Origins

Flying squirrels are the oldest living line of modern squirrels (modern, as opposed to their precursors, the early-Eocene squirrel-like rodents called "paramyids"). Evidence of their relative's existence goes back to the late Eocene period, between 38 and 55 million years ago! Tree squirrels made their first appearance on this earth during the late Oligocene period, about 30 million years ago. Ground squirrels came in waves, with some appearing 28 million years ago (very late Oligocene period), late Miocene period (8 million years ago) and very late Pliocene period (2.5 million years ago).

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North American flying squirrels are not very well represented in the fossil record, for several reasons.

Their fine bone structures do not fossilize well, and due to their arboreal lifestyle, dead specimens rarely were situated so that fossilization could occur.  Flying squirrel teeth are often the only fossil record that survive the ravages of time. Attempts have been made, in the past, to identify extinct flying squirrel species' via dentition characteristics, but it was found that using this method alone was inaccurate more often than not. A high degree of expertise is required to make distinctions between extinct tree and flying squirrel teeth, so skeletal (including the skull) AND dental characteristics combined present the most accurate method of identification.

So, there being a paucity of information to be gleaned from fossil records (these records are virtually all Pleistocene Era records, by the way), we have little data to enlighten us about the flying squirrel's time here on this continent.

What we do know, however, is:

What we now call the southern flying squirrel likely emigrated to what is now know as  North America via the Bering Land Bridge via what is now know n as Asia roughly 25 million years ago during the early Miocene era. This squirrel was adapted for life in temperate mixed deciduous-coniferous forests, and migrated north and south with the glacier activity through the eons.

More adapted to warmer climes than the northern, southern flying squirrels found themselves migrating as far south as Central America during the Pleistocene era, roughly 100,000 years ago. Relict populations exist to this day in higher-elevation areas of Central America and Mexico, their connectivity forever lost due to climate changes and more recently, large-scale deforestation.

What we now call the northern flying squirrel is a relative newcomer to North America, having emigrated to North America during another incarnation of the Bering Land Bridge roughly 12 million years ago, during the early Pliocene era. This route of emigration via the Bering Land Bridge is not unusual, as many of North America's mammals, both extinct and extant, followed a similar route. In fact, it is generally accepted that our First Nations people emigrated to this continent via the Bering Land Bridge roughly 12,000 years ago, although there is some evidence that there may have been "ancient mariners" who rafted across the Pacific Ocean

The general thinking today is that the northern flying squirrels came from different stock than that of the southern flying squirrel. The most convincing argument for this theory is the squirrel's baculum, a small supporting bone of the penis that aids in the mating process and is present in many mammal species. The northern flying squirrel's baculum is structurally much more comparable in shape possessed by the Asian genus Hylopetes than that of the southern flying squirrel's. Shown on the right is the baculum of the northern flying squirrel (much enlarged!).


The Bering Land Bridge (Beringia)

The Bering Sea, Bering Strait and Beringia (a term to describe an area ranging from the Kolyma River in the Russian far east to the Mackenzie River in the Northwest Territories of Canada) was named for Vitus Bering, a Danish explorer for a Russian czar in the 18th Century.

At some points, this land mass, which at times sported a rich mixed forest environment, was thousands of miles wide, depending upon sea levels in the Bering Sea.

An animated video on how post-glacial flooding affects the size and scope of the Bering Land Bridge can be found via the links below. These animations detail the last "flooding" of the Bering Land Bridge, beginning approximately 21,000 years ago. Roughly 12,000 to 14,000 years ago, human beings crossed the Bridge from Asia into North America. A thousand or so years after that the Bridge was under water.