Squirrels are not well known in the fossil record. However,
paleontologists have worked assiduously, screen washing
and extracting fossil teeth from sediment, which has re-
sulted in many fi nds showing the distinctive features of
squirrel teeth. From these, a general picture of squirrel evo-
lution has emerged.
One of the earliest fossil squirrels, dating from the late
Eocene of North America, 36 million years ago (MYA), is
Jeff erson’s squirrel, Douglassciurus jeff ersoni. Remarkably, a
complete skeleton of the species was found. Jeff erson’s
squirrel was clearly a tree squirrel, with its body propor-
tions and most details of its postcranial anatomy extremely
similar to that of modern eastern fox squirrels (Sciurus niger).
However, the rostrum was shorter than in other squirrels,
and the jaw musculature was extremely primitive, with the
masseter muscle originating only from the ventral surface
of the zygomatic arch under the eye (protrogomorphy) and
not from the zygomatic plate and the side of the rostrum in
front of the eye (sciuromorphy), as it does in other modern
and fossil squirrels. Because the more anterior origin of the
masseter musculature has been the main defi ning character
of the Sciuridae, there was some initial reluctance on the
part of paleontologists to consider Douglassciurus to be a
sciurid, but in all other respects it is very much a squirrel. It
probably looked like a snub-nosed modern tree squirrel. It is
closely related to Protosciurus, found from the early Oligo-
cene (33 MYA) to the early Miocene (23 MYA) in North
America. Protosciurus diff ers in having at least the beginning
of the typical squirrel masseter muscle arrangement (sciuro-
morphy, instead of protrogomorphy). Presumably they also
looked very much like modern North American tree squir-
rels. Miosciurus (approximately 23 MYA), in turn, is closely
related to Protosciurus, but Miosciurus is suggested to be close
to the divergence of tree squirrels and ground squirrels, al-
though it is not ancestral to either. Tree squirrels attributed
to the modern genus Sciurus, which was probably derived
from Protosciurus, are found from the Miocene (approxi-
mately 16 MYA) to the present in North America. They are
also found from the late Pliocene (3 MYA) to the present in
Europe, and from the middle Pleistocene (0.7 MYA) to the
present in Asia. They probably reached South America at
the time that the Panamanian isthmus formed (3 MYA).
Other New World tree squirrels are known only from Pleis-
tocene fossils (the North American red squirrel, Tamia s c i-
urus) or not at all as fossils (the dwarf and pygmy squirrels
of Central and South America, Microsciurus and Sciurillus).
Similarly, the Bornean Rheithrosciurus is not known as a
fossil.
Asian tree squirrels are very rare as fossils. In Europe, a
large tree squirrel in the early Miocene (23 MYA) is attrib-
uted to the genus Ratufa. This genus includes the four spe-
cies of giant tree squirrels presently living in southern Asia,
but no Recent (~10,000 years ago to the present) species in
Europe. Other paleontologists have questioned this attribu-
tion and have considered this fossil to be “a large general-
ized tree squirrel” of unknown affi liation. Ratufa is known
with more certainty from the middle Miocene (16 MYA) of
southern Asia. Late Miocene (11 MYA) fossils from Asia are
attributed to the callosciurine squirrels, which are now the
dominant tree squirrels in Southeast Asia. These include
Dremomys, Callosciurus, and Tamiops, which are known with
more certainty from the early to the mid-Pleistocene, less
than 2 MYA (Dremomys); the early Pleistocene (Callosciurus);
and the late Pliocene to the middle Pleistocene, less than 3
MYA (Tamiops). All of the other genera of callosciurine squir-
rels in the Recent fauna are not known as fossils.