During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which inhabited the New and Old Worlds, migrated south of the Polar Circle, they will have been completely cut off from each other.
Pliozeno aldian eguraldia hoztuz joan zenean, mundu zahar eta berrian bizi ziren espezie komunak polo zirkuluaren hegora migratu bezain laster, erabat banandurik geratuko ziren.