Geology Reference
In-Depth Information
to the Arctic when the climate became warmer. The second
problem is the apparent lack of correlation between extinc-
tions and earlier glacial advances and retreats throughout
the Pleistocene Epoch. Previous changes in climate were not
marked by episodes of mass extinction.
Proponents of the prehistoric overkill hypothesis ar-
gue that the mass extinctions in North and South America
and Australia coincided closely with the arrival of humans.
Perhaps hunters had a tremendous impact on the faunas of
North and South America about 11,000 years ago because
the animals had no previous experience with humans. The
same thing happened much earlier in Australia soon after
people arrived about 40,000 years ago. No large-scale extinc-
tions occurred in Africa and most of Europe because animals
in those regions had long been familiar with humans.
One problem with the prehistoric overkill hypothesis
is that archaeological evidence indicates that early human
inhabitants of North and South America, as well as Austra-
lia, probably lived in small, scattered communities, gather-
ing food and hunting. How could a few hunters decimate
so many species of large mammals? However, humans have
caused major extinctions on oceanic islands. For example, in
a period of about 600 years after arriving in New Zealand,
humans exterminated several species of the large, fl ightless
birds called moas.
A second problem is that present-day hunters concen-
trate on smaller, abundant, and less dangerous animals. The
remains of horses, reindeer, and other small animals are
found in many prehistoric sites in Europe, whereas mam-
moth and woolly rhinoceros remains are scarce.
Finally, few human artifacts are found among the remains
of extinct animals in North and South America, and there is
little evidence that the animals were hunted. Countering this
argument is the assertion that the impact on the previously
unhunted fauna was so swift as to leave little evidence.
The reason for the extinctions of large Pleistocene mam-
mals is still unresolved and probably will be for some time.
It may turn out that the extinctions resulted from a combi-
nation of different circumstances. Populations that were al-
ready under stress from climatic changes were perhaps more
vulnerable to hunting, especially if small females and young
animals were the preferred targets.
Chapter Summary
Cenozoic tectonism was concentrated in the Alpine-
Himalayan and circum-Pacifi c belts. Each belt is com-
posed of smaller areas of deformation called orogens.
The Cenozoic evolution of the North American Cordil-
lera included deformation during the Laramide orogeny,
extensional tectonics that yielded basin-and-range struc-
tures, extensive intrusive and extrusive igneous activity,
and uplift and erosion.
One model for the Laramide orogeny involves near-
horizontal subduction of the Farallon plate beneath
North America, resulting in fault-bounded uplifts in the
area of the present-day Rocky Mountains.
As the North American plate drifted westward, it collided
with the Pacifi c-Farallon Ridge, at which time subduc-
tion ceased and the continent became bounded by large
transform faults, except in the Pacifi c Northwest where
subduction continues.
Sediments eroded from Laramide uplifts were deposited
in intermontane basins in the Great Plains, whereas a
wedge of sediments pierced by salt domes is found on the
Gulf Coastal Plain.
Cenozoic uplift and erosion were responsible for the
present topography of the Appalachian Mountains. As
the Appalachians eroded, much of the sediment was
deposited on the Atlantic Coastal Plain.
Vast glaciers covered about 30% of Earth's land surface
during the Pleistocene. About 20 warm-cold Pleistocene
climatic cycles are recognized from evidence found in
deep-sea cores.
Large parts of North America and other Northern
Hemisphere continents were covered by continental
glaciers several times during the Pleistocene.
Cenozoic mineral resources include sand and gravel,
placer deposits of gold, some evaporite minerals such as
borax, and oil and natural gas.
Marine invertebrate groups that survived the extinctions
at the end of the Mesozoic continued to diversify, giving
rise to the present-day marine fauna.
The Paleocene mammalian fauna was composed of
Mesozoic holdovers and several new orders. During this
time of mammal diversifi cation, several orders became
extinct, but by Eocene time, most living mammal orders
had evolved.
Shrewlike placental mammals that evolved during the
Late Cretaceous were the ancestors for the placental
mammalian orders that evolved during the Cenozoic.
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