life on earth on the land

Life on land10 ALITTLE BEFORE 400 million years ago plants began to grow on land.. egg-Although living things had been numerous and diverse in the waters of the world for hundreds of mil

ON THE LAND

LIFE ON EARTH

THE DIAGRAM GROUP

Copyright © 2004 by The Diagram Group

Written, edited, and produced by Diagram Visual Information Ltd

Editorial director: Denis Kennedy

Editors: Bender Richardson White, Gordon Lee

Contributor: John Stidworthy

Indexer: Martin Hargreaves

Art director: Roger Kohn

Senior designer: Lee Lawrence

Designers: Anthony Atherton, Christian Owens

Illustrators: Julian Baker, Pavel Kostal, Kathleen McDougall, Coral Mula, Graham Rosewarne

Picture researcher: Neil McKenna

All rights reserved No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher.

For information contact:

Facts On File, Inc.

You can find Facts On File on the World Wide Web at: http://www.factsonfile.com

Printed in the United States of America

EB Diagram 10 9 8 7 6 5 4 3 2 1

This book is printed on acid-free paper.

20 Snails and worms

22 Spiders and scorpions

24 Millipedes and centipedes

26 Invertebrates

AMPHIBIANS AND REPTILES

28 Frogs and toads

74 Primitive hoofed mammals

76 South American hoofed mammals

78 Odd-toed hoofed mammals

80 Even-toed hoofed mammals

4 Introduction

This bookis a concise, illustrated guide to living things

that evolved on, and now inhabit, the land Texts,explanatory diagrams, illustrations, captions, and featureboxes combine to help readers grasp important information

A glossary clarifies the more difficult scientific terms foryounger students, while a list of websites provides links toother relevant sources of additional information

Chapter 1, The Land, looks at the living conditions that

animals face on land, and briefly reviews the course ofevolution among land animals It also covers the topic of massextinctions throughout the Earth’s history

Chapter 2, Fossils, tells how fossils are formed, how they are

dated, and how some are important to us as fuels

Chapter 3, Animals without Backbones, gives an outline of

the main groups of invertebrates, both living and extinct, thathave taken to life on land These include snails, worms, andvarious groups of arthropods—animals with jointed legs

Chapter 4, Amphibians and Reptiles, looks at the evolution of

these two groups of vertebrates, with examples of theirmodern species

Chapter 5, Mammals, is the longest section, examining the

ancient history of mammals, and then taking a closer look atthe various main groups, or orders, that make up this

important group of land animals

Chapter 6, Birds, describes those birds that have forsaken

flying, to parallel the lifestyle of the running mammals

Chapter 7, Biomes, looks at the various main habitats on

Earth, and how living things are adapted to them, withexamples of characteristic species

On the Land is one of six titles in the Life On Earth series

that looks at the evolution and diversity of our planet, itsfeatures, and living things, both past and present

The series features all life-forms, from bacteria and algae totrees and mammals It also highlights the infinite variety ofadaptations and strategies for survival among living things,and describes different habitats, how they evolved, and the

communities of creatures that inhabit them Individual

chapters discuss the characteristics of specific taxonomic

groups of living things, or types of landscape, or planetary

features

Life On Earth has been written by natural history experts,

and is generously illustrated with line drawings, labeled

diagrams, and maps The series provides students with a solid,

necessary foundation for their future studies in science

6

THE MAIN CONTINENTAL BLOCKS have

not always been arranged as they aretoday Over millions of years, slowgeological processes have gradually shifted the

“plates” of the Earth’s crust that carry thecontinents At various times in the past theycame together in different ways For example,Australia, Antarctica, and South America wereonce connected The geography of the pasthad great influence on the evolution of variousanimal and plant groups, and governed theirability to spread

Continental movements led to collisions thatsometimes crumpled the edges of the main continental blocksslowly over millions of years The Himalayas are the result ofIndia moving up from the south, and colliding with the mainblock of Asia The Andes have been thrust up where the PacificOcean plate meets South America The Himalayas and Andesare “young” mountains, and contain many of the world’s

Land makes up about 30 percent

of the Earth’s surface It is a vast

area of 57.5 million square miles

(149 million sq km) Two-thirds of

the land area in the present world

is in the Northern Hemisphere,

with Australia, most of South

America, part of Africa, and some

outlying islands of Asia in the

O N T H E L A N D T H E L A N D

highest peaks Older mountains, such as those in

Scotland, have been worn down over hundreds of

millions of years, and are relatively low

The average height of the continents above sea

level is about 2,756 feet (840 m), but there is a huge

variation in height from the tallest mountain,

Everest, at 29, 140 feet (8,882 m), to some parts of

the land, such as the shore of the Dead Sea, that

are as much as 1,299 feet (396 m) below sea level

The land contains some areas, such as parts of

Australia, or Eastern Europe, with flat plains

stretching far and wide The enormous variation in

landforms entails a similar variation in the

adaptations of the animals that live there This is

one reason for the existence of a huge range of

animals in the modern world, and throughout the

many millions of years that animals have lived on

1 Tropical forest biome

Found near the equator where conditions are warm and very wet.

2 Desert biome

Very dry, often hot, and with few plants Covers one fifth of the land.

3 Coniferous forest biome

Forests with long winters and short summers.

4 Grassland biome

Warm or temperate, but with not enough water f0r trees to grow.

5 Temperate forest biome

Temperate, with enough water for tree growth.

Many trees drop their leaves in winter.

6 Tundra biome

Frozen for much of the year with dwarf plants.

b

Climatic zones

8

BECAUSE OF THE TILT of the rotating Earth, the

North and South Poles are plunged intodarkness in the middle of their winters Thecirculation of winds and ocean currents also affectsclimate, but the basic pattern is clear The tropics arevery warm throughout the year At higher latitudesthere is a temperate zone, with warm summers butcooler winters At the highest latitudes, near thePoles, there is a cold climate all year with, at most, abrief summer during which some ice may melt

It is not surprising that, in general, life is moreabundant and varied in the warm parts of the world,

The world has a series of

marked climatic zones.

Equatorial regions are

warmed by sunlight

throughout the year.

Temperatures are lower

toward the Poles, where

sunlight has to penetrate a

thicker layer of atmosphere

before reaching the surface.

Temperature belts in

January and July

Seasonal temperatures differ

more the greater the

distance from the equator.

(Temperatures are given

below in both Fahrenheit

and Celsius.)

Below -30 0

F -34 0

c -30 0

F

January

July

as long as there is water available The icy wastes of

Antarctica are least likely to support life, but there are

no places on the Earth entirely devoid of living things

In mountains, temperature drops with height, so

different climatic zones are found at different heights

On the highest mountains the summits tend to have an

Arctic feel

The world can be divided into a number of “biomes,”

defined by climate and rainfall Each has its own typical

vegetation and animal life, although the species may

not be the same on different continents

But it was not always warm in the past There is

evidence of great ice ages 445 million years ago, and

again about 300 million years ago It is only recently,

geologically speaking, that the world emerged from an

ice age Ice cover disappeared from North America

about 11,000 years ago Some people believe that we

are now in a short, warm period within this ice age

It cannot be assumed that today’sclimates are typical ofthe past For much ofthe Cretaceous period(144 to 65 millionyears ago) the Earthhad a warm climate.Even near the Poles itwas warm, so thattemperatures were farmore even across theworld than now Thiswas the heyday of thedinosaurs and

pterosaurs, and theylived from the equator

to the Antarctic, eventhough the polarwinters must havebeen dark

9

Life on land

10

ALITTLE BEFORE 400 million years ago plants

began to grow on land Their ancestors wereprobably green algae, but some of these newland plants developed water-conducting tissue and,soon after, supporting tissue They could then growupward, rather than just form a crust or flat carpet.Arthropods, the jointed-legged animals with hardouter skeletons, were the first land animals This is notsurprising, as their skin/skeleton could support themout of water, and may also have provided someprotection against water loss Their breathingapparatus could be adapted to breathing air

Some of these early land arthropods, such asscorpions, were surprisingly similar to types still livingtoday Few of the new land animals were adapted foreating plants directly, although some could feed on decayingvegetable matter Most seem to have been predators

Millions of years after these first colonists of the land, somefishes took their first steps as tetrapods, four-legged

“amphibians.” It was many more millions of years before theamphibians familiar to us today (frogs and salamanders)evolved Before these arrived on the scene, some tetrapodsbecame able to breed on dry land By 300 million years agothese so-called amniotes gave rise to others—reptiles and thesynapsid forerunners of mammals

Mammals originated from synapsids at the time ofthe dinosaurs The earliest mammals did not

belong to groups you could see today

The main groups of mammal we

recognize now—the laying monotremes, themarsupials, and placentalmammals like us—did notappear until much later

egg-Although living things

had been numerous and

diverse in the waters of

the world for hundreds

of millions of years, there

is little evidence of life on

land before about 400

million years ago With

its bare, weather-beaten

surface, the land must

originally have been a

challenging place to live.

First colonists of the land

Our early mammal

ancestors lived in the

shadow of the dinosaurs.

PermianTriassic

No life on the land

No life on the land

Proterozoicperiods

Period Millions of

years ago

Who lived at that time?

Omithischian Bennettitalean

Mass extinctions

12

NON-BIRD DINOSAURS disappeared completely

65 million years ago Not only dinosaurs, but alsoother reptiles, such as plesiosaurs in the sea andpterosaurs in the air, vanished Many fishes becameextinct, along with many invertebrates Scarcely anyland animals bigger than wolf-size survived Thedisappearance of so many kinds of animals at the sametime is known as a mass extinction

Why did so many animals disappear? We do not knowfor sure, but there have been many theories It appearsthat a large asteroid hit the Earth at about this time, atwhat is now the coast of Mexico It would have thrown

up vast clouds of dust, and produced heat and fires The Earth’sclimate was probably disrupted, possibly for many years

On the other side of the world, in India, at about the sametime, there was a huge outpouring of lava that covered manythousands of square miles (sq km), also affecting the climate.Even before these events though, dinosaurs were declining.Numbers had been dropping for 20 million years or so Someshellfish also disappeared millions of years before the layer ofrock that marks the “mass extinction.” Climates had beenchanging as sea levels dropped, making continental interiors

For about 150 million

years, the dinosaurs were

the dominant large

animals on land Diverse,

adapted for many ways

of life, and advanced for

their time, they seemed

destined to rule the

drier and harsher, and

creating land that animals

could cross between formerly

separate areas, increasing

competition Perhaps the meteorite strike was

a sudden event that provided the final blow to

the declining groups of animals

The mass extinction that saw the end of the dinosaurs

was not the only one in the Earth’s history, nor was it

the most catastrophic in terms of the percentage of

animals wiped out From the rocks laid down

hundreds of millions of years ago come tantalizing

clues about factors that may have caused

the extinctions, but from this distance

in time we will probably never be sure

What was affected?

Enormous cut in diversity of sea life

Trilobites; many kinds of fish; sponges

9o percent of species lost, including the last trilobites

65 percent of marine species lost; over

30 percent of land vertebrates; most land plants

All non-bird dinosaurs; other large reptiles; many

65 million years ago.

Becoming a fossil

14

OCCASIONALLY, conditions are just right for the

remains of animals and plants to be preserved

in rocks Even then, they may be destroyed bygeological processes, such as erosion, on the surface.But some fossils remain preserved in rocks for millions,

or even hundreds of millions, of years, and may be dug

up to give us an insight into life long ago.Small, soft-bodied animals stand leastchance of preservation Animals inwater may die, fall to the bottom, and

be buried in mud Their skeletons orshells may not decay Coveredeventually by a great thickness ofsediment, new minerals maygradually replace bone or shell,making a hard replica of the original.Sometimes the original hard partsare instead dissolved away by acidsseeping through the sediment, leavingjust a hole, but one that retains the shape

of the animal that was formerly there

On land, becoming a fossil is even rarer than inwater Many land animal fossils are preserved becausethey died in, or near, water and have been carried into mud andpreserved On land, most animals are consumed, or decayrapidly This is particularly true of forest animals Sometimesburial by wind-blown sand or dust, or covering by volcanic ash,

Layers of mud cover the animal.

Layers of mud are crushed into hard rock.

The rock wears away and the fossil

is exposed.

The great majority of

animals and plants that

die do not become fossils.

They are eaten by other

organisms, or decay

away completely.

Amber (above)

Millions of years ago, this

fly was trapped in resin

and thus preserved.

can lead to fossilization Small animals can be

trapped in tree resin The resin itself becomes

fossilized as amber, with insects or spiders

trapped within it

Relatively recent animals, from thousands

rather than millions of years ago, may also be

preserved by mummification in a dry climate,

or by freezing in a cold one

15

O N T H E L A N D F O S S I L S

Fossilized tetrapods in New Mexico

These “amphibians” died 200 million years ago, and

were preserved in the mud of a drying-up pond.

STRANGE BUT TRUE

It is not justanimals’

bodies thatbecomefossils

Burrowscan also bepreserved inrocks Some

of the earliest land fossils areburrows It is not always easy toguess what made them

Footprints and trackways canalso be preserved in mud Ifthey can be matched to aparticular animal, we can learnhow, and sometimes how fast,

an animal moved millions ofyears ago Fossil droppings,called coprolites, are also found.Again, if their maker can beidentified, it may provideinformation about eatinghabits Even eggs and nests can

be buried in the sand tobecome fossils

Dating fossils

16

SOMETIMES, areas

of rock aredeformed or foldedupside-down over acertain area, but, as long

as geologists recognizewhat has happened,older and younger layerscan be traced with precision

From studies over large areas of theworld, a picture of the sequence of rocksthrough the ages was built up The major geologicalperiods, such as the Cretaceous, were recognized by therock formations laid down in them It was possible to makeestimates of the time it took for the huge thicknesses ofsediments to be laid down, and so calculate the probable datesand duration of these periods

Widespread animal species that existed for a limited timeaid the recognition of sediments of the same period, even if farapart Detailed detective work allowed scientists to put mostrocks they found in sequence But until the properties ofradioactive elements were discovered, there was no way ofputting a date on a rock with any degree of certainty Usingradioactive isotopes, absolute dating is possible, at least forigneous rocks (those formed by volcanic activity)

Minerals in these rocks may contain a radioactive elementsuch as uranium-235 Atoms of this element “decay,” losingpart of their nucleus, and turn into lead-207 at a steadyrate Half of a sample of this uranium will turn into lead

in 713 million years If you measure the relativeamounts of uranium-235 and lead-207 in the rock,you can establish an absolute measure of how

The simplest way of

establishing when a fossil

lived is to examine the

layer of rock in which it

was found Layers of

sedimentary rock lie on

top of one another In

undisturbed rocks, upper

layers are younger than

lower ones.

Dating fossils

Layers in sedimentary rocks allow us to tell the relative dates of fossils Lower layers are older than upper ones.

Archaeopteryx

This fossil comes from the Jurassic period (150 million years ago).

Fallen rocks

much time has elapsed since the rockwas originally formed.

However, measurement andcalculation is rarely as easy as thissuggests Different radioactive elementshave different “half-lives” of decay, andare useful for different periods of theEarth’s history Where igneous rocksoccur between layers of sediment, theycan give a date to the sediment

Nowadays, the ages of enough rockshave been calculated for us to have aclear idea of the likely date of mostfossils, although there may still be roomfor some adjustments

34–23.8 55–34 65–55

2 1

Different aged strata (left)

By virtue of the relative position of strata, the comparison of fossils, and radiometric dating, the ages of strata, and their position in the geological timescale, have been established.

By radiation potassium-40 loses half its mass every 1,310 million years (one half-life) Thus a sample‘s potassium-40 content can indicate its age,

A Original sample

B After 1.3 billion years (one half-life) half remains.

C After 2.6 billion years (two half-lives) one quarter

Fossil fuels

18

CONDITIONS HAVE NOT always been

suitable for the formation of fossil fuels.Most of the deposits come from just afew of the many geological periods Nodeposits of significance are being laid downnow Even though there are vast quantities inthe ground, they are being used A time will

come when it is no longerpossible to power civilizationfrom fossil fuels

Much of the best coal wasformed over 300 million yearsago A geological period,known as the Carboniferous(354–290 million years ago),was named because of theabundance of carbon—coal—

in its rocks During thisperiod, plenty of swampforests grew on low ground,with trees up to 100 feet (30m) tall They were clubmosses and horsetails Theirpresent-day relatives aremuch smaller When theydied, they fell into acidicwater and were partlypreserved, instead ofdecaying away

Coal, and underground oil and gas

deposits, are all fossil fuels They

formed long ago from the remains

of dead organisms These remains

were concentrated into a form

which is useful to people.

Legacy

The swampy forests of 300 million years ago are the source of much of the good quality coal in use today.

From time to time, seawater flooded some of the

low-lying forests, and sediment was dumped on top of the plant

remains, compressing them Then the water level dropped

again, plants grew, and the cycle repeated itself Most of the

time there was forest, with briefer periods when sediment

was dumped Even so, the coal seams are thin compared to

the sediments surrounding them

Oil is also formed from the remains of living things Often

these were tiny sea organisms that died and accumulated

at the bottom of still water The carbon compounds in their

bodies seeped down and were trapped in the rock

Deposits of natural gas are often found above oil

reserves, derived from the same creatures Particular

limestone formations are associated with oil, as are some

salt deposits More than half of the world’s oil—in the rich

deposits of the Middle East and the Gulf of Mexico—

started forming during the second half of the time of the

dinosaurs Texan oil comes from more than 100 million

It is estimated that athickness of about

100 feet (30 m) ofplant remains must

be compressed toprovide a 3 foot (1 m)seam of coal Itwould have taken5,000 years, or evenmore, to producethese plants

Where they are

This map shows some of

the main areas in the

world where fossil fuels

are found.

 ?

DID YOU KNOW?

Most earthworms are 12 inches (30 cm) long or less, but in western NorthAmerica there are bigger species In SouthAfrica and Australia there are worms thatgrow to over 10 feet (3 m) in length!

20

SOMETIMES TRACES OF THE soft bodies of worms, or

fossils of hard jaw parts or burrows, are found Butnearly all are remains of marine worms Land worms areeven less likely to become fossils Gastropod mollusks (snails)are known from over 500 million years ago, but these firstsnails were sea creatures Far from fading out, snails havebeen increasingly successful over the last 100 million years or

so Shells fossilize well, but most gastropod fossils are fromeither sea or fresh water A few are land snails related to thosefound in our gardens

Earthworms are the most familiar worms on land Theyburrow in soils all over the world They have bodies dividedinto rings called segments, sometimes as many as 200 ormore Many segments are similar, but the front segments

contain the mouth and small “brain,” hearts,and reproductive organs Most worms eatpieces of dead plants in the soil, or rottingleaves brought from the surface Their

Worms are some

simple, segmented body

plan, but are still

successful soil animals.

Mouth

Segments

Anus

Body wall

activities in burrowing andchurning the soil help toenrich and aerate it Worms are also an important food formany other kinds of animal.

Even more common than earthworms, but far less visible,are roundworms These have a totally different body plan,without segments, and with smooth tough skins They arepointed at both ends, and the outside of the body is almostfeatureless Although some are large, most are tiny or evenmicroscopic Some live freely in the soil,

but many live as parasites inside thebodies of other animals and plants

Roundworms are largely hidden

Snails are also unsegmented A snailmoves on a large muscular “foot.” Glandsproduce slime to smooth its way Thedigestive system and other organs aretucked inside a hard, coiled shell Thesnail retreats into this for protectionfrom enemies The head bears senseorgans In the mouth is a tongue coveredwith many horny teeth that the snail uses to rasp at plants as it feeds

21

Like most mollusks, snails have a complex internal structure (above), and shells made of calcium carbonate (below).

Lung

Digestive gland (liver)

 §

22

THE FIRST FOSSILS of actual animals date from as

much as 415 million years ago, and include a

scorpion and a tiny arachnid named Eotarbus This

was not in fact a spider, but it did resemble present-dayspiders and mites

Around 395 million years ago, a place called Rhynie inScotland was full of volcanic springs, around which grewminiature forests of primitive plants They, and the animals

living in them (including mites and relatives of Eotarbus),

became fossilized In some, so-called book lungs arepreserved; these are folded structures that are found in livingscorpions (and some spiders) They are used for breathing air,and were probably modified from gills of aquatic ancestors.The earliest known “true” spider was found in rocks fromthe eastern United States about 375 million years ago It hadfangs with poison glands, and also a spinneret—an organ thatfeeds out the silk of the web It looks as though, even then,spiders were spinning webs or trap lines

Spiders, scorpions, and mites are all arachnids Theseanimals have segmented bodies (though the segmentation is

not obvious in many spiders) and eightwalking legs In front of these, other pairs

of legs are turned into jaws In scorpions

they include a pair of largepincers for seizing prey In

Tracks and burrows

suggest that there may

have been animals on

land as long as 450

million years ago.

Eotarbus (above)

This was one of the first

creatures to inhabit the

land about 415 million

years ago.

Bird spider (below)

This bird-eating spider

measures about two and

a half inches (6 cm) in

length, and comes from

Panama.

 §

spiders there are downward-stabbing fangs in the moreprimitive types, or pincerlike fangs in the more advanced Allspiders produce venom from their fangs, but although lethalfor small prey, few kinds are dangerous to humans Scorpionshave a venom gland attached to the sting at the tip of the tail

Again, the venom kills small animals, but few scorpions are athreat to us

Both spiders and scorpions have a long history, but nowadaysthere are only about 1,200 kinds of living scorpion More

than 35,000 kinds of spider have been named

There are other types of arachnid including “falsescorpions,” sun spiders, and at least 30,000 kinds ofmite There are probably many more tiny mites inexistence, many living on, or else inside, other livingorganisms, but we have a tendency to notice

mainly those that causediscernible diseases inboth domestic animalsand plants

23

Dust mite (right)

Although this creature is microscopic in size, some people are allergic to it.

O N T H E L A N D A N I M A L S W I T H O U T B A C K B O N E S

Scorpion (left)

While seizing prey with their pincers, scorpions may also use the sting in their tail

to subdue it. False scorpion

These are tiny arachnids that are found in leaf litter.

Millipedes and centipedes

24

ON LAND, fossil burrows from 450 million

years ago have been interpreted asbelonging to millipedes, and pellets ofplant remains from 410 million years ago could bemillipede droppings Remains of centipedes fromover 400 million years ago, and millipedes from atleast 375 million years ago, confirm them as among the

earliest-known land animals Giant relatives of millipedes 6feet (1.8 m) long and 20 inches (50 cm) wide trundled acrossthe land 315 million years ago, leaving marks resemblingrailway tracks

Both centipedes and millipedes have a head with specializedsegments carrying jaws and sense organs, but behind the headthe segments are mostly very similar to one another, and bearpairs of walking legs These animals hatch from an egg withonly a small number of segments, and add more until theyreach adulthood

With a few exceptions, the 10,000 or so species of millipedeare plant eaters, feeding on dead or dying vegetation Theytend to tunnel into the earth, or else push through theleaf litter Even after millions of years of evolution, theyhave not developed a waterproof coat to the skin, andare usually found in damp places Most millipedes have simple

Centipedes and millipedes are

ancient animal types, whose

relatives lived in the sea 500

million years ago.

For posterity (above)

An ancient millipede is

trapped in amber.

Arthropleura (above)

This millipede lived 300

million years ago, and

was 6 feet (2 m) long. Millipede head (below)

Jaws that have adapted for nibbling plants remain.

Walking legs Antenna

Millipede head

Eye

mouthparts adapted for

chewing bits of plant, although

some tropical species can suck

plant juices Their eyes are simple,

and it is doubtful if they can form images

Feelers are used to touch and scent the

surroundings Many millipedes can repel

attackers with foul-tasting or poisonous fluids from

glands along the side of the body Although most

millipedes are inconspicuous, some tropical species

grow to 10 inches (25 cm) or more

Centipedes are carnivores, and feed on insects and

other small animals They have jaws with poison

fangs Some of the largest tropical species, 13 inches

(33 cm) long, can give a nasty bite to a human, or kill

a small lizard or a mouse Food is mainly detected by

touch through the

antennae and legs In

contrast to

millipedes, which

have no need to

move fast, centipedes

have longer legs, and

move quickly to hunt and

seize prey There are about

Centipedes can live six years

or more, and millipedes livemore than ten

A different view (left)

The underside of this millipede shows its large number of legs Poison claw

Giant African millipede (below)

Although huge in comparison with other species, this

is a harmless plant eater Centipede

26

INSECTS ARE THE MOST SUCCESSFUL of all

land arthropods in terms of numbers andvariety of ways of life Most fly well, butseveral groups of primitive wingless insects areearthbound Some insects have become winglessbecause they have no need of flight, as they liveinside plants or burrow in the ground Fleas, whichare parasites on other animals, are wingless

There are other arthropods living on land, but none rival theinsects in number Crustaceans, such as shrimps and crabs, areextremely successful in the sea, but few of the 50,000 specieslive on land Some crabs do so, but most of these live nearcoasts and return to the sea to spawn The coconut crab of thePacific islands is a spectacular creature Related to hermit crabs,

it climbs trees and can open coconuts

Like spiders, millipedes and

centipedes, insects have jointed

legs Invertebrates with such

legs are known as arthropods.

Woodlice are successful land crustaceans They belong to a

group called isopods Most isopods crawl or swim in the sea,

where one species reaches 17 inches (42 cm), but most woodlice

are only o.4 inch (1 cm) or so long Woodlice have gills, but

many also have breathing tubes like those of insects These

help reduce water loss in breathing, but most woodlice still live

in damp places and are active at night, as their skins are not

waterproof Woodlice feed on dead and living plants

Bacteria in the gut help to break this food down

There are only 80 species of velvet worms

Attractively colored in blue, green, or orange,

and with a velvety sheen, they are

nocturnal and shun strong light They

live on forest floors and other damp

places in the tropics Few are more

than 6 inches (15 cm) long They walk

using a pair of legs on each body

segment, and prey on small animals

The jaws and antennae are

reminiscent of insects and other

jointed-legged animals, as is the habit

of molting the skin periodically They

have breathing tubes and a blood system

like that of an insect On the other hand, their

legs are not jointed and much of the body is rather

like that of a worm At one time, they were thought

to be intermediates in evolution between worms and

insects, but their exact relationship is uncertain Early

velvet worms lived in the sea 500 million years ago

of the head of a bloodsucking flea.

Frogs and

toads

28

THE SKULL, HEAD, AND MOUTH of a frog or toad are

relatively large The back legs are long, and most speciescan straighten them suddenly to leap The body is short,with nine or fewer vertebrae, no ribs, and hip bones that arelarge and rigidly joined to the backbone These are

adaptations for jumping The shorter front legs act as shockabsorbers Frogs are meat-eaters The large eyes help them tofind prey Muscles also pull the eyeballs down to help in theprocess of swallowing

The earliest known froglike amphibian is Triadobatrachus,

from 220 million years ago Soon after, frogs took on theirmodern shape Their great success, though, seems to havecome in the last 65 million years There are about 3,500 species

of living frogs and toads They range in size from adults onlyo.4 inch (1 cm) long to the Goliath frog of West Africa, whichcan grow up to 14 inches (35 cm) long

Some frogs and toads burrow Others live on the surface

Some are good tree-climbers, andothers always live in, or around,water All have moist, scale-lessskins, one reason why most live indamp places and are active atnight Many have glands in theskin that secrete substances thatdeter attackers In some cases, as

in poison-arrow frogs, the secretion is deadly poisonous

Both frogs and toads have

unusual shapes for a vertebrate.

Cane toad (right)

One of the largest land amphibians, it

is now a major pest

in Australia.

Skeletons (above)

Frogs have a very short

backbone The fossil (top)

shows the beginning of

Amphibians typically lay eggs in water These are fertilized

externally and hatch into aquatic larvae with gills These later

turn into adults that live on land, breathing air with lungs and

through the skin Some may lay as many as 10,000 eggs,

allowing for great losses at the egg and tadpole stage But there

is a surprising variety in frog reproduction Some poison-arrow

frogs guard their eggs on damp ground until they hatch, then

carry them to water attached to their bodies Other frogs beat a

body secretion into a mass of foam on a branch above the water

and deposit their eggs in it The tadpoles later fall into water Yet

other frogs carry eggs or young in skin pouches of various types

Surprisingly, a fifth of all frogs lay eggs on land that develop

straight into little frogs

29

O N T H E L A N D A M P H I B I A N S A N D R E P T I L E S

Poison-arrow frog (left)

This creature transports its tadpoles on its back.

Bug-eyed tree frog (left)

Large toe-pads and big eyes are adaptations that help tree frogs to climb more easily.

 §

STRANGE BUT TRUE

Darwin’s frogswallows eggsinto its vocal sacand keeps theminside until theyhave developedinto adults

30

IN GENERAL, SALAMANDERS probably look much

like some of the earliest tetrapods hundreds ofmillions of years ago In many ways, though, theyhave changed compared to these remote ancestors.Only about 350 species of salamander are alivetoday, nearly all living north of the equator They aremost abundant in North America Tennessee is said

to have more kinds than the whole of Asia andEurope Some salamanders have reverted to apermanently aquatic life, but most are land animals,

6 inches (15 cm) long or less They feed on small preysuch as insects, worms, and slugs Because of their thin, moistskins, salamanders live in damp places Many spend a largepart of their lives doing nothing, hiding in burrows or understones or logs

Salamanders and newts

are amphibians with long

bodies, long tails, and small

heads They have two pairs

of short legs sticking out

sideways from the body.

They walk flexing the body

from side to side.

Terrestrial salamander (above)

This salamander lives on the ground and also in burrows.

Newts move to water in spring, where they court and mate

before laying eggs that hatch into larvae These feed in water

before changing into the adult form Adults spend most of

their time on land A few salamanders lay eggs on damp land

or even produce living young

On land, salamanders and newts breathe through the skin,

the mouth lining, and the lungs Lack of lungs might be

thought a handicap in an air-breathing animal However, the

200 species in the biggest family of salamanders are entirely

lungless They are mostly small,

and absorb enough oxygen

through their skins Many

are skilled hunters, and

can shoot out a long

tongue to trap an insect

The caecilians are

strange amphibians, close to

neither salamanders nor frogs

They burrow through the soil, have no limbs,

and have very reduced eyes They look like earthworms,

complete with rings, and feed on worms and insects In size

they range from 4 to 28 inches (11 to 70 cm) They have very

solid skulls, which help them push through the soil Many

aspects of their lifestyle remain unknown

Taking it easy

Salamanders have an inconspicuous lifestyle but, in some places, make up a large part

of the local animal life.

ONE GROUP OF “TETRAPODS,” including

Seymouria and its relatives, had strong legs that held the body off the ground Seymouria was a

24-inch-long (60 cm) animal that lived about 280million years ago Some aspects were quite reptilian; infact, they were once thought to be reptiles However,the skull still had lateral line grooves In fishes andamphibians these hold sense organs for feeling movement inwater Fossil aquatic larvae have also been found, so thesewere not yet reptiles, but possibly the group from which thereptiles evolved

The next great innovation was the shelled egg, allowingreptiles and synapsids to be full-time land animals The shell,either hard or leathery, gave the embryo its own private pod inwhich to develop Membranes surround the embryo to

protect it, and allow it to breathe Unfortunately, fossileggs and skin, characterized by scales, are rare Thismeans that we have to distinguish between fossilreptiles and non-reptiles by their bones and, inparticular, the skull

The earliest animal recognized as a reptile is a little

creature called Hylonomus that lived about 310

million years ago in what is now Nova Scotia.About 8 inches (20 cm) long, it was shaped like

a lizard, although it was not closely related Itssmall head had the small sharp teeth of an

Soon after tetrapods

became established on

land, some became even

better at living on land

than in the water.

Hylonomus

This was one of the

very first reptiles.

Weighing as much as a cow, this

was a very early plant-eater.

First inhabitants

Pareiasaurs, up to 5 feet (2.2 m) l0ng, were some

of the first large eaters on the land.

plant-The pelycosaur group

This group of synapsids included both

plant-and meat-eaters Meat-eaters such

as Dimetrodon had long skulls, and their

teeth were adapted for biting They had

long fanglike teeth at the front of the jaw.

Plant-eaters, such as Cotylorhynchus, had

short jaws with peglike teeth all round.

Some pelycosaurs grew more than 10 feet

(3 m) long These early synapsids died

out well over 250 million years ago.

Dimetrodon (left)

This creature had teeth specially adapted for catching prey.

How did pelycosaurs without sails manage?

Pareiasaurus

insect-eater After this reptiles evolved

rapidly Some remained small, while

others were bulky animals Many stayed

as insect or flesh-eaters, but others had

teeth that were adapted for eating plants

Pareiasaurs were heavy, with massive

legs to support their weight They

reached 8 feet (2.5 m) long or more They

had leaf-shaped teeth, like modern

plant-eating lizards

Rise of the dinosaurs

34

DINOSAURS rapidly became the

dominant large land animals, andremained so until 65 million yearsago when, with the exception of birds,they became extinct During that timemany different dinosaurs evolved anddisappeared, succeeded by new kinds They had

different body shapes, different sizes, andadaptations to many different ways

of feeding But the thing thatprobably began their success wasquite simple The dinosaurs had athighbone with a ball-like

projection sticking out at anangle at the top This fitted snuglyinto a socket in the hip bones, andallowed them to use their legs in avertical position right underneath the

Coelophysis (right)

This was an early, small, bipedal dinosaur.

Tyrannosaurus rex (above)

This was a huge,

meat-eating dinosaur Scientists

differ over how fast it

could move.

Dinosaurs evolved about 235 million

years ago, probably from small

reptiles like Euparkeria Little over

20 inches (50 cm) long, this predator

had teeth in jaw sockets, which was a

standard feature for dinosaurs.

Adapted for feeding

Camptosaurus

and its relatives may have had tongues that could pull plant food into their mouths.

body They could easily be swung back and forth

when walking or running In most reptiles the legs

stick out in a sideways position

These improved legs were also good at supporting

weight Some of the dinosaurs were much heavier

than any land animals before or since Others were

small, lightly built, and good runners Many of the

earliest dinosaurs had hind limbs bigger than the

front ones, and were undoubtedly bipedal Later,

heavier dinosaurs tended to support their weight on

four limbs, but many dinosaurs remained bipedal The

huge meat-eater Tyrannosaurus rex, weighing six

tons, had tiny arms and was bipedal

Most scientists recognize two divisions within

the dinosaurs The first includes the

“lizard-hipped” dinosaurs, or saurischians The earliest

known dinosaurs belong to this group

Saurischians include various carnivorous

types, and also one major group of plant-eaters, the

sauropods These had huge bodies on elephantlike

legs, and long necks and tails Their heads were

comparatively small They tore off plants with their

teeth then swallowed them into a gizzard, where they

were ground with stones, as in a bird Sauropods

included such familiar dinosaurs as Diplodocus and

IT’S A FACT

For many years

Brachiosaurus was the

largest known dinosaur,

at up to 90 feet (27 m)long and 80 tons(tonnes) in weight—asmuch as 20 largeelephants Now bones

of even larger sauropodsare known, such as

Argentinosaurus, maybe

115 feet (35 m) long and

up to 100 tons (tonnes)

in weight Seismosaurus

was even longer, but not

so heavily built Mostgiant dinosaurs are onlyknown from a fewbones, so relative size isestimated by making

a comparison withbetter-known animals

Dinosaurs in variety

36

THE BIRD-HIPPED DINOSAURS were, with no

known exceptions, plant-eaters They were themain herbivores toward the end of the dinosaurage, when the giant sauropods had disappearedfrom many parts of the world The ornithischians

include familiar animals such as Stegosaurus, Iguanodon, and Triceratops.

Iguanodon was one of the most widespread

dinosaurs, and its remains have been found in NorthAmerica, Europe, and Asia It had a toothless beak,but many cheek teeth to break up its food The warmclimate and plentiful food allowed herds to flourish.Many of the plant-eaters, from the giant sauropods

to Triceratops, seem to have been herd animals that

found safety in numbers

Some duck-billed dinosaurs were able to deal withreally tough food such as pine needles Behind thetoothless bill was a huge battery of grinding teeth.New teeth constantly grew from below, replacing theones that ground down A duckbill could have 2,000teeth, the highest number in any backboned animal

A surprising amount is known about breeding in one

The dinosaurs that are not

“lizard-hipped” are

“bird-hipped” or ornithischian.

Iguanodon

Maiasaura

This creature laid eggs

in mud nests within

a protective rim.

duckbill, Maiasaura A whole fossilized breeding colony

was discovered with eggs and young in mud nests

shaped like little volcanoes

A large number of ornithischian dinosaurs were

ankylosaurs Not as large or spectacular as some

dinosaurs, they were nevertheless successful animals

They had small teeth and probably fed on soft plants

Their notable feature, and probably what made them

successful, was body armor Bony plates covered their

backs and bony spikes, covered with horn, added extra

protection Some had a bony club on the end of the

tail as an extra defense

The bird-hipped plant-eaters were hunted by

meat-eaters, all of which were lizard-hipped dinosaurs They

ranged from the enormous Tyrannosaurus rex to fast

runners such as Velociraptor and Deinonychus The

latter was about 10 feet (3 m) long and had an

enormous killing claw on its hind foot Smaller still

was Saurornithoides, which lived about 80 million

years ago It had a large brain, large eyes that could

judge distances as it pounced on small creatures, and

it may have hunted at night

Although the thick boneprevented damage, it leftlittle room for a brain

Triceratops Stegosaurus

Tortoises and tuataras

38

ONE LINE OF AMNIOTES developed a single

main skull opening Early amniotes includedthe ancestors of reptiles and synapsids.These synapsids are all extinct, but we and othermammals are their descendants

Most reptiles had two main openings in the side

of the skull that made it lighter, and gave space forbigger jaw muscles.These diapsid reptiles included

many extinct types, such as non-birddinosaurs Snakes and lizards are

living diapsid reptiles

The first fossils of thetortoise group come fromover 200 million years ago.They were already very similar in structure

to those of today There is little clue to theirimmediate ancestors

Tortoises have scales, and solid shells likeearly reptiles They lay shelled eggs on land They have unusualbodies; the shell has an outer horny layer equivalent to thescales, with an inner layer of bone This is made from bonyplates in the skin plus the ribs and backbone All these fusetogether to make a strong box The hip and shoulder bones

end up inside the ribs, rather than outside As theycannot move their ribs, tortoises have special muscles

to fill their lungs There are no teeth, just a horny beak.The shell is good protection, but limits mobility

The skull of the earliest

reptiles had solid bone along

the sides and over the top,

except for eye sockets and

nose The only living reptiles

with this type of skull are

the tortoises and turtles.

Anapsids (left)

Apart from those for eyes and nostrils, anapsid reptiles lack holes

in the sides of the skull.

Anapsid skull

Tortoise shell and skeleton

Proganochelys

From about 200 million

years ago, this creature

already had an armored

outer shell.

Land tortoises are usually slow-moving

vegetarians Some are not much more than

4 inches (10 cm) long when fully grown,

but the giant tortoises of the Indian

Ocean and Galapagos islands may

be 4.5 feet (1.4 m) long and

weigh 550 pounds (250 kg)

The tuatara is the sole relic

of a group of diapsid reptiles

that were widespread over

200 million years ago

Now it lives only on

islands off the coast of

New Zealand The tuatara

itself has hardly changed

in 140 million years Its

slow-motion life is

interesting: it can be active

at cooler temperatures than

most reptiles, and its eggs may

take 15 months to hatch

39

O N T H E L A N D A M P H I B I A N S A N D R E P T I L E S

Seychelles giant tortoises

Giant tortoises live only

on islands free of major land predators.

Tuatara

This creature now only survives on a few small islands.

IT’S A FACT

Tuataras can livefor 100 years Spur-thighed tortoisesand box turtles canalso live for 100years or more ASeychelles tortoise,possibly 50 yearsold at the time,was taken to theisland of Mauritiusand lived there for

a further 152 years

!