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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.