Skull anatomy of S. rhombeus (photo by SH) and fossil teeth of M. paranensis (inset, reprinted from Cione et al.).
Teeth of S. rhombeus and M. paranensis share similarly shaped cusps that form labio-lingually compressed triangular blades. Arrow (inset) demarks fine serrations on M. paranensis tooth for slicing flesh. Homologous landmarks were measured as length of labial base (mm) for the 5th premaxillary tooth (A). Allometric relationship describing growth of the 5th tooth relative to body size, measured as total length (mm) in the black piranha, S. rhombeus (B). Tooth length scales isometrically with increasing body size (slope = 1.02). Data were log10 transformed and fitted to a least-squares linear regression model: Log10 (tooth) = 1.02 Log10 (TL) − 1.91 (r2 = 0.96, F (1,14) = 339.8; p < 0.0001). Using this regression equation, a conservative body size of 71 cm TL was back-calculated for M. paranensis and was subsequently used to estimate its lower bound of bite forces (Fig. 2A).