If turtles are known for anything, it's for their shells and for being slow. However, a fossilized turtle discovered in China shows a species of turtle without a shell. But how could that be?
The team of researchers estimate the nearly complete fossil skeleton to be 228 million years old and believe its evidence of the early evolutionary history of turtles.
"This impressively large fossil is a very exciting discovery that gives us another piece in the puzzle of turtle evolution," said Dr. Nick Fraser, keeper of Natural Sciences at National Museums Scotland, in a statement. "It shows that early turtle evolution was not a straightforward, step-by-step accumulation of unique characteristics but was a much more complex series of events that we are only just beginning to unravel."
When the fossil was first discovered, only a faint outline of the skeleton was visible.
"Even then it was clear that this was a bit of a monster and unlike anything else I had seen in these very rich deposits," said Fraser. "A turtle was just one of many things that went through my mind, but I was truly stunned when I saw the whole fossil fully prepared."
The research team named the fossil Eorhynchochelys sinensis, which means "dawn beaked turtle from China." The believe this species lived in coastal waters and foraged on both land and water using its limbs to dig through the muddy waters much like pond turtles do today.
So why do modern turtles have shells?
An international group of paleontologists discovered a common evolutionary link in 2016 between turtles having shells and moving slowly that offers an explanation for the origin of the turtle shell that you might not expect, reported Phys.org.
Turtles today use their shells for protection, but that may not have been the shell's original purpose. By studying the characteristics of early proto-turtle fossils, the researchers believe that shell-like characteristics first evolved to help turtle ancestors burrow underground.
"Why the turtle shell evolved is a very Dr. Seuss-like question and the answer seems pretty obvious — it was for protection," explained Dr. Tyler Lyson, the lead author of the study. "But 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."
What were these early proto-turtles like? Scientists have identified Eunotosaurus (just like the fossilized one discovered in China), an extinct group of reptiles that lived during the late Middle Permian, as close relatives of modern turtles. The key trait that links these ancient reptilians to turtles is their broadened ribs, which are unusual among all vertebrates, not just among reptiles.
They're unusual because broadened ribs have a number of structural disadvantages, such as labored breathing and slower locomotion. Ribs support the body when a creature walks on all fours, so by splaying them out, it makes quadrupedal motion awkward.
"The integral role of ribs in both locomotion and breathing is likely why we don't see much variation in the shape of ribs," said Lyson. "Ribs are generally pretty boring bones. The ribs of whales, snakes, dinosaurs, humans, and pretty much all other animals look the same. Turtles are the one exception, where they are highly modified to form the majority of the shell."
Early proto-turtles hadn't fully formed a shell yet, however. So why should they have evolved broadened ribs — a prerequisite for forming a shell — when there were so many disadvantages associated with the trait? It turns out that there's one niche that pre-shell broadened ribs may have been useful for: burrowing. The rib shape provides a stable base that may have allowed Eunotosaurus, with its large hands and spatula-shaped claws, to burrow into the ground.
Since Eunotosaurus was likely a slow animal, burrowing also would have offered a way for the creature to hide from predators. Shells may have formed over time to enhance this protection.
It's a fascinating evolutionary story that proves how natural selection often stumbles, by accident, upon useful traits through some other adaptation. If it wasn't for the burrowing behavior of Eunotosaurus, turtle shells may never have evolved.
Editor's note: This article has been updated since it was originally published in July 2016.