During their long separation, the two New Worlds might almost have belonged to different planets, not least because the Cretaceous extinction occurred while they were independent of each other, and they were repopulated by different sets of survivors. These species radiated into new forms as the world recovered, and when the Americas finally joined hands again, their separate casts of living things met for the first time, in a moment one paleontologist called “one of the most extraordinary events in the whole history of life.”
