Page 176 - Introduction to Paleobiology and The Fossil Record

MASS EXTINCTIONS AND BIODIVERSITY LOSS 163
Extinction, long studied by paleontologists to
inform them of the past, is now a key theme
in discussions about the future. Will Cuppy,
the famous American humorist, was able to
talk about the extinction of dinosaurs, plesio- the 1980s onwards, and it has wide interdis-
saurs, the woolly mammoth and the dodo,
all of them icons of obsolescence and failure.
The dodo is perhaps the most iconic of icons
(Fig. 7.1), and it used to be held up as a
moral tale for children: here was a large
friendly bird, but it was simply too friendly
and stupid to survive. The message was: be
careful, take care, and don’t be as improvi-
dent as the dodo! The dodo is now an icon of
human carelessness rather than of avian
extinction.
The most spectacular extinctions are known
as mass extinctions, times when a large cross- change across this interval?”). The excitement
section of species died out rather rapidly.
There may have been only five or six mass
extinctions throughout the known history of
life, although there were many extinction
events, smaller-scale losses of species, often in
Figure 7.1 An image of a dodo from another
era. Lewis Carroll introduced the dodo as a
kindly and wise old gentleman in Alice Through
the Looking Glass, although at the time most
people probably regarded the dodo as rather
foolish. Driven to extinction in the 17th century
by overhunting, the dodo is now an image of
human thoughtlessness.
a particular region or involving species with
a particular shared ecology.
The serious study of mass extinctions is a
relatively new research field, dating only from
ciplinary links across stratigraphy, geochem-
istry, climate modeling, ecology, conservation
and even astronomy. The study of mass extinc-
tions involves careful hypothesis testing (see
p. 4) at all levels, from the broadest scale
(“Was there a mass extinction at this time?
Was it caused by a meteorite impact or a vol-
canic eruption?”) to the narrowest (“How
many brachiopod genera died out in my fi eld
section? Does their extinction coincide with a
negative carbon isotope anomaly? Do the
sediments record any evidence for climate
of studies of mass extinctions, and smaller
extinction events, is that these events were
hugely important in the history of life, and yet
they are unique paleontological phenomena
that cannot be predicted from the modern-
day standpoint. In practical terms, the fi eld
involves such a broad array of disciplines that
research involves teamwork, often groups of
five or 10 specialists who pool their expertise
and resources to carry out a study.
In this chapter, we will explore what we
mean by extinctions and mass extinctions,
and whether there are any general features
shared by these times of crisis. We shall then
explore the two most heavily studied events,
the Permo-Triassic mass extinction of 251
million years ago, and the Cretaceous-
Tertiary mass extinction of 65 million years
ago, in most detail. Finally, it is important to
consider how paleobiology informs the current
heated debates about extinctions now and in
the future.
MASS EXTINCTIONS
Defi nition
Extinction happens all the time. Species have
a natural duration of anything from a few
thousand years to a few million, and so they
live for a time and then disappear. This means
that there is a pattern of normal or back-
ground extinction that happens without any
broad-scale cause. In any segment of time,
perhaps 5–10% of species may disappear
every million years. In fact, more species have