There are 28 species of piranha living exclusively in the fresh waters of South America. Piranha are found as far north as the Orinoco River in Venezuela to Argentina's Parana River in the south. These flesh-eating fish are distinguished by a single row of razor sharp teeth, a blunt snout and an underbite. Ancient fossils of piranha resemble present-day species, suggesting this fish subspecies existed in South America 25 million years ago.
Pacu, or tambaqui, is a close cousin to piranha. Fish in this family share the same blunt nose and body shape as piranha. However, pacu are much larger. While a piranha is usually 6 to 10 inches long, pacus can grow as long as 3 feet and weigh 60 pounds. The pacu eats only plants and nuts. It has two rows of blunt teeth and is considered an early relative of the modern-day piranha.
A 5-centimeter-long jaw bone found in a drawer at a museum in Argentina may be proof of a transitional fish, a missing link, in piranha evolution. The jawbone, first discovered on a riverside cliff in Argentina in 1900, is from an extinct species of piranha called megapiranha paranensis. This huge, extinct fish species existed 8 million to 10 million years ago. Scientists estimate this early piranha was 3 feet long, which is four times as long as modern piranha, but similar in length to pacu.
The megapiranha is remarkable as being the earliest example of a piranha with a single row of teeth. Early piranha and pacu fossils had two sets of teeth and were probably herbivores. The megapiranha jaw has one set of teeth set in a zigzag pattern, as if the second row of teeth is merging with the first. This may have been the first step in the evolution of piranha as carnivores. Scientists believe the megapiranha was an omnivore, but that is a striking difference from earlier species that were strictly herbivores.