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Ecomorphological theory predicts a relationship between the morphology of a given trait and its ecological performance. In turn, variation in ecomorphology is viewed as integral to the structuring of animal communities. This reasoning is practically axiomatic, but the full logic chain is seldom integrated into a single study. We tested the functional relationship between premolar tooth size and chewing performance across a diverse community of wild primates, including chimpanzees and seven monkey species. We found that relatively large premolars were associated with improved food fracture, and that the chewing performance of granivores (seed predators) exceeded that of sympatric folivores and frugivores by 51–56% and 64–68%, respectively. This finding is robust when controlling for variation in chewing effort and seasonal grit ingestion. Evidence of convergent evolution speaks to the fitness advantages of enlarged premolars among granivorous species, and we show that premolar-mediated seed-eating shapes the composition of primate communities across nine African forest sites. Our findings are relevant to palaeoanthropology and the puzzling megadontia of some fossil hominin lineages, as our data favour a diet of stress-limited brittle foods, not ductile foods, as the principal selective pressure favouring the performance benefits of enlarged premolars.