The Serrasalmidae are a family of Neotropical freshwater fishes that includes carnivorous piranhas as well as their herbivorous relatives, the pacus. Pacu diets consist of leaves, stems, fruits, seeds, and algae, as well as insects, benthic invertebrates, plankton. Likewise, some piranha species are actually more omnivorous than carnivorous, feeding on fruits and seeds in particular and only to a lesser degree the fins and scales of other fishes. These diverse prey materials appear concomitant with diverse jaws and dentitions in both pacus and piranhas, suggesting that some species are ecomorphologically specialized for feeding on certain prey. We investigated how the pattern and tempo of feeding morphological specialization in herbivorous serrasalmids reflects the ecological diversity of their food resources. Pacu and piranha species were first categorized as either algivores, frugivores, folivores, phytophages, planktivores, or omnivores based on a meta-analysis of published gut content data. We used computed tomography (CT) scanning and morphometrics to describe the primary morphological axes of jaw and dental variation and any correlates these phenotypes may have with each species’ primary prey. We found significant differences in the occlusional offset, mechanical advantage, size and shape of the lower jaw among different diet guilds. Phytophages tended to have scissor-like dental occlusion, resembling piranhas more than other pacus. We also found significant differences in the rate of morphological evolution among different diet guilds, notably folivores had morphological rates over 40 times faster than that of planktivores, suggesting different selective regimes acting on each dietary guild.