Slow Turtle Faster Than Expected With Shell Evolution

Turtles are notorious for doing things slowly, but they started developing shells a lot sooner along the evolutionary timeline than anyone suspected, a new fossil discovery has revealed.

Unique among Earth's creatures, turtles are the only animals to form a shell outside of their bodies through a fusion of modified ribs, vertebrae and shoulder girdle bones. The characteristics of the skeletal modification, as well as how it originated, have intrigued the science world for centuries.

But, a research team from the Smithsonian National Museum of Natural History recently found evidence that the evolutionary emergence of the turtle shell started 40 million years earlier than previously believed. 

The team's work is published in the May 30 issue of Current Biology.

For many years, the oldest known fossil turtle dated back about 210 million years, but it already had a fully-formed shell, which didn't provide any clues about early shell creation.

Then, in 2008,  an important clue arrived when the 220 million-year-old remains of an early turtle species, Odontochelys semitestacea, were discovered in China with a fully-developed plastron (the underside portion of a turtle's shell), but only a partial carapace, or shell top, made of broadened ribs and vertebrae on its back.

With data from the new discovery, scientists then turned to newly discovered specimens of Eunotosaurus africanus, a South African species 40 million years older than O. semitestacea that also had distinctively broadened ribs.

The research team's study of South America's Eunotosaurus determined it shared many features only found in turtles, such as no muscles that run between the ribs, paired belly ribs and a specialized process of rib development. Proving those similarities ultimately demonstrated 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," Smithonsian postdoctoral fellow Tyler Lyson was quoted saying by Science Codex.

"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," Lyson said. "As such, Eunotosaurus helps bridge the morphological gap between turtles and other reptiles."