Flying Squirrels: Origins

A second issue has been whether gliding evolved twice within tree squirrels from both South-East Asian tree squirrels (Callosciurini) and from Holarctic tree squirrels (Sciurini), or whether flying squirrels are derived from a single ancestor. An apparent high diversity of fossil flying squirrel teeth early in squirrel history has been taken as evidence that flying squirrels have an origin independent of tree and ground squirrels, justifying the division of the squirrel family into two distinct groups and even promoting suggestions that flying squirrels belong in a separate family. Morphological and molecular evidence, however, has found that flying squirrels are derived from tree squirrels and are now recognised as a tribe, Pteromyini, within the Family Sciuridae. Molecular studies indicate the divergence of the five major groups of squirrels, including the flying squirrels, took place during the late Eocene and Oligocene (34–23 mya).

A third uncertainty is the grouping of the different genera of living flying squirrels and their affinities with the genera of fossil squirrels that are thought to have been able to glide. At present, none of the fossil genera can categorically be allocated as gliders because at least some of the features used to describe them are also found in at least some tree squirrels. It will not be until the discovery of further fossil remains with adequate postcranial material, such as the limbs that are long and thinner in gliding mammals, that the ability of the fossil attributed to gliders can be confirmed.