The Origin and Evolution of Mammals - Moodle

284 THE ORIGIN AND EVOLUTION OF MAMMALS
giraffid, bovid, suid, and tragulid families of
Artiodactyla. These various ungulates were accompanied
by the main families of the Carnivora, felids,
canids, viverrids, and later mustelids. Several families
of Eurasian rodents and Eulipotyphla were
introduced. A number of the African mammals
made the reverse migration, northwards from
Africa, and of these the most prominent were proboscideans,
with representatives of the gomphotheres,
mastodonts, and deinotheres arriving in
Eurasia in what has been termed the ‘proboscidean
event’ of about 20 Ma (Agustí 1999; Rögl 1999). They
were accompanied by African lineages of anthracotheriid
artiodactyls, and members of the last group
of surviving hyaenodontid creodonts, which still
flourished in Africa. By mid-Miocene, other groups
of African origin had also spread into Eurasia,
including the pliopithecid anthropoids, dryopithecid
apes, and hyracoids.
Relatively free dispersal across the Bering Straits
between Asia and North America was also possible
during the Early and Middle Miocene, allowing the
introduction of, notably, proboscideans, felid carnivores,
and antilocaprid deer into North America.
The warming phase reached its peak in mid-
Miocene, about 15 Ma, which coincides with the
second peak of Cenozoic mammal diversity. As at
the time of the Eocene peak, the rise in temperature
is believed to have resulted from an increase in CO 2
levels (Pearson and Palmer 2000). The associated
rise in plant productivity would have created the
opportunity for diversification of herbivorous
mammals and their predators. In North America
the diversity of ungulates, dominated by tayassuid,
camelid and antilocaprid artiodactyls, equids, and
proboscideans increased from about 30 genera
at the commencement of the Miocene to a peak
of about 65 at the mid-Miocene temperature
maximum. Most of the increase was in mammals
with cheek teeth that were intermediate between
the low crowned version of pure browsers and the
high crowned, hypsodont version found in grazers
(Janis et al. 2000, 2002). In Africa, the Miocene was
marked by the radiation of the newly introduced
bovids, suids, and giraffids amongst the artiodactyls,
and rhinocerotid perissodactyls.
From this time on, temperatures started to fall,
and dry, more seasonal conditions began to prevail.
During the second half of the Miocene, the tropical
and subtropical forests were increasingly displaced
by the spread of more open woodland, and eventually
grassland savannas. The mammal diversity fell,
and there was an especially large drop in browsing
ungulates, and forest adapted forms. Close to the
end of the Miocene, about 7 Ma, there was a dramatic
acceleration of this trend, and it was associated
with a floral revolution. The photosynthetic pathway
of most modern grasses is C 4 metabolism,
which is more efficient under conditions of low CO 2
levels coupled with high temperatures. The proportion
of C 4 plants in a fossil animal’s diet can be estimated
from the 13 C/ 12 C ratio preserved in its bones,
and Cerling et al. (1997, 1998) revealed that there was
a worldwide increase in the utilisation of C 4 plants
by mammals about 7 Ma. It coincides with a worldwide
faunal change in which hypsodont-toothed
mammals became far and away the dominant ungulates:
equids and proboscideans in North America;
antelopes, hippos, and giraffids in Africa; hypsodont
deer, hippos, and giraffids in Eurasia; several kinds
of notoungulates and rodents in South America.
This inferred spread of temperate grasslands was
part of the final important episode of the Miocene,
known as the Messinian Crisis. After a sequence
of relatively minor glaciations and intervening
warmer times, the end of the Miocene was marked
by a much more substantial arctic glaciation. As well
as the direct effect on mammalian faunas, the associated
fall in sea level combined with the continuing
northward movement of Africa to cause the closing
off of the Mediterranean Sea from the Atlantic
Ocean. Evaporation followed creating a huge, Dead
Sea-like hypersaline lake surrounded by highly arid,
desert conditions. It created a land bridge between
North Africa and the Iberian Peninsula, across which
passed a range of taxa. They included the hippos,
camels, cercopithecoid monkeys, and gerbils entering
into Europe (Agustí 1999).
The Plio-Pleistocene: exchanges and
extinctions
For the first 2 million years of the Pliocene, after the
Messinian Crisis, the world underwent another
brief warm phase in which tropical and subtropical
forests expanded once more and mammal diversity