The case of the mysterious limb

Several years ago, when paleontologist Peter Van Roy examined a seven-foot-long fossil that he’d spent more than 500 hours freeing from rock and putting back together, he couldn’t believe what he was seeing. The 480-million-year-old creature, Aegirocassis benmoulae (for Mohamed ‘Ou Said’ Ben Moula, the collector who found it), belongs to a long-extinct group of aquatic invertebrates called anomalocaridids—the earliest ancestors of arthropods—and Van Roy had just discovered a feature that may solve an old puzzle in arthropod evolution.

“The fossil had two sets of swimming flaps per body segment on its trunk,” says Van Roy. “I was in shock.” That may be enigmatic to most of us, but he explains: modern arthropods typically have a two-branched limb. Evolution has in many cases modified each branch for a separate purpose—and therein lies the evolutionary flexibility that accounts for the spectacular success of this group. But previously studied anomalocaridids had only one swimming flap per trunk segment, so scientists had puzzled over how the two-branched structure evolved. Van Roy’s finding offered “evidence that the modern arthropod limbs formed through the fusion of both flap structures,” he says.

Re-examination of other fossils showed that the two flaps were often just overlooked. In the March 11 online edition of Nature, Van Roy and colleagues reported other surprises—such as sieve-like anatomy indicating that A. benmoulae was a filter feeder. Plankton was burgeoning in the ancient oceans, and evolution remodeled these giant animals to take advantage of it, says Van Roy—“the earliest example of an overarching ecological trend.”