In modern piranhas, teeth are arranged single file, whereas related, plant-eating pacus have two rows. This three-inch (eight-centimeter) fossil jaw fragment from a Megapiranha
shows a zigzagging tooth arrangement--an evolutionary midpoint between the pacu and piranha patterns, experts announced in a June 2009 study.
At least one mystery remains, however.
"The fossils have seven teeth and [modern] piranhas have six," said study co-author Wasila Dahdul, an evolutionary biologist at the University of South Dakota. Did the seventh tooth "just get completely lost over time, or maybe two teeth fused? That's something we don't know."
Photographs courtesy Mark Sabaj-Perez