The Ecology Book

220

Ordovician

Global cooling

leads to the

extinction of

85 percent of

marine life.

T

here have been five periods

in Earth’s history when

abnormally large numbers

of multicellular organisms have

died off in a relatively short time.

These mass extinctions are defined

by the loss of multicellular animals

and plants because their fossils are

far easier to detect than those of

single-celled organisms.

The general (“background”) rate

of extinction is between one and five

species a year. Fossil records show,

for example, the extinction of two to

five families of marine animals every

million years. This is far exceeded

during mass extinctions, which

always mark the boundary between

two geological periods. Scientists

do not understand all the factors

responsible for these events, though

they are agreed on some. Increased

MASS EXTINCTIONS

IN CONTEXT

KEY FIGURE

Luis Alvarez (1911–88)

BEFORE

1953 American geologists

Allan O. Kelly and Frank

Dachille suggest in their book

Target: Earth that a meteor

impact may have been

responsible for the extinction

of the dinosaurs.

AFTER

1991 The Chicxulub Crater

in the north of the Yucatan

Peninsula in southeastern

Mexico is proposed as the site

of a massive comet or meteor

impact at the end of the

Cretaceous period.

2010 An international panel

of scientists agrees that the

Chicxulub impact led to

the Cretaceous–Paleogene

mass extinction, around

65 million years ago.

The meteor that hit Earth at the end

of the Cretaceous period was traveling

at 40,000 mph (64,000 kph). Its power

was a billion times greater than the

Hiroshima atomic bomb.

Mass extinction events from

499 million years ago to the present

Late Devonian

A rapid drop in sea level

is one of a number of

possible causes for the

loss of 70–80 percent

of animal species.

Permian

Huge volcanic

activity helps to

wipe out 96 percent

of all marine species.

Triassic

Climate change or

an asteroid hit are

potential causes for

the extinction of round

75 percent of species.

Cretaceous

A meteor strike and

volcanic activity drive

up to 80 percent of

animals, including

most dinosaurs,

to extinction.

CRETACEOUS (K)

145–66

PALAEOGENE66–23 NEOGENE23–03

(OR ANTHROPOCENE) PERIODHOLOCENE

100,000 YEARS AGO–PRESENT

SILURIAN

444–419

DEVONIAN

419–359

CARBONIFEROUS

359–299

PERMIAN

299–252

TRIASSIC

252–201

JURASSIC

201–145

ORDOVICIAN

485–444

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