at the Smithsonian's National Museum of Natural History.

“Eunotosaurus helps bridge the morphological gap between turtles and other reptiles.”