The Origin and Evolution of Mammals - Moodle

and thus remain in their preferred habitat.
However, no such option of extending their geographical
range was available to the rain-forest
fauna of Central America, which merely experienced
a decline in the area of their available habitat.
The Great American Biotic Interchange was
unique and dramatic because it brought together
two biotas that had been virtually completely isolated
from one another for the whole of the previous
60–70 Ma. There were nevertheless significant
biotic changes in other parts of the world. The
severe glaciation 2.4 Ma, shortly after the establishment
of the Panamanian land bridge, was the start
of the particular cycle of ice ages and interglacial
periods that is still the dominant feature of the
Earth’s climate. In Europe, approximately 17
such ice ages are recorded in the 1.7 Ma of the
Pleistocene. Typically, an ice age is associated with
a reduction in the snowline by about 1000 m and a
fall in average global temperature of about 6 o C.
Sea-levels fall by between 100 and 150 m as water is
locked up in the ice caps. The effects on the environment
are worldwide, but differ from place to
place. In the higher latitudes of the Holarctic continents,
North America and Eurasia, there is an oscillation
between temperate forest and steppe. In
lower latitudes warm, moist canopied forest alternates
with drier, more open woodland and grassland
conditions, and it was indeed during the Pleistocene
that the fully modern version of savannah, prairie,
steppe, and pampas developed. Superimposed on
these general changes were particular events affecting
the environment. The uplifting of the Himalayas
and associated formation of the Tibetan Plateau
had a profound effect both directly by the increase
in the area of habitable upland, and indirectly by
modifying the pattern of the Monsoon winds affecting
the Asian continent. Further north, the Bering
Strait dried up from time to time, allowing free dispersal
of tundra and cool forest mammals, with the
result that the Holarctic mammalian fauna became
relatively homogenous.
The outcome of this complex oscillation of environmental
factors on the mammalian fauna was a
series of waves of interchanges and extinctions. An
enormous amount of detailed information is available
from the Pleistocene palaeoenvironmental and
fossil record (e.g. Potts and Behrensmeyer 1992;
LIVING AND FOSSIL PLACENTALS 287
Graham 1997; Agustí and Wenderli 1995; Agustí
and Antón 2002), and there is space here for no
more than a single taste of it. Of the 130 genera of
Early Pleistocene mammals in North America,
Webb (1985b) found that 40 of them were new. Of
these, 20 were immigrants, composed of 8 from
South America, and 12 from Asia via the Bering
land bridge; the latter notably included mammoths
and musk oxen. The other new genera, many of
them cricetid rodents, had presumably evolved in
situ. In similar vein, Maglio (1978) quotes figures
for the Early Pleistocene of Africa. Fifty-three new
genera appeared, of which about half were immigrants
from the north, including members of such
disparate taxa as bovids, primates, bats, pigs, and
rodents. Simultaneously, there were 33 generic
extinctions.
The end-Pleistocene: megafaunal extinction
The end of the Pleistocene was marked by the
abrupt extinction of many mammals, with a strong
bias against species of larger body size, the mammalian
megafauna (Martin and Klein 1984; McPhee
1999). The timing and severity of this extinction
varies from continent to continent (Fig. 7.28), and
its unusual features led to a controversy concerning
the cause that has continued for the last century
and a half (Grayson 1984). Was it due to the direct
effect of human over-hunting (Martin and
Steadman 1999), or was it due to the kind of environmental
change that caused all the mass extinction
events prior to this one (Webb 1985b). Leaving
aside the case of Africa and southern Asia, all mammals
of body size greater than 1000 kg disappeared
and about 80% of those weighing 100–1000 kg.
Africa is the most glaring exception, with only
about 10% loss of its mammalian megafauna; elephants,
hippos, and rhinos continue to represent
mammals of over 1 tonne body weight. Were this
continent to be considered in isolation, no particularly
significant increase in extinction would even
be noted at the end of the Pleistocene. Survival of
mammals over 1 tonne in weight also occurred in
southern Asia, but the fossil record is presently too
poor to be able to quantify the extinction pattern.
The timing of the extinction was also variable. In
North America it is well dated to within a narrow