How a hard shell ultimately helped the tortoise dig in and survive
How did the tortoise get its shell?
For biologists, it is a puzzle. Having a rigid shell makes you slow and renders it harder to breathe. Once fully developed, it gives you protection, but for those reptiles that made the first evolutionary shuffle down this path there seemed only to be downsides.
Now scientists believe they have found an answer.
The first reptile to broaden its ribcage and stiffen its back, the adaptations that would eventually lead to the rigid shell found in today’s turtles and tortoises, did so not for protection but for burrowing.
That lizard, the Eunotosaurus africanus, looks to the untrained eye much like the lizards of today.
Living 260 million years ago, around the time its cousins would turn into dinosaurs, it chose a different evolutionary path.
Many palaeontologists think that with its broadened ribs, this “stem turtle” represents the common ancestor of all shelled animals today.
But this theory has presented problems.
“The current protective function conferred by the shell in extant turtles fails to adequately explain the impetus for initially broadening the ribs,” US scientists say, writing in the journal Current Biology.
“Broadened ribs and overall increased thoracic rigidity in early stem turtles have consequences for both respiration and locomotion.”
This means there need to be considerable upsides to bothering to develop them.
“For this specialised morphology to have evolved via natural selection, an adaptive advantage that outweighs these costs was required.”
After examining fossils of the creature, they propose that the unusual alignment of ribs developed to help it to dig into the sandy soil of its natural habitat.
“The original expansion of the ribs was an adaptation for stiffening the skeleton to provide a stable base from which to operate a powerful forelimb digging apparatus,” they write.
This, ultimately, might be the reason its descendants survived at all.
When it was living, the world was gearing up for the great Permian extinction, associated with rapid climate change and an abrupt drying in environment.
One way to survive was to find protection underground. If the scientists’ theory is correct, the stem turtle sat out the end of this period by burrowing and emerged blinking into a wetter, more hospitable environment. Then it began the ponderous path to adapting its digging apparatus into armour.