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The owl monkey, Aotus, is the only modern nocturnal anthropoid with monogamous social structure. It has been demonstrated by the fossil species, Aotus dindensis, discovered from La Venta, Colombia, that the Aotus lineage had emerged as early as the middle Miocene (12-15 Ma). The type specimen of A. dindensis, which was discovered in 1986, preserves extremely large orbits, indicating a nocturnal habit. However, a few anatomical traits in living Aotus, such as the lack of a tapetum lucidum, indicates that nocturnality is a secondary adaptation from diurnal ancestry in this genus. Here we report new fossil specimens of A. dindensis from La Venta. The specimens include maxillary teeth and a mandibular fragment preserving lower molars. The detailed analysis of the specimen suggests that A. dindensis exhibits strong sexual dimorphism in the maxillary canine and premolars, which is traditionally associated with intense intermale competition for mates and/or food resources in non-monogamous, diurnal societies. As a result, the new fossil materials of A. dindensis demonstrate the first osteological evidence for the diurnal ancestry of the night monkey, Aotus. Moreover, the coexistence of large orbits and canine dimorphism suggests the presence of mosaic evolution in the craniodental characters of the Aotus lineage. [ABSTRACT FROM AUTHOR]