Scientists from Argentina, the United States and Venezuela have discovered a new fossil piranha that helps explain how the teeth of modern piranhas have evolved.
Discovered in the late Miocene (about 7"11 million years ago) deposits in the riverside cliffs of the Paran River near the city of Paran in Argentina, the fossil consists of part of the upper jawbone (premaxilla) and several teeth.
The new fossil piranha, described in a study published in a recent issue of the Journal of Vertebrate Paleontology as Megapiranha paranensis, displays seven teeth arranged in a zig-zag row in its upper jaw.
This is a condition intermediate between the piranhas, which have six teeth arranged in a single row, and their closest relatives, the pacus, which have seven teeth arranged in two rows.
This suggests that the two rows of teeth in the pacus were compressed to form a single row in piranhas. However, it is still unclear as to how piranhas came to have six instead of seven teeth.
It has been traditionally assumed that this was due to the loss of one tooth, but more recent hypotheses suggest that the six-toothed condition in piranhas resulted from the fusion of two of the teeth.
Megapiranha is one fish you might not want to encounter, though. By comparing the size of the fossil jawbone and teeth with modern counterparts, the authors estimate that it reached a size between 95 and 128 cm, much larger than modern piranhas.
However, even though the teeth of Megapiranha are triangular and bladelike as in most modern piranhas, the authors conclude that he ecology of Megapiranha is difficult to reconstruct. Despite their reputation as carnivores, some piranha species are primarily herbivorous, and several species include plant material and scales in their diet.
This suggests that the extremely compressed tooth morphology seen in piranhas did not evolve in correlation with carnivory. In light of this plasticity, the compressed crowns and round tooth bases of the Megapiranha specimen may have been suited to a range of feeding behaviors.
For more information, see the paper: Cione, AL, WM Dahdul, JG Lundberg and A Machado-Allison (2009) Megapiranha paranensis, a new genus and species of Serrasalmidae (Characiformes, Teleostei) from the upper Miocene of Argentina. Journal of Vertebrate Paleontology 29, pp. 350"358.