Squirrels of the World

Squirrels constitute a family, the Sciuridae, of the order Ro-
dentia, all being descendants of a common ancestor that
lived some 30–40 million years ago. All share certain ana-
tomical features of their teeth and jaw muscles by which
they can be recognized, but distinguishing diff erent groups
within the Sciuridae and discerning interrelationships among
them have been diffi cult. A particularly interesting question
has been the relationship of fl ying squirrels to the others.
Beginning in the nineteenth century and continuing still,
there has been debate about whether fl ying squirrels evolved
from tree squirrels or represent an independent lineage with
a much longer history and thus are not closely related to tree

squirrels. Comparative anatomists usually considered them

to be true squirrels, whereas paleontologists contended that

they were the sister group of all the other squirrels, one

which evolved independently at about the same time as the

others. Molecular data have recently been used in this de-

bate, and the evidence adduced from the DNA clearly sup-

ports the hypothesis that living fl ying squirrels are most

closely related to the Sciurini (consisting of the American

and the northern Eurasian tree squirrels), and thus that

fl ying squirrels are not an independent lineage. However,

this conclusion leaves paleontologists with the problem of

determining what the fossil squirrels are, since those occur-

Ta xo n o mi c In t r o du c ti o n

tribe Protoxerini

tribe Marmotini Holarctic ground squirrels

tribe Xerini African ground squirrels

African tree squirrels

subfamily Callosciurinae Southern Asian tree squirrels

tribe Sciurini Holarctic tree squirrels

tribe Pteromyini Flying squirrels

subfamily Sciurinae

Other rodents

subfamily Sciurillinae

subfamily Ratufinae

subfamily Xerinae

South American pygmy squirrels

Giant tree squirrels

Phylogeny of the Sciuridae, based on research by Steppan, Storz, et al. (2004).