CHAPTER 20 HISTORY OF THE VERTEBRATES. THE PALEOZOIC ERA Scientists divide the earth’s past into different time periods. Large blocks of time are called

CHAPTER 20

HISTORY OF THE VERTEBRATES

THE PALEOZOIC ERA

• Scientists divide the earth’s past into different time periods.• Large blocks of time

are called eras.• Eras are divided

into blocks of time called periods.

• Some periods are divided into epochs, which in turn can be divided into ages.

Paleozoic EraPrecambrian

Devonian CarboniferousSecond massextinction

SilurianOrdovicianFirst massextinction

Fourth massextinction

Cretaceous

Jurassic

Permian Triassic

Cretaceous

NowCenozoic EraTertiary

Cambrian

400 M.Y.A.500 M.Y.A. 450 M.Y.A.

300 M.Y.A.350 M.Y.A.400 M.Y.A.

300 M.Y.A.

200 M.Y.A.

100 M.Y.A. 3.5 M.Y.A.50 M.Y.A.

100 M.Y.A.150 M.Y.A.

200 M.Y.A.250 M.Y.A.

600 M.Y.A. 500 M.Y.A.550 M.Y.A.

Mesozoic Era

Fifth massextinction

Third massextinction

THE PALEOZOIC ERA

• Virtually all of the animals that survive at the present time originated in the sea at the beginning of the Paleozoic era.• The diversification of animal life began soon

after the Cambrian period (545490 M.Y.A.).• Some Cambrian animals, such as trilobites,

have no surviving close relatives.• The first vertebrates evolved about 500 M.Y.A.

THE PALEOZOIC ERA

• While most of the animal phyla that evolved in the Cambrian remained marine, a few phyla have successfully invaded land.• Fungi and plants were the first terrestrial

organisms, appearing over 500 M.Y.A.• Arthropods were the first terrestrial animals,

invading land about 410 M.Y.A.

THE PALEOZOIC ERA

• Vertebrates invaded the land during the Carboniferous period (360280 M.Y.A.).• Amphibians were

the first terrestrial vertebrates, followed by the reptiles, which were successful and dominant.

THE PALEOZOIC ERA

• Mass extinctions are particularly sharp declines in species diversity.• Five mass extinctions

have occurred.• The most drastic

occurred during the last ten million years of the Permian period, the end of the Paleozoic era.• An estimated 96% of

all species of marine animals became extinct.

• The most well-studied extinction occurred at the end of the Cretaceous period (65 M.Y.A.).• Probably triggered by a large asteroid hitting the earth.• Dinosaurs went extinct at this time.

THE PALEOZOIC ERA

• Mass extinctions left vacant many ecological opportunities.• Extinctions are often followed by rapid evolution

among the relatively few species that survived.

THE PALEOZOIC ERA

• We are currently experiencing a sixth mass extinction event.• The number of species is declining at a rapid

rate due to human activity.• Some predict that as many as 25% of all

species will become extinct in the near future.

THE MESOZOIC ERA

• The Mesozoic era (248–65 M.Y.A.) has traditionally been divided into three periods:• Triassic• Jurassic• Cretaceous

• During the Jurassic period, the supercontinent of Pangaea began to break up, sea levels were rising, and the world’s climate became warmer and wetter.

http://youtu.be/cQVoSyVu9rk

THE MESOZOIC ERA

• The Mesozoic era (248–65 M.Y.A.) was a time of intensive evolution of terrestrial plants and animals.• Reptiles continued their

success and diversified greatly.

• The flowering plants evolved in the early Cretaceous.

• From reptile ancestors, dinosaurs, mammals, and birds evolved.

THE MESOZOIC ERA

• Dinosaurs and mammals appeared at about the same time, 200–220 M.Y.A.• But the dinosaurs filled the evolutionary niche

for large animals.• For over 150 million years, dinosaurs were the

dominant land vertebrates.

THE MESOZOIC ERA

• About 65 M.Y.A., at the end of the Cretaceous period, dinosaurs disappeared.• This loss included flying

reptiles (pterosaurs) and the great marine reptiles.

• Mammals occupied the niches left open by the loss of the dinosaurs.

THE MESOZOIC ERA

• Many explanations have been advanced to explain the demise of the dinosaurs.• The most widely accepted, proposed by Luis W.

Alvarez, blames an asteroid impact.• Iridium is an element rare on earth but

abundant in meteorites.• A layer of iridium is abundant in many parts of

the world in a layer of sediment that dates to the end of the Cretaceous period.

THE CENOZOIC ERA

• Early in the Cenozoic era (65 M.Y.A. to present), the climate was relatively warm compared to today’s colder and drier climate.• The first half of the era was very warm with

jungle-like forests at the poles.• A gradual cooling caused ice caps to form

at the poles.

THE CENOZOIC ERA

• This was followed by a series of glaciations, the most recent ending about 10,000 years ago.• Many very large mammals

evolved during the ice ages, including:• Mastodons, mammoths,

saber-toothed tigers, and cave bears.

MammothsAlthough only two species of elephants survivetoday, the elephant family was far more diverseduring the late Tertiary. Many were cold-adaptedmammoths with fur.

Giant ground slothsMegatherium was a giant 6-meter groundsloth that weighed 3 tons and was as largeas a modern elephant.

Irish elkNeither Irish nor an elk (it is a kind of deer),Megaloceros as the largest deer that ever lived, withantlers spanning nearly 4 meters. Seen in French cavepaintings, they survived until at least 7,700 years ago.

Cave bearsNumerous in the ice ages, this enormousvegetarian bear slept through the winterin large groups.

SOME GROUPS OF EXTINCT ANIMALSTABLE 20.1

Saber-tooth catsThe jaws of these large, lionlike catsopened an incredible 120 degreesto allow the animal to drive its hugeupper pair of saber teeth into prey.

FISHES DOMINATE THE SEA

• A series of key evolutionary advances allowed vertebrates first to conquer the sea and then the land.

• About half of all vertebrates are fishes.• Fishes provide the evolutionary base for the

invasion by land by amphibians.

VERTEBRATE FAMILY TREE

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Tim

e (m

illio

ns

of

year

s ag

o)

Mammals

Birds

Reptiles

Amphibians

Modern bonyfishes

Cartilaginousfishes

Jawlessfishes

(two classes)

Primitive reptiles(extinct)

Primitive amphibians(extinct)Acanthodians

(extinct)

Ostracoderms(extinct)

Placoderms(extinct)

Chordate ancestor

Ordovician(490–438)

Silurian(438–408)

Devonian(408–360)

Carboniferous(360–280)

Permian(280–248)

Quaternary(2–Present)

Tertiary(65–2)

Cretaceous(144–65)

Jurassic(213–144)

Triassic(248–213)

100

200

300

400

500


• All fishes have four important characteristics in common:• Gills are used to extract dissolved

oxygen from water.• Vertebral column - all fishes have

an internal skeleton with a spine.

• Single-loop blood circulation - blood is pumped in a single loop, from the heart to the gills, then to the body, then back to the heart.

• Nutritional deficiencies:• Fishes are unable to synthesize some amino acids and

must consume them in their diet.• This trait has been inherited by all of their vertebrate

descendants.A

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Chordate ancestor


• The first fishes were jawless ostracoderms that appeared in the sea about 500 M.Y.A.• Agnathans are the

only jawless fishes found today.• They include

hagfish and lampreys.


• Jawed fishes appeared around 410 M.Y.A.• Jaws evolved from the frontmost of a series of

cartilaginous arch supports that reinforced the tissue between gill slits.

Skull Anterior gill arches

Gill slits


• The earliest jawed fishes had spines (acanthodians) or heavy armor (placoderms), and some reached enormous sizes.


• Sharks and bony fishes appeared about 400 M.Y.A. and shared the seas with spiny fish and placoderms for 150 million years.

• Eventually, the less maneuverable spiny fish and placoderms went extinct.

• Sharks and bony fish have dominated the seas for the last 250 million years.


• Sharks, along with skates and rays, belong to the class Chondrichthyes.• Sharks have a flexible skeleton made of

cartilage and are fast and maneuverable swimmers.

• There are 750 species in this class today.


• While some are filter feeders, most sharks are predators and have a mouth armed with rows of sharp teeth.

• Sharks also have many special sensory adaptations that suit their predatory life.


• Bony fishes have a heavier internal skeleton made of bone.• But they achieve

maneuverability through the aid of a swim bladder, a gas-filled sac that allows fish to regulate their buoyancy.• Sharks gain buoyancy

with oil in their livers, but they must keep swimming to counteract their denser-than-water bodies.

Muscularvalve

Toheart

Dorsal aorta

Gasgland

Swim bladder


• Bony fishes comprise the class Osteichthyes.• Some bony fishes are lobe-finned (subclass

Sarcopterygii).• This group includes the ancestors of the first

tetrapods (four-legged animals).


• Other bony fishes are ray-finned (subclass Actinopterygii).• This group includes the vast majority of

today’s fishes.• Bony fishes are the most successful of all

fishes, indeed of all vertebrates – nearly 30,000 species.


• Bony fishes have several adaptations that have helped make them such evolutionary successes:• Lateral line system

• A special sensory system that enables fish to detect changes in water pressure.

• Also present in sharks.• Operculum

• A bony covering on top of the opening of the gills.

• This allows the fish to ventilate the gills while remaining stationary.

AMPHIBIANS INVADE THE LAND

• The amphibians include frogs, salamanders, and caecilians.• They were the first

terrestrial vertebrates and evolved from the lobe-finned fishes.

Am

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Chordate ancestor

1

2

3

Lobe-finned Fish

Tiktaalik

Early Amphibian

Humerus

Pelvis

Tibia

Fibula

FemurUlna

Shoulder

Radius

Radius

Shoulder

Ulna

Humerus

Pelvis

Shoulder

Tibia

Femur

Humerus

Ulna RadiusFibula

• A fossil of the genus Tiktaalik (discovered in 2006) exhibited limb morphology that was transitional between fish and amphibians.


• Amphibians have five key characteristics that allowed them to invade land successfully:

• legs• lungs• cutaneous respiration• pulmonary veins• partially divided heart


• Amphibians were the dominant land vertebrates for 100 million years.• By the mid-Permian

period, fully terrestrial groups of amphibians had evolved, some very large and with extensive body armor.


• The amphibians of today must reproduce in water and are not completely terrestrial.

• Approximately 4,850 species exist today in the class Amphibia.

REPTILES CONQUER THE LAND

• Reptiles are more fully terrestrial than amphibians.

• All living reptiles share the following fundamental characteristics:• Amniotic egg

• This innovation is a watertight environment that offers the embryo protection against drying out.

• Dry skin• Reptiles are covered by scales

or armor in order to prevent drying out.

• Thoracic breathing• Reptiles increase their lung

capacity by expanding their chest cavity when breathing in air.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Am

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Chordate ancestor

THE WATERTIGHT EGG

Chorion

Leathery skinEmbryo

Yolk sac

AllantoisAmnion


• Today some 7,000 species of reptiles belong to the class Reptilia.

• Reptiles improved on the evolutionary innovations of amphibians to terrestrial life.– Reptilian legs were arranged to better support

body weight and to facilitate more efficient locomotion.

– Lungs and heart became more efficient in reptiles than in amphibians.


• Reptiles evolved around 300 million years ago; three lineages formed:• In one lineage, predatory reptiles called the

pelycosaurs evolved and were dominant for 50 million years; therapsids replaced the pelycosaurs as the dominant land vertebrate, and one group eventually gave rise to the mammals.


• A second lineage gave rise to the ancestors of turtles.


• A third lineage gave rise around 230 M.Y.A. to the ancestors of snakes, lizards, and tuataras, to marine reptiles (ichthyosaurs and plesiosaurs), and to the archosaurs.


• Early archosaurs resembled crocodiles, but later forms called thecodonts were the first reptiles to be bipedal.


• Early archosaurs gave rise to four groups: • Dinosaurs, many of which grew to immense

sizes• Crocodiles, which have changed little• Pterosaurs, the flying reptiles• Birds

• Dinosaurs were the most successful of all land vertebrates but became extinct around 65 million years ago, along with the marine reptiles and pterosaurs.

BIRDS MASTER THE AIR

• Birds evolved from bipedal dinosaurs about 150 M.Y.A.• They only became

common after the pterosaurs became extinct.

Am

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Chordate ancestor


• Many scientists consider birds to be feathered dinosaurs, given their similarity in so many respects to dinosaurs.


• Modern birds lack teeth and have only vestigial tails.

• They retain many reptilian characteristics• Birds lay amniotic eggs (but with hard shells).• Birds have reptilian scales on their feet and

lower legs.


• Birds are different than reptiles in that they have:• Feathers

• Derived from reptilian scales but adapted for flight.

• Flight skeleton• The bones of birds are

thin and hollow, reducing weight while providing enhanced points for flight muscle attachment.

Barbules

Quill

Shaft

HooksBarb

Shaft


• Birds are endothermic• Their high body

temperatures enhance metabolism, satisfying the large energy requirements of flight.

• The oldest bird of which there is a clear fossil is Archaeopteryx.

• There are about 8,600 species of birds in the class Aves today.

MAMMALS ADAPT TO COLDER TIMES

• Mammals evolved about 220 M.Y.A. and belong to the class Mammalia.• Members of this

class share three key characteristics:• Mammary glands

• Hair• Middle ear

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Am

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Chordate ancestor


• The first mammals evolved from therapsids.

• Early mammals were small shrewlike creatures.• They lived

inconspicuously in an age dominated by dinosaurs.


• Mammals were a minor group during the time of the dinosaurs, but rapidly diversified when dinosaurs and many other land and marine animals became extinct 65 M.Y.A.

• Over 4,500 species of mammals exist now, half of them rodents, and one-quarter of them bats.


• Modern mammals have a number of characteristics that make them successful.• Endothermy allows for mammals to be active

at any time of day or night and to colonize harsh environments.

• Teeth type varies in mammals, which have heterodont dentition, allowing specialization to eating habits.

• Hooves help with locomotion in running mammals; hooves, claws, and fingernails are made of keratin; horns are composed of a bony core surrounded by a sheath of compacted keratin.


• Placenta is an adaptation for nourishing developing young that will be born live.

PlacentaChorion

Embryo

Uterus

Amnion

Yolksac

Umbilicalcord


• Today’s mammals include.• Monotremes

• Egg-laying mammals

• Marsupials• Pouched mammals

• Placental mammals

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CHAPTER 20 HISTORY OF THE VERTEBRATES. THE PALEOZOIC ERA Scientists divide the earth’s past into different time periods. Large blocks of time are called