<Tr> <Td_colspan="2"> <Ul> <Li> Callitrichidae </Li> <Li> Cebidae </Li> <Li> Aotidae </Li> <Li> Pitheciidae </Li> <Li> Atelidae </Li> </Ul> </Td> </Tr> <Ul> <Li> Callitrichidae </Li> <Li> Cebidae </Li> <Li> Aotidae </Li> <Li> Pitheciidae </Li> <Li> Atelidae </Li> </Ul> <P> New World monkeys are the five families of primates that are found in the tropical regions of Central and South America and Mexico: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae . The five families are ranked together as the Ceboidea, the only extant superfamily in the parvorder Platyrrhini . Platyrrhini means flat - nosed, and their noses are flatter than those of other simians, with sideways - facing nostrils . Monkeys in the family Atelidae, such as the spider monkey, are the only primates to have prehensile tails . New World monkeys' closest relatives are the other simians, the Catarrhini ("down - nosed," comprising Old World monkeys and apes). New World monkeys descend from African simians that colonized South America, a line that split off about 40 million years ago . </P> <P> New World monkeys are small to mid-sized primates, ranging from the pygmy marmoset (the world's smallest monkey), at 14 to 16 cm (5.5 to 6.5 in) and a weight of 120 to 190 g (4.2 to 6.7 oz), to the southern muriqui, at 55 to 70 cm (22 to 28 in) and a weight of 12 to 15 kg (26 to 33 lb). New World monkeys differ slightly from Old World monkeys in several aspects . The most prominent phenotypic distinction is the nose, which is the feature used most commonly to distinguish between the two groups . The clade for the New World monkeys, Platyrrhini, means "flat nosed". The noses of New World monkeys are flatter than the narrow noses of the Old World monkeys, and have side - facing nostrils . New World monkeys are the only monkeys with prehensile tails--in comparison with the shorter, non-grasping tails of the anthropoids of the Old World . </P>

What is one of the hypotheses concerning the origin of new world primates