What happens when you poke a trilobite? Since these ocean scuttlers went extinct 250 million years ago, no humans have been able to conduct this vital experiment. But the fossil record reveals that when trilobites were scared, they acted much like today’s pillbugs—tucking in their legs and antennae and rolling themselves into tight little balls. Finding fossils of rolled-up trilobites is usually no big deal for paleontologists, but most examples of this distinctive rolling, exoskeleton side out, come from after the Cambrian period, which ended about 485 million years ago. Now, scientists digging in the Rocky Mountains of western Canada have found much older enrolled trilobites that date back to the middle of the Cambrian, they report online today in Biology Letters. In fact, they represent the oldest known example of the defensive behavior in any kind of animal. These early practitioners of rolling-like-a-ball hadn’t fully mastered the technique: While their descendants could snap their trunks around to encapsulate their entire bodies, the best these older trilobites could do was to basically fold themselves in half, shown in the image above. What’s more, they had to keep their muscles engaged the whole time they wanted to hold the position. That means that when they died, their carcasses usually stretched out again—which explains why the Middle Cambrian fossil record isn’t full of folded-up trilobites. The rare specimens found by the researchers were likely quickly buried alive, preserving their positions for eternity.
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