A well-preserved 260-million-year-old fossil of an extinct reptile known as Eunotosaurus fills a 30- to 55-million-year gap in the turtle fossil record, according to paleontologists reporting in Current Biology.

Unique among Earth’s creatures, turtles are the only animals to form a shell on the outside of their bodies through a fusion of modified ribs, vertebrae and shoulder girdle bones. The turtle shell is a unique modification, and how and when it originated has fascinated and confounded biologists for more than two centuries.

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

Then a 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.

With this knowledge the scientists turned to newly discovered specimens of Eunotosaurus africanus, a South African species 40 million years older than Odontochelys semitestacea that also had distinctively broadened ribs.

The 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 lead author Dr Tyler Lyson from the Smithsonian’s National Museum of Natural History, Yale Peabody Museum of Natural History and Yale University.

“There are several anatomical and developmental features that indicate Eunotosaurus is an early representative of the turtle lineage; however, its morphology is intermediate between the specialized shell found in modern turtles and primitive features found in other vertebrates. As such, Eunotosaurus helps bridge the morphological gap between turtles and other reptiles.”

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Bibliographic information: Tyler R. Lyson et al. Evolutionary Origin of the Turtle Shell. Current Biology, published online May 30, 2013; doi: 10.1016/j.cub.2013.05.003