Transitional. These zigzag teeth are from an extinct fish related to both piranhas and pacus.

Carolina State University

How the Piranha Got Its Teeth

Piranhas have long been a staple of horror movies, and it's no wonder. Their razor-sharp teeth can tear chunks of flesh from creatures many times their size. Now scientists have rediscovered a fossil piranha jaw that shows how the fish got those choppers.

The closest living relatives of piranhas are pacus, South American river fish that eat mostly plants. (In fact, some rainforest seeds can only germinate after passing through the gut of a pacu.) But pacu teeth aren't nearly as pointy and terrifying as those of the piranha. Another key difference is that pacu teeth are arranged in two rows, whereas piranha teeth are lined up in a single row. In the 1950s, a scientist proposed that the common ancestor of piranhas and pacus had two rows of teeth, which eventually merged into a single row in piranhas. But nobody had ever seen a fossil showing an intermediate arrangement.

Now such a fossil has been discovered--collecting dust in an Argentine museum drawer. The specimen, which was unearthed early in the 20th century, is nothing but a 5-centimeter-long upper-right jawbone. But that's enough. The fossil teeth are in a zigzag formation, just as expected for teeth moving from two rows in an ancestor fish to single file in the piranhas. In a paper published this month in the Journal of Vertebrate Paleontology, researchers christen the fossil fish Megapiranha paranensis, which basically means "really big piranha from along the Rio Paraná." It's clearly a new species, says ichthyologist Mark Westneat of The Field Museum in Chicago, Illinois, who was not involved with the study.

There are other clues too that M. paranensis was a transitional fish, says ichthyologist Wasila Dahdul, one of the authors of the paper and now a visiting scientist at the National Evolutionary Synthesis Center in Durham, North Carolina. Like modern piranhas, Megapiranha had serrated edges on its teeth, and like modern pacus, the bone has a hinge at the front where it would attach to the other half of the upper jaw. "It was really neat," says Dahdul. "The more we looked at it, the more things came out of it."

To estimate the extinct fish's size, Dahdul measured dozens of skeletons from modern piranhas and pacus. Megapiranha was probably more than 1 meter long, as big as the biggest modern pacus, and a record for piranhas. Modern piranhas reach only 45 centimeters or so. And, by the way, they are not nearly as vicious as rumored, she says. Some only eat insects or plants, for example, and even the flesh-eaters rarely bite people. "You do have to watch your fingers, though, when you've got a piranha and it's flipping around on the ground," says Dahdul.

No one knows for sure what Megapiranha ate, says ichthyologist John Lundberg, a co-author of the paper and Dahdul's graduate adviser at the Academy of Natural Sciences in Philadelphia, Pennsylvania. But he is willing to make one guess about the species, having seen its modern relatives in Amazonian markets: "I would predict that it would be a fine eating fish."