The Origin and Evolution of Mammals - Moodle

effect of the surrounding sea. Tropical rain forest
with its rich fauna of arboreal and browsing mammals
persisted. However, by the Early Oligocene,
30 Ma, the fall in sea-level had caused the coalescence
of the western European islands, and even
more significantly had caused the drying up of the
Turgai Straits, thereby establishing a land connection
with Asia. The eventual effect was one of the largest
turnovers of mammal faunas of all. Around 60% of
the Late Eocene species went extinct, including tropical
tree dwellers like the haplorhine primates,
some of the uniquely European rodents such as the
forest browsing theridomyids, and the browsing
palaeothere perissodactyls. The archaic insectivorous
leptictidans disappeared, as did most of the
hyaenodontid creodonts. In their place came a great
wave of immigrants from Asia, most of them directly,
others possibly via North America. Notable amongst
the new arrivals were families of Carnivora such as
bears, weasels, cats, and the cat-like nimravids; the
perissodactyl families of rhinos, indricotheriids, and
chalicotheres; and the artiodactyl anthracotheriids
and ruminants. New families of Asian rodents
including squirrels and cricetids amongst several
others, and the lagomorph pikas arrived.
Clearly a key to understanding the events of the
Eocene–Oligocene transition lies in Asia, but the
picture here is nowhere near as well-known as in
North America and Europe. Certainly there was a
faunal change from archaic to modern groups,
much as in North America (Prothero 1994; Lucas
2001), and by the Early Oligocene, the fauna was
dominated by modern eulipotyphlan, rodent, lagomorph,
perissodactyl, artiodactyl, and carnivoran
families as in the rest of the northern hemisphere.
Relative dating is difficult, however, and while
there is little doubt that these new taxa occurred in
Asia before Europe, it is less certain that this is the
case with respect to North America.
The Late Eocene and earliest Oligocene of Africa
is known adequately only from the Fayum deposits
of Egypt (Rasmussen et al. 1992). This part of Africa
was in the tropical belt along the southern shore of
the Tethys Sea, and appears to have been relatively
immune from the effects of the cooling episodes of
the time elsewhere. A fauna unique to Africa,
whose presence is hinted at by the few earlier
Eocene fossils of North Africa, persisted (Maglio
LIVING AND FOSSIL PLACENTALS 283
1978). It was dominated by the hyracoids,
proboscideans, and embrithopods that were part of
the afrotherian radiation, and African taxa of hyaenodontid
creodonts, anthracotheriid artiodactyls,
rodents, and primates, which had entered the continent
earlier, by an unknown route from the north.
This fauna remained unchanged until at least the
Oligocene–Miocene boundary, to judge from the
Chilga fauna of Ethiopia (Kappelman et al. 2003).
The Eocene–Oligocene transition is represented
in South America by the Tinguirirican beds of Chile
(Wyss et al. 1994; Flynn and Wyss 1998), in which a
number of archaic groups disappeared, and more
modern versions of the South American indigenous
meridiungulates appeared. It was around this time
that the New World rodents arrived, probably by
rafting from Africa across what was still a relatively
narrow Atlantic Ocean (Wyss et al. 1993). The
platyrrhine anthropoids are not recorded until the
Late Oligocene when they too arrived, probably
also from Africa.
Finally, and needless to say, the flourishing
Eocene flora and fauna of Antarctica disappeared
permanently and without trace with the formation
of the ice-cap.
The Miocene: second flourishing and
second decline
The Early Miocene was a time of temporarily
increased global temperature and relative dryness.
The climate change was accompanied by the expansion
once again of areas of tropical and subtropical
forest, while sea level falls and tectonic events
resulted in the opening up of several land connections.
The consequence was a period during which
several major groups of placentals spread and diversified
(Potts and Behrensmeyer 1992). The most
important dispersal event was the immigration into
the hitherto largely isolated Africa of many groups
of Eurasian mammals which had never occurred
there before, but which were to be the basis of the
development of the rich modern African fauna.
According to Maglio (1978), 29 new families of
mammals entered Africa at this time, compared to a
mere 14 families remaining from the Oligocene.
Amongst the most prominent arrivals were
rhinocerotid and chalicothere Perissodactyla, and