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