This fossil, however, provides definitive proof that these giant flying squirrels, whichweighed about 1.4 kg—three times heavier than today’s typical gray squirrels—once roamed North America.

The last time a wild red squirrel was spotted in Cornwall was in 1984; most of Britain’s 140,000 are in Scotland, but there are also pockets in the north of England, Wales and on the Isle of Wight. By ...