Why did turtles evolve to have shells? This seems like a really redundant question with a really obvious answer – for protection against predators – but as with many evolutionary biology tales, things are more complex than that.

As a new study in the journal Current Biology reveals, the shell first emerged as part of a burrowing adaptation, whereupon the ribs widened to make digging easier. The ribs eventually fused to turn into the shell we know today. The rigid, shield-like nature of the shell also just happened to provide protection from hungry hunters, despite the fact that this was not its original “purpose”.

“Just like the bird feather did not initially evolve for flight, the earliest beginnings of the turtle shell was not for protection but rather for digging underground to escape the harsh South African environment where these early proto-turtles lived,” Tyler Lyson, a paleontologist at the Denver Museum of Nature and Science and lead author of the study, said in a statement.

Thanks to a newly-excavated 260-million-year-old proto-turtle fossil – Eunotosaurus africanus – found by an 8-year-old on his father’s South African farm, researchers now have a very clear picture of how the oldest known ancestral turtles evolved during the Permian period of time, from 299 to 252 million years ago.

Although several other E. africanus fossils have been found before, this particular specimen, just 15 centimeters (5.9 inches) long, is the most well-preserved find to date, and includes fully articulated hands and feet. A painstaking analysis of this fossil confirmed that appearance of the turtle shell is a direct consequence of the bizarre and rather novel evolutionary history of ancestral turtle ribs.

^{Gif in text: A computed tomography scan of the fossil, peeling away the extraneous rock layers and revealing the skeletal features within. Gabriel S. Bever}

The Karoo Basin of South Africa was a harsh, arid place back in the Permian, bathed in heat and full of fearsome predators, including the large, carnivorous Gorgonopsia. Quickly digging into the ground was one way to avoid the hazardous world at the surface, and ancestral turtles like E. africanus began to evolve adaptations to this over a relatively short space of time.

Not only did their legs gradually reposition themselves, but their rib cages significantly broadened to make them flatter overall. Ultimately, this gave them a more mechanically stable platform from which to operate their digging forelimbs, even if it restricted their breathing efficiency and slowed their pace down. Better diggers produced more survivors compared to their surface-dwelling counterparts.

This highly modified rib cage formed the basis of the shell that would, from this point onwards, begin to appear in the fossil record. The younger, 220-million-year-old ancestral turtle Odontochelys semitestacea, for example, revealed that a shell had already appeared out of fused lower ribs just 40 million years later – for now, just on the belly and not on the back.

Additional fossils demonstrate that the upper ribs later followed suit. They also eventually fused with the spine, leading to the formation of the carapace, the first type of shell. Afterward, the ribs grew over the shoulder blades, which formed the hardened, “complete” shell seen on all modern turtles.

This is a wonderful example of something known as an “exaptation”, an evolutionary process where a physiological feature evolves to serve one purpose, but is then used to perform an additional function. In this case, a better digging stance became an armored shield, and thanks to this, the turtle lineage has survived three mass extinction events, one of which wiped out over 90 percent of all life on Earth.

^{Image in text: The new 260-million-year-old E. africanus fossil. Tyler R. Lyson}