The technical distinction between the New World platyrrhines and Old World catarrhines is the shape of their noses. The platyrrhines (from Ancient Greekplatu-, "flat", and rhin-, "nose") have nostrils which face sideways. The catarrhines (from Ancient Greek kata-, "down", and rhin-, "nose") have nostrils that face downwards. Catarrhines also never have prehensile tails, and have flat fingernails and toenails, a tubular ectotympanic (ear bone), and eight, not 12, premolars, giving them a dental formula of: , indicating 2 incisors, 1 canine, 2 premolars, and 3 molars on each side of the upper and lower jaws.
Most catarrhine species show considerable sexual dimorphism and do not form a pair bond. Most, but not all, species live in social groups. Like the platyrrhines, the catarrhines are generally diurnal, and have grasping hands and (with the exception of bipedal humans) grasping feet.
According to Schrago & Russo, New World monkeys split from their Old World kin about 35 million years ago (Mya). They use the major catarrhine division between cercopithecoids and hominoids of about 25 Mya (which they argue is strongly supported by the fossil evidence), as a calibration point, and from this also calculate the gibbons separating from the great apes (including humans) about 15-19 Mya.
According to Begun and Harrison, the Catarrhini split from their New World monkey kin about 44 - 40 Mya, with the first catarrhines appearing in Africa and Arabia, and not appearing in Eurasia (outside Arabia) until 18-17 Mya.
The distinction between apes and monkeys is complicated by the traditional paraphyly of monkeys: Apes emerged as a sister group of Old World monkeys in the catarrhines, which are a sister group of New World monkeys. Therefore, cladistically, apes, catarrhines and related contemporary extinct groups such as Parapithecidae are monkeys as well, for any consistent definition of "monkey". "Old World Monkey" may also legitimately be taken to be meant to include all the catarrhines, including apes and extinct species such as Aegyptopithecus, in which case the apes, Cercopithecoidea and Aegyptopithecus emerged within the Old World Monkeys.
Below is a cladogram with extinct species in which the crown Catharrhini, which emerged in the Propliopithecoidea. Also, Saadanioidea is sister of the Cercopithecoidea rather than of the Crown Catarrhini here. It is indicated how many million years ago (Mya) the clades diverged into newer clades.
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