It has long been thought that turtles evolved their shells for protection from predators. But a new study suggests they may have once served another purpose: burrowing. The Daily Mail reports that scientists studying the fossils of prototurtles—the early ancestors of turtles who roamed the earth 220 million years ago—noticed a funny thing about their shells, which are typically formed by the slow broadening of the ribs and fusing of the bones. These early species found in modern-day South Africa did not have fully fused bones, meaning they didn’t get the protective benefit. This led researchers to question what purpose the shells originally had. Because they also found that the early turtles had eyes similar to those of animals that dig burrows for shelter, they propose this week in Current Biology that the shells may have helped them in this task. Scientists believe that the discovery may give insight as to how the turtles survived a mass extinction event 250 million years ago.