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Do turtle shells actually consist of modified ribs? The answer, surprisingly, is yes. This weird-but-true claim went viral in mid-June 2021 when Twitter user and post-doctorate turtle expert Ceri Weber shared the strange graphic below. At the time of this writing, the image had been retweeted nearly 10,000 times. Whereas a crab shell is made of hard bone (called chitin) and snail shells are composed of calcium carbonate, a 2013 study published in the peer-reviewed journal Current Biology described the unique evolutionary process that transformed turtle ribs into shells. Turtles are the only known animal on Earth to form a shell on the outside of their body through a fusion of modified ribs, vertebrae, and shoulder girdle bones — a process believed to have started some 250 million years ago. In 2008, researchers discovered the fossilized remains of a 220 million-year-old turtle species, Odontochelys semitestacea, in China. The specimen had a fully-developed plastron (the belly area of a turtle’s shell) but its partially developed outer shell, known as the carapace, was made up of broadened ribs and vertebrae on its back. Scientists applied this newfound information to a previously discovered South African species known as Eunotosaurus africanus, which was 40 years older and also had distinctively broadened ribs. They found E. africanus was one of the earliest 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, then a scientist at the Smithsonian National Museum of Natural History, in a news release at the time. 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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