Life and limb. Crinoids fossilized while regrowing damaged arms (arrow) bear witness to ancient dangers.

Crinoid Call to Arms

New fossils have given paleontologists a view of an ancient arms race between predator and prey. By counting the chewed-off arms on fossil crinoids, researchers have shown that these filter-feeding cousins of starfish suffered ever fiercer attacks during a period when fish and other major predators were diversifying.

Just as swords inspired the invention of chain mail, the history of life hints at many arms races between predators and prey. But with the remnants of the carnage long turned to stone, it can be difficult to prove that the evolution of bigger teeth, for instance, actually did encourage the evolution of defenses such as thicker armor. Most studies of predation intensity have focused on marine snails, which drill into bivalves and brachiopods. Such holes are common in fossil shells and sometimes show signs of repair. That indicates that the prey survived an attack and could have passed on genes for a thicker shell or other defense, thus ratcheting up the arms race.

In the new study, paleontologists Forest Gahn of the Smithsonian Institution's National Museum of Natural History in Washington, D.C., and Tomasz Baumiller of the University of Michigan, Ann Arbor, looked at another set of predators and prey during what's called the Middle Paleozoic Marine Revolution. At that time--about 380 million years ago--sharks and fishes were diversifying wildly. Invertebrates in shallow waters were changing, too; crinoids, for example, were evolving thicker armor and spines.

The crinoids' tricks also included the ability to regenerate lost body parts. So when a fish chomped off several of the tentacle-like appendages, crinoids grew new ones. Looking at slabs with beautifully preserved crinoids, primarily from eastern North America, Gahn and Baumiller could spot new arms growing from stumps. By counting the stumps, they calculated the rate of predation. For approximately 100 million years before the Middle Paleozoic Marine Revolution, the researchers found that fewer than 5% of crinoids sported regenerating arms. By the time the predator revolution was in full swing, however, more than 10% were growing replacement arms, the pair reports in the 3 September issue of Science.

The evidence for increasing predation is "straightforward and convincing," says paleontologist Geerat Vermeij of the University of California, Davis, who showed that a burst of predator evolution called the Mesozoic Marine Evolution spurred prey to respond.

Related Sites
Tomasz Baumiller and Forest Gahn's Science paper
More about fossil crinoids
Information about living crinoids