The South American monkeys (platyrrhines) first appear in the fossil record of that continent in the Oligocene, and we deduce that they must have arrived there at least by the late Eocene
This reasoning involves some rather complex arguments about crossing the Atlantic Ocean. Why don't we think that the platyrrhines might have reached South America a lot earlier when the Atlantic Ocean was narrower and traversing it would have been easier?
Platyrrhines are anthropoid primates that are more derived than the late Eocene African anthropoids, such as the parapithecids. They could not have originated before this group itself evolved, which ties down their earliest possible time of origin to the late Eocene.
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