HISTORY OF THE
VERTEBRATES
CHAPTER 20
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.
Precambrian
Paleozoic Era
Cambrian
600 M.Y.A.
550 M.Y.A.
Ordovician
500 M.Y.A.
Silurian
First mass
extinction
500 M.Y.A.
450 M.Y.A.
Devonian
400 M.Y.A.
Carboniferous
Second mass
extinction
400 M.Y.A.
350 M.Y.A.
300 M.Y.A.
Mesozoic Era
Permian
Triassic
Third mass
extinction
300 M.Y.A.
250 M.Y.A.
Jurassic
200 M.Y.A.
Cretaceous
150 M.Y.A.
200 M.Y.A.
Cenozoic Era
Cretaceous
Fourth mass
extinction
Tertiary
100 M.Y.A.
Now
Fifth mass
extinction
100 M.Y.A.
50 M.Y.A.
3.5 M.Y.A.
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 (545-490 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 (360-280
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 junglelike forests at the poles.
• A gradual cooling caused ice caps to form
at the poles.
THE CENOZOIC ERA
TABLE 20.1
• 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, sabertoothed tigers, and cave
bears.
SOME GROUPS OF EXTINCT ANIMALS
Cave bears
Numerous in the ice ages, this enormous
vegetarian bear slept through the winter
in large groups.
Irish elk
Neither Irish nor an elk (it is a kind of deer),
Megaloceros as the largest deer that ever lived, with
antlers spanning nearly 4 meters. Seen in French cave
paintings, they survived until at least 7,700 years ago.
Mammoths
Although only two species of elephants survive
today, the elephant family was far more diverse
during the late Tertiary. Many were cold-adapted
mammoths with fur.
Giant ground sloths
Megatherium was a giant 6-meter ground
sloth that weighed 3 tons and was as large
as a modern elephant.
Saber-tooth cats
The jaws of these large, lionlike cats
opened an incredible 120 degrees
to allow the animal to drive its huge
upper 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
Quaternary
(2–Present)
0
Jawless
fishes
(two classes)
Birds
Amphibians
Cartilaginous
fishes
Modern bony
fishes
Reptiles
Mammals
Time (millions of years ago)
Tertiary
(65–2)
Cretaceous
100
(144–65)
Jurassic
(213–144)
Triassic
(248–213)
200
Permian
(280–248)
Primitive reptiles
(extinct)
300
Carboniferous
(360–280)
Devonian
(408–360)
400
Silurian
(438–408)
Placoderms
(extinct)
Ordovician
500
(490–438)
Chordate ancestor
Ostracoderms
(extinct)
Primitive amphibians
(extinct)
Acanthodians
(extinct)
Chordate ancestor
• 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.
Mammals
Birds
Reptiles
• 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:
Amphibians
• All fishes have four important
characteristics in common:
Fish
FISHES DOMINATE THE SEA
FISHES DOMINATE THE SEA
• 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.
FISHES DOMINATE THE SEA
• 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
FISHES DOMINATE THE SEA
• The earliest jawed fishes had spines
(acanthodians) or heavy armor
(placoderms), and some reached enormous
sizes.
FISHES DOMINATE THE SEA
• 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.
FISHES DOMINATE THE SEA
• 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.
FISHES DOMINATE THE SEA
• 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.
FISHES DOMINATE THE SEA
• 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.
Dorsal aorta
To
heart
Gas
gland
Swim bladder
Muscular
valve
FISHES DOMINATE THE SEA
• 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).
FISHES DOMINATE THE SEA
• 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.
FISHES DOMINATE THE SEA
• 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.
Chordate ancestor
Mammals
Birds
Reptiles
• They were the first terrestrial
vertebrates and evolved
from the lobe-finned fishes.
Amphibians
• The amphibians include
frogs, salamanders, and
caecilians.
Fish
AMPHIBIANS INVADE THE LAND
1
Lobe-finned Fish
Shoulder
Pelvis
Tibia
Humerus
Ulna
• A fossil of the genus
Tiktaalik (discovered
in 2006) exhibited
limb morphology
that was transitional
between fish and
amphibians.
Femur
Fibula
Radius
2
Tiktaalik
Shoulder
Humerus
Ulna
3
Radius
Early Amphibian
Shoulder
Pelvis
Femur
Humerus
Fibula
Ulna
Tibia
Radius
AMPHIBIANS INVADE THE LAND
• Amphibians have five key characteristics
that allowed them to invade land
successfully:
• legs
• lungs
• cutaneous respiration
• pulmonary veins
• partially divided heart
AMPHIBIANS INVADE THE LAND
• 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.
AMPHIBIANS INVADE THE LAND
• 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
Mammals
Reptiles
Amphibians
Fish
• 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
Chordate ancestor
• Reptiles increase their lung
capacity by expanding their
chest cavity when breathing in
air.
Birds
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THE WATERTIGHT EGG
Embryo
Leathery skin
Amnion
Allantois
Chorion
Yolk sac
REPTILES CONQUER THE LAND
• 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 CONQUER THE LAND
• 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.
REPTILES CONQUER THE LAND
• A second lineage gave rise to the ancestors of
turtles.
REPTILES CONQUER THE LAND
• 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.
REPTILES CONQUER THE LAND
• Early archosaurs resembled crocodiles, but
later forms called thecodonts were the first
reptiles to be bipedal.
REPTILES CONQUER THE LAND
• 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.
Chordate ancestor
Mammals
Birds
Reptiles
• They only became
common after the
pterosaurs became
extinct.
Amphibians
• Birds evolved from
bipedal dinosaurs
about 150 M.Y.A.
Fish
BIRDS MASTER THE AIR
BIRDS MASTER THE AIR
• Many scientists
consider birds to be
feathered dinosaurs,
given their similarity in
so many respects to
dinosaurs.
BIRDS MASTER THE AIR
• 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 MASTER THE AIR
• Birds are different than
reptiles in that they
have:
Shaft
Barbu
Shaft
• 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.
Quill
Hooks
Barb
BIRDS MASTER THE AIR
• 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
• Mammary
glands
• Hair
• Middle ear
Chordate ancestor
Mammals
Birds
Reptiles
Amphibians
• Members of this class
share three key
characteristics:
Fish
• Mammals evolved
about 220 M.Y.A.
and belong to the
class Mammalia.
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MAMMALS ADAPT TO COLDER
TIMES
• The first mammals
evolved from
therapsids.
• Early mammals
were small shrewlike
creatures.
• They lived
inconspicuously in an
age dominated by
dinosaurs.
MAMMALS ADAPT TO COLDER
TIMES
• 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.
MAMMALS ADAPT TO COLDER
TIMES
• 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.
MAMMALS ADAPT TO COLDER
TIMES
Embryo
• Placenta is an
adaptation for Chorion
nourishing
developing young
that will be born live.
Uterus
Amnion
Umbilical
cord
Placenta
Yolk
sac
MAMMALS ADAPT TO COLDER
TIMES
• Today’s mammals
include.
• Monotremes
• Egg-laying
mammals
• Marsupials
• Pouched mammals
• Placental mammals