13.3 Mesozoic Era: Age of Reptiles
Mesozoic
Era : Age
of
Reptiles
Mesozoic Era : 251 – 65
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After the Permian extinction,
millions of years passed before the
number and diversity of living
things began to rise again.
One group that became very
successful and diverse was the
ammonites.
These extinct mollusks were
related to squid and octopus.
Ammonites have distinct spiral
shells; each type of ammonite
having a slightly different shell.
Mesozoic Era : Age of reptiles
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There were many changes in oceans
during the Mesozoic era.
As a result, many different types
of ammonites evolved and became
extinct after a few million years.
For this reason, they make
excellent index fossils.
Ammonite index fossils help
geologists date sedimentary rock
layers from the different periods
of the Mesozoic: the Triassic,
Jurassic and Cretaceous.
Triassic period ( 251 - 200 )
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Pangaea continued as a single, large
landmass through most of the
Triassic period. (251 – 200 million)
After a slow recovery from the
Permian extinction, many kinds of
reptiles evolved. Late in the
period, the first mammals
appeared.
Several important new types of
organisms evolved during the
Triassic. For example, the first
dinosaurs were among the many
types of reptiles that evolved.
Triassic period ( 251 - 200 )
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Most landmasses were fused
together in the supercontinent
Pangaea during the Triassic.
A huge ocean called Panthalassa
(meaning “all sea”) occupied the
rest of earth’s surface.
In the interior of Pangaea,
climates on land varied greatly
from season to season. Pangaea
had hot summers and cold
winters, similar to those in the
interior of North America or
Asia today.
Triassic period ( 251 - 200 )
Triassic Earth:
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Toward the end of the Triassic,
Pangaea began to split apart
slowly.
The split began as Africa pulled
away from North and South
America.
The breakup of Pangaea
continues today, about 200
million years later.
Triassic period ( 251 - 200 )
Triassic Life:
 Several important new types of
organisms evolved during the
Triassic.
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For example, the first dinosaurs
were among the many types of
reptiles that evolved. Later in
the Triassic, the first mammals
evolved from a surviving group of
mammal-like reptiles.
Unlike reptiles, most mammals do
not lay eggs. Mammals are warmblooded animals that nourish
their young with milk. Triassic
mammals, such as Megazostrodon
were small rodent-like animals.
First mammals: megazostrodon
Triassic period ( 251 - 200 )
Triassic Life:
 The fossil record shows that
gymnosperms became common
during the Triassic.
Gymnosperms are a group of
plants with seeds that lack a
protective outer coat.
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Unlike sporebearing plants,
gymnosperms do not require
standing water, such as dew, in
their reproductive cycle. For this
reason, they can use nutrients
and occupy space in drier areas.
conifers
Conifers, which first appeared
during the Permian period, are
gymnosperms. Other
gymnosperms include cycads and
ginkgoes.
cycads
Jurassic period ( 200 -145 )
Jurassic period: (200 – 145)
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As rifts formed between parts
of Pangaea, seas opened between
continents.
At the same time, mountain
building occurred in some areas,
such as western North America.
There, the North American plate
moved west over the oceanic
plate.
Tectonic activity started a wave
of deformation that moved
inland along the entire western
part of the continent.
Jurassic period ( 200 -145 )
Jurassic period: (200 – 145)
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The newly formed Jurassic seas
helped to moderate the harsh
climate conditions of Pangaea.
Generally, climates were mild at
all latitudes during the Jurassic.
Warmer conditions contributed
to the development of new life
forms.
Jurassic period ( 200 -145 )
Jurassic Life: (200 – 145)
 New developments occurred in
the oceans, on land, and in the air
during the Jurassic.
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In the oceans, fish with modern
characteristics evolved. Their
jaw structure, scales, and fins
were similar to those of fish
today.
Reptiles adapted to life in the
oceans. Fish eating plesiosaurs
and ichthyosaurs became good
swimmers but kept reptilian
teeth and breathed by means of
lungs.
Jurassic period ( 200 -145 )
Jurassic Life: (200 – 145)
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During the Jurassic period,
dinosaurs developed into a varied
and widespread group.
Some dinosaurs, such as
Diplodocus, Brachiosaurus, and
Stegosaurus were plant eaters.
Meat eating dinosaurs of the
Jurrasic included Allosaurus.
diplodocus
Jurassic period ( 200 -145 )
Evolution of Birds
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During the Jurassic, pterosaurs
evolved. These flying reptiles
had “wings” formed from
membranes of skin attached to
their forelegs.
But scientists think that birds
developed from dinosaurs. In
fact, some dinosaurs had
feathers.
The feathers may have served
for display or to help regulate
body temperature.
Jurassic period ( 200 -145 )
Evolution of Birds
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The early ancestors of birds
were probably small, feathered
dinosaurs that could glide.
Later, more birdlike animals, like
Archaepteryx evolved.
Archaepteryx still had reptile
like teeth, claws on it’s wings,
and a tail.
It lacked the large breastbone
that modern birds have to
anchor flight muscles, but it
could fly short distances.
Cretaceous period ( 145 - 65 )
Cretaceous period:
 The landmasses that had made
up Pangaea continued to move
apart during the Cretaceous
period. At the same time, Earth’s
climates continued to be
relatively warm.
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Early in the Cretaceous, shallow
seas invaded much of the
western North America. These
shallow seas created great
swamps like those of the
Paleozoic era.
The swamps formed the
Cretaceous coal deposits of the
western United States and
Canada.
Cretaceous period ( 145 - 65 )
Cretaceous Life:
 Dinosaurs of the Cretaceous
period evolved many different
sizes, shapes, and ways of living.
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Large slow moving plant eaters
waded on four legs in the
shallows of swampy areas.
Smaller, faster, meat-eaters ran
on two hind legs.
The Cretaceous was the time of
the fierce Tyrannosaurus rex,
the horned Triceratops and
gigantic plant eaters like
Apatosaurus.
Cretaceous period ( 145 - 65 )
Cretaceous Life:
 Other animals groups also
appeared and flourished. Snakes
evolved early in the Cretaceous.
Mammals with more modern
characteristics evolved.
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Flowering plants, or angiosperms,
also evolved during this period.
Angiosperms are plants that
produce flowers and seeds with
an outer covering.
Many modern angiosperms;
include trees such as willow,
birch, and sassafras, evolved and
became common during the
Cretaceous.
sassafras
Weeping
willow
Cretaceous period ( 145 - 65 )
Cretaceous Extinction:
 About 65 million years ago, the
Cretaceous period ended in a
mass extinction.
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The dinosaurs and most other
large animals became extinct.
Smaller reptiles, birds and
reptiles were also affected, with
some species of these surviving.
In the oceans, ammonites were
among the species that became
extinct.
Cretaceous period ( 145 - 65 )
Cretaceous Extinction:
 Most scientists think that a
large meteorite collided with
earth and caused the mass
extinction.
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The collision would have created
huge quantities of dust that
blocked out the sun, perhaps for
years. This in turn caused plants
to die and food to vanish for
many dinosaurs.
With the plant eating dinosaurs
perishing, the meat eating
dinosaurs would thus have no
food as well.
Cretaceous period ( 145 - 65 )
Cretaceous Extinction:
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Another hypothesis to explain
the Cretaceous extinction is that
huge volcanic eruptions in India
may have led to climate changes.
Some scientists think that
disease may have wiped out the
dinosaurs pointing to a decline in
species even before the mass
extinction.
It is likely that several factors
combined together to create the
Cretaceous extinction.