Transitional Vertebrate Fossils
GAP: One of the least understood groups of modern mammals -- there are no known bat fossils
from the entire Paleocene. The first known fossil bat,
, is from the (later) Eocene,
and it was already a fully flying animal very similar to modern bats. It did still have a few
"primitive" features, though (unfused & unkeeled sternum, several teeth that modern bats have
Fruit bats and horseshoe bats first appear in the Oligocene. Modern little vespertiliontids
(like the little brown bat) first appear in the Miocene.
Creodonts -- early placental mammals with minor but interestingly carnivore-like
changes in the molars and premolars. Had a carnivore- like shearing zone in the teeth,
though the zone moved throughout life instead of staying in particular teeth. Also had a
carnivore- like bony sheet in the brain dividing cerebrum & cerebellum, details of ankle.
Closely related to & possibly ancestral to carnivores. The origin of the creodonts is
unclear. They probably were derived from condylarths.
(late Cretaceous) -- This creodont (?) lost the last molar & then later enlarged
the last upper premolar and first lower molar. (In modern carnivores, these two teeth are
very enlarged to be the wickedly shearing carnassial teeth, the hallmark of carnivores.)
Still unfused feet & unossified bulla. This genus is probably ancestral to two later lines of
Eocene carnivores called "miacoids". Miacoids were relatively unspecialized meat-eaters
that seem to have split into a "viverravid" line (with cat/civet/hyena traits) and a "miacid"
line (with dog/bear/weasel traits). These two lines may possibly have arisen from these
slightly different species of
(Cretaceous) -- These are two species that
lost their third molar, and may have given rise to the viverravid line of miacoids (see
(Paleocene) -- A later, as yet unnamed species that has very miacid-like
(mid-Paleocene) -- A very early viverravid. The upper carnassial was
large; the lower carnassial was of variable size in different individuals.
(early Eocene) -- Early miacids. Enlarged carnassials now
specialized for shearing. Still had unfused foot bones, short limbs, plantigrade feet,
GAP: few miacoid skulls are known from the rest of the Eocene -- a real pity because for early
carnivore relationships, skulls (particularly the skull floor and ear capsule) are more useful than
teeth. There are some later skulls from the early Oligocene, which are already distinguishable as
canids, viverrids, mustelids, & felids (a dog-like face, a cat-like face, and so on). Luckily some
new well-preserved miacoid fossils have just been found in the last few years (mentioned in
Szalay et al., 1993). They are still being studied and will probably clarify exactly which miacoids