Inferences about the diet of Miocene platyrrhine monkeys have relied upon the morphology of the molar teeth, specifically the crests on the molars. Using a library of Micro-CT images of a broad comparative sample of living platyrrhines (callitrichines, cebines, pitheciids and atelids), late early Miocene Homunculus, and the early Miocene Tremacebus and Dolichocebus, we extend these inferences by examining the surface areas of the tooth roots, anchor points for the periodontal ligaments. From muscle scars on the skull, we estimate the mechanical leverage of the chewing muscles at bite points from the canine to the last molar. Extant platyrrhines that gouge bark to obtain exudates do not have especially large canine roots or anterior premolar roots compared with their less specialized close relatives. Extant platyrrhines that have more folivorous diets have much larger molar roots than do similar-sized more frugivorous species. Homunculus patagonicus has large postcanine roots relative to body size and poor masticatory leverage compared to the extant platyrrhines in our sample. The large postcanine roots, heavy tooth wear, and moderately-long shearing crests suggests a diet of abrasive, resistant foods. However, relatively poor jaw adductor leverage would have put the masticatory apparatus of Homunculus at a mechanical disadvantage for producing high bite forces compared to the condition in extant platyrrhines. Tremacebus and Dolichocebus, like Homunculus, have larger tooth root surfaces than comparable-sized living platyrrhines. They also resemble Homunculus in being more prognathic and having posteriorly-located temporalis origins - all features of a relatively poor leverage system.