Over the past century, the Santa Cruz Formation of coastal Argentina (late Early Miocene) has yielded a remarkable collection of platyrrhine primates. With few notable exceptions, most of the specimens have been included in Homunculus patagonicus Ameghino, 1891, a stem platyrrhine. Homunculus patagonicus was approximately 1.5 to 2.5 kg in body mass, about the size of a living saki monkey (Pithecia) or a female Cebus. Molar structure indicates that the diet consisted of a mixture of fruit and leaves. A deep jaw, large postcanine tooth roots, large postglenoid processes and moderately large chewing muscle attachments (i.e. massive zygomatic arches, sculpted temporalis origins) suggest that physically resistant foods were key components of the diet. Heavy tooth wear suggests large amounts of ingested silica or exogenous abrasives. Incisor morphology suggests that exudate harvesting may have been part of the behavioral repertoire, although not a specialization. The canines were small, providing no evidence of sclerocarpic foraging. Canines were sexually dimorphic, suggesting that the taxon experienced some intrasexual competition rather than being solitary or pair-bonded. Brain size was small and the frontal cortical region was proportionately small. From the small size and structure of the orbits, the structure of the organ of hearing, the reduced olfactory fossae and the relatively large infraorbital foramina, we infer that Homunculus was probably diurnal, with acute vision and hearing, but with a poor sense of smell and little reliance on tactile vibrissae. Homunculus was an above-branch arboreal quadruped with leaping abilities. The semicircular canals show evidence of considerable agility, reinforcing the inference of leaping behavior. The overall locomotor repertoire is not unlike that of the forest-dwelling extant saki monkey Pithecia. Considered together, the mosaic of dietary and locomotor morphology in Homunculus suggests that Homunculus inhabited an environment – as compared with earlier Colhuehuapian and Pinturan primate habitats – shifting towards greater seasonality in patchy forests near river courses.