Scientists have now discovered why turtles evolved to have shells. The earliest turtles developed their unique body shape to help them burrow into the ground, helping them survive the Permian/Triassic extinction event. The details are in a new study published in the journal Current Biology.
The turtle shell is a unique adaptation that offers the reptiles protection from predators. The evolution of the turtle shell has always been a bit of a mystery, however. Developing a shell requires drastic body changes, including broadened ribs. Broadened ribs on their own didn’t appear to provide any benefits yet the earliest turtles had developed them before the evolution of a hardened shell. This confused scientists since the body plan offered no protection and had serious costs. Broadened ribs negatively affect locomotion, decreasing overall speed. They also make it harder for the turtle to breathe by decreasing costal ventilation.
Well-preserved and intact fossil material provided the research team with some clues. The researchers analyzed newly found fossils from the earliest stem turtle species, Eunotosaurus africanus. This species lived about 260 million years ago and had broadened ribs but no hard shell. In their analysis, the team found that the turtles had powerful forelimbs for digging. These ancestors of the modern turtle had to burrow underground to avoid the harsh heat of their home in South Africa. The broad ribs of Eunotosaurus africanus provided a base to make it easier to dig with their front claws. Over time, the turtle shell evolved as a protective adaptation.
Turtles originally developed their body type in order to facilitate digging underground. Burrowing allowed them to escape harsh environmental conditions, leading the authors to speculate that this adaptation helped them survive the Permian/Triassic extinction event. As time went on, turtles evolved to have the hard shells they’re known for today. These findings are helping scientists understand turtle evolution and ecology.
Tyler R. Lyson et al. Fossorial Origin of the Turtle Shell. Current Biology (2016).