The Ecology Book

222

impact would have also released

sulfuric acid into the atmosphere,

which produced acid rain,

acidifying the oceans and killing

off marine life. Around the same

time, a huge amount of volcanic

activity flooded 193,000 sq miles

(500,000 sq km) of southern India

with lava, forming the Deccan

Plateau and further changing the

climate and atmosphere.

The K-Pg event is best known

for the extinction of all nonflying

dinosaurs. It was also responsible

for the death of nearly all four-

legged animals (tetrapods) that

weighed more than 55 lb (25 kg).

An exception were crocodiles,

which may have survived because

they are ectotherms (cold-blooded

animals), able to survive for a long

time without food. Dinosaurs were

endotherms (warm-blooded animals),

with a fast metabolism that

demanded regular meals. Many

plant species died because they

could not photosynthesize, leaving

herbivorous dinosaurs with little

vegetation to eat, while predatory

species starved for lack of prey. In

contrast, fungi, which do not depend

on photosynthesis, proliferated.

In the oceans, phytoplankton, a

vital food source that also relied on

photosynthesis, died out. Creatures

that fed on phytoplankton then

faced extinction. These included

cephalopods, such as belemnites

and ammonites, and the marine

reptiles known as the mosasaurs

and the sauropterygians.

Marine annihilation

The earliest mass extinction,

and the second-most catastrophic,

occurred when our planet cooled

MASS EXTINCTIONS

dramatically, toward the end

of the Ordovician period, around

444 million years ago. At this time,

most organisms on Earth lived in

the oceans. As the supercontinent

Gondwana drifted slowly over the

South Pole, a giant ice cap formed,

lowering global temperatures.

Much of the planet’s water became

“locked up” as ice, depressing sea

levels and reducing the area of

Earth’s surface covered by ocean.

As a result, marine organisms

living in shallow continental-shelf

water suffered particularly high

rates of extinction. In at least two

peak die-off periods, separated

by hundreds of thousands of years,

nearly 85 percent of marine species

died out, including brachiopods,

bryozoans, trilobites, graptolites,

and echinoderms.

Slow extinction

By the Late Devonian period,

around 359 million years ago, the

continents had been colonized by

Although many flying dinosaurs

survived the K-Pg mass extinction

at the end of the Cretaceous period,

all pterosaurs perished, ending their

162-million-year stay on Earth.

The current

extinction has its

own novel cause: not

an asteroid or a massive

volcanic eruption but

“one weedy species.”

Elizabeth Kolbert

American journalist

US_218-223_Mass_Extinctions.indd 222 12/11/18 6:25 PM