6 Paleontologymyoides—developed marmotlike characteristics in the late
Miocene (12–5 MYA). They are probably derived from early
ground squirrels of the Otospermophilus group. Similarly,
Marmota seems to have evolved from this group in the late
Miocene. Marmots are known from the late Miocene (
MYA) to the Recent in North America, and in the Pleisto-
cene and the Recent in Europe and Asia. There are now six
species in North America and eight species in Eurasia.
Hesperopetes thoringtoni (36 MYA), slightly older than Dou-
glassciurus, and hence the oldest known fossil squirrel, is
known only from teeth and is attributed to the “fl ying” squir-
rel group, a group distinguished by having wrinkled or crenu-
lated enamel on their molars. Squirrels of this group are rep-
resented by fossils from late Eocene until the Pliocene (36–
MYA) and, if they are related to true fl ying squirrels, are still
extant. Hesperopetes includes three species in North America
from the late Eocene (36 MYA) and the late Oligocene (
MYA). Oligopetes, with two species in Europe, is an early Oli-
gocene (33 MYA) “fl ying” squirrel. However, molecular data
suggest that modern fl ying squirrels diverged from the Sci-
urini tree squirrels in the early Miocene (23 MYA). Crenulated
molars did not evolve in this lineage until the middle or late
Miocene (10 MYA), which is discordant with the dates of
these fossils reported from the Eocene and the Oligocene.
Unfortunately, there are no postcranial bones of these pur-
ported “fl ying” squirrels to show whether these animals were
gliders, like modern fl ying squirrels.
Within this “fl ying” squirrel group there are early Mio-
cene fossils of four genera: Petauristodon, Aliveria, Miopetau-
rista, and Blackia. Petauristodon, with six species existing in
North America from the early to the late Miocene (19–
MYA), probably did not glide and most likely was not related
to modern fl ying squirrels (tribe Pteromyini). Aliveria con-
sists of two species in the early Miocene (18–16 MYA) and is
considered to be a reasonable ancestor of fl ying squirrels,
together with the similar Shuanggouia of the middle Mio-
cene and Albanensia (with four species) in the middle and the
late Miocene (15–11 MYA). Miopetaurista, with six species
ranging from the early Miocene to the Pliocene (22–3 MYA),
has smooth enamel but is otherwise similar to Aliveria.
Blackia, with two species ranging from the late Oligocene to
the late Pliocene (30–2 MYA), was a small squirrel with ex-
treme pitting of the enamel and small cusps. No other squir-
rels exhibit this pattern, suggesting that Blackia was not
ancestral to any other fossil “fl ying” squirrels or modern
squirrels.
Other middle Miocene “fl ying” squirrels include For-
sythia, a single species in the middle Miocene (14–11 MYA)
in Europe, and Meinia and Parapetaurista in Asia. Meinia was
described as being similar to Blackia. Parapetaurista is con-
sidered to be similar to Miopetaurista or Albanensia. Pliope-
taurista (with four species) and Pliosciuropterus are late Mio-
cene and Pliocene (11–2 MYA) genera, with similarities to
modern giant fl ying squirrels (Petaurista). However, one
species of Pliopetaurista (P. meini) appears to be more simi-
lar to Callosciurus.
Three species from the early Miocene to the early Plio-
cene (23–3 MYA) in Europe are attributed to the genus Hylop-
etes, congeneric with species in the modern fauna of South-
east Asia.
Flying squirrels of the Pliocene and the Pleistocene are
attributed to four of the genera described above (Blackia,
Miopetaurista, Pliopetaurista, and Pliosciuropterus) or to mod-
ern genera.
It is doubtful whether one can recognize true fl ying
squirrels (tribe Pteromyini) in the fossil record by means of
the pitting and crenulations on the teeth. These features are
found on the teeth of 3 of the 15 genera of living fl ying squir-
rels, but also on 7 genera of tree squirrels. In fact, other
characteristics of the teeth of fl ying squirrels that are touted
as diagnostic are found among tree squirrels as well, leading
to the suggestion that the gliding habits of fossil squirrels
can be recognized only by characteristics of their postcra-
nial skeleton, which is unfortunately seldom preserved. In
the Pliocene and the Pleistocene, however, the close simi-
larity of the teeth of some fossils to those of modern genera
makes it highly probable that these were indeed true fl ying
squirrels.