Unearthing the megapiranha

Ichthyologist John Lundberg has been nipped by his share of piranhas. It's an occupational hazard of doing field work in South American rivers. Luckily for him, the sharp- toothed predators no longer grow to three feet long, as they apparently did nine million years ago.

Lundberg and colleagues at the Academy of Natural Sciences in Philadelphia just finished describing the fossil jaw of a prehistoric river monster known as the megapiranha. The jaw, he says, helps explain how the modern piranha evolved.

Piranhas today have unique teeth, which are arranged in a single row like a saw blade. "It's a very efficient biting mechanism," he said, from firsthand knowledge.

The fossil jaw was unearthed in Argentina in the early 20th century but was left in a drawer. Argentine biologist Alberto Luis Cione rediscovered it a few years ago. "There's an old saying in paleontology that museums are the best places to find unknown things," Lundberg said.

From the size of the jaw, he estimated the fish's total length at about three feet, he said. Piranhas today are thought to have evolved from fish with a double row of teeth, like the pacu, a piranha relative that evolved millions of years earlier.

How did piranhas get a single row of teeth?

Working with colleague Wasila Dahdul, Lundberg showed that the megapiranha had an intermediate tooth arrange-

ment. "The neat thing about this fish is that its teeth are arranged in a zigzag row," he said, as if the teeth from the ancestral double row were in the midst of moving forward, as they are today.

Since they branched off from a common ancestor, piranhas and pacus diverged to take advantage of different foods. While piranhas eat mostly other fish, pacus eat mostly fruit that drops from trees into flooded forests. But hungry fish are not that choosy. "Any fish will eat anything that comes its way no matter what it's specialized in," Lundberg said. "It's hard out there so you eat what you can." - Faye Flam