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Why Do Piranhas Have Sharp Teeth?

The closest living relatives of piranhas are pacus, South American river fish that eat mostly plants. But pacu teeth arent 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 1950's, 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, in an Argentine museum drawer. The specimen, unearthed early in the 20th century, is just a 5 centimeter long upper right jawbone, but thats enough. The fossil teeth are in a zigzag formation, just as expected for teeth moving from 2 rows in an ancestor fish to single file in the piranha.

Source: {http://news.sciencemag.org/archaeology/2009/06/how-piranha-got-its-teeth}