Origin and early dispersal of the squirrels and their relatives

The first appearances of squirrels and aplodontids in both Europe and North America point to an unknown origin in between both continents. More recent analyses of the tectonic history of the North Atlantic and Arctic ocean basins are consistent with the hypothesis, that an exchange between both sides of the Atlantic has been possible at least until the early Oligocene, the time of origin of both groups. The long lasting isolation of Northern Europe from both the southern parts of the continent and Asia gives enough room for the assumption of a separate evolutionary centre.