Britannica Student Encyclopedia - 2010

They live in water and are often covered

in scales. Most fish have a bony skeleton.

However, sharks and rays have a

skeleton made of cartilage, a strong and

flexible tissue.

Animals that live part of their life on

land and part of their life in water are

called amphibians. Frogs and salamanders

are examples of amphibians.

Most young amphibians live in water

and breathe with gills. As they grow they

form lungs and legs and move onto

land.

The first vertebrates to live completely

on land were reptiles—for example, lizards,

snakes, and crocodiles. Reptiles

have dry, scaly skin. Some reptiles, such

as turtles, spend a lot of time in water,

but they breathe air.

Vertebrates that have wings and feathers

are called birds. Most, but not all, birds

can fly.

Vertebrates that feed their babies with

milk from the mother are called mammals.

Mammals include mice, deer,

seals, whales, monkeys, and humans.

BRITANNICA STUDENT ENCYCLOPEDIA Animal 125

 

Mammals live on land in all parts of the

world. A few types, such as whales, live

in the ocean but still must breathe air.

#More to explore

Amphibian • Bird • Fish • Insect

• Living Things • Mammal • Mollusk

• Plant • Reptile •Worm

Animal Disease

#see Disease, Animal.

AnimalMigration

#see Migration, Animal.

Animals, Extinct

When an entire species, or type, of animal

dies out, that species is extinct.

Once a species becomes extinct, it is

gone forever.

Causes of Extinction

The most common cause of extinction is

a sudden, serious change in a species’

habitat. A habitat is the surroundings in

which an animal lives. Animals can

rarely survive such sudden change. Their

food supply may be wiped out. They

may also lose shelter or other things that

they need to survive.

Many things can change a species’ habitat.

Floods, fires, droughts, volcanoes,

and other natural events may be causes.

People also change the environment in

ways that drastically affect animals.

People clear forests and drain wetlands.

They build dams that disrupt the flow of

rivers. They build cities on land that

animals need to survive. They also create

harmful pollution.

Some changes that cause extinction

affect only a small area. Others are large

enough to affect the entire world. A fire

or other local event may cause the

extinction of animals that live only in

that region. A sudden change in the

global climate might wipe out an animal

species that lives in many parts of the

world.

People can cause extinctions more

directly as well. Some species have been

hunted to extinction. The passenger

pigeon is one example. Humans killed

millions of the birds over many years.

The last one died in the early 1900s.

Mass Extinctions

Sometimes many different species

become extinct in a short time. This is

A bird called the dodo once lived on an

island in the Indian Ocean. Europeans took

control of the island in the 1500s and

began to hunt the bird. By 1681 the dodo

was extinct.

126 Animal Disease BRITANNICA STUDENT ENCYCLOPEDIA

 

called a mass extinction. Several major

mass extinctions have occurred in the

past. Each time many animal species

were wiped out. Some survived, however,

and over millions of years new species

developed.

The worst mass extinction happened

about 248 million years ago. This

extinction included mainly animals

without backbones that lived in water.

Another mass extinction occurred about

65 million years ago. It wiped out many

of the planet’s land animals, including

the dinosaurs.

Dinosaur Extinction

Dinosaurs first appeared on Earth about

215 million years ago. They were the

most important land animals for more

than 150 million years. By 65 million

years ago, however, the dinosaurs had

died out.

Many scientists believe that a large asteroid,

or rock from space, caused this mass

extinction. When the asteroid hit Earth,

the impact caused drastic changes. Thick

dust and other materials blocked the

sun. Temperatures dropped, and plants

could not grow. The dinosaurs could not

survive the cold temperatures and lack

of food. But early species of birds and

mammals did survive.

Large Mammal Extinction

About 10,000 years ago another mysterious

animal extinction occurred. This

extinction was especially dramatic in

North America, where many large mammals

disappeared. Among them were

woolly mammoths and saber-toothed

cats.

Scientists do not know what caused this

extinction. Some believe that growing

numbers of human beings hunted and

killed too many of the animals or their

prey. Another theory is that the climate

changed and affected the availability of

food.

Endangered Species Today

In the past 200 years the world has lost

many animal species. Hundreds of others

are on the verge of extinction. Animals

at risk of dying out are called

endangered species.

Governments today are working to protect

the world’s endangered species.

Laws protect some animals’ habitats

from being polluted or destroyed. Other

laws make it illegal to hunt endangered

animals. Some species respond well to

these protective measures and increase in

number. Others are not as successful.

#More to explore

Animal • Dinosaur • Endangered

Species • Mastodon and Mammoth

The Carolina

parakeet and

the passenger