Not to belittle sharks—they have their own week, after all—but the most voracious eaters in the oceans may not even have a spine. Sea urchins are omnivores and, when they dine on kelp, they can completely wipe out a kelp forest, leaving urchin-filled barrens on the ocean floor. But they don't limit themselves to plants. Sea urchins will happily eat any animals that don't get out of the way. Now, researchers have examined the fossils of a marine invertebrate, and find that it actually evolved the ability to get out of the way. Any of these creatures, called crinoids, that share a habitat with sea urchins can now go mobile.

Crinoids like the sea lilly have a simple body plan, consisting of a body with multiple arms and a mouth, attached to a long stalk. They've been around for hundreds of millions of years, having survived two mass extinctions. Early on in their history, they were generally sessile, meaning they attached themselves to a solid surface and remained stationary for the rest of their lives. Over time, however, that changed. The species that lived in shallow waters started evolving the ability to detach from their stalks, or even move their base of operations to new locations on the ocean floor. Now, the only sessile species exist in very deep waters.

What drove this change? The authors of a new paper make a compelling case that sea urchins did. They've actually gone through and matched bite marks on fossil crinoids to the damage on crinoid stalks that have been partly digested by sea urchins. They've also gone through the fossil record and compared the diversity of crinoid species to that of sea urchins. As sea urchin diversity goes up, the number of sessile crinoids drops, and the number of species that can either detach or move to new locations rises.

It probably doesn't take much to outrun a sea urchin, but it still represents a pretty radical change in lifestyle for the crinoids. And, if the sea urchin's pursuit of its prey is a bit slow motion, it's got nothing on the millions of years it took this evolutionary shift to play out.

PNAS, 2012. DOI: 10.1073/pnas.1201573109 (About DOIs).