Geologic Time
The Paleozoic Era
Early Paleozoic
The Phanerozoic eon has three eras. The Paleozoic
(pay lee uh ZOH ihk) era is the oldest era of the Phanerozoic eon. The
Mesozoic (mez uh ZOH ihk) era is the middle era of the Phanerozoic
eon. The Cenozoic (sen uh ZOH ihk) era is the youngest era of the
Phanerozoic eon.
As shown below, the Paleozoic era lasted for more than
half of the Phanerozoic eon. The Paleozoic era is divided
into three parts: early, middle, and late. The Cambrian and
Ordovician periods make up the Early Paleozoic.
Paleozoic Era
542 mya 251 mya 0
Paleozoic
Eras
542 mya
Paleozoic
Eras
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Periods
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Precambrian
Phanerozoic Eons
The Age of Invertebrates
The organisms from the Cambrian explosion were
invertebrates (ihn VUR tuh brayts). Invertebrates are animals
without backbones.
The invertebrates of the Cambrian lived only in the
oceans. So many kinds of invertebrates lived in the Early
Paleozoic oceans that this time is often called the age of
invertebrates.
Geology of the Early Paleozoic
If you could have visited Earth during the Early Paleozoic,
it would have seemed unfamiliar to you. There was no life
on land. All life was in the oceans.
The shapes and locations of Earth’s continents also would
have been unfamiliar to you if you could have visited Earth
during the Early Paleozoic. The landmass that would become
North America was on the equator.
Earth’s climate was warm during the Early Paleozoic.
Rising seas flooded the continents. The water formed many
shallow, inland seas. An inland sea is a body of water formed
when ocean water floods continents. Most of North America was
covered by an inland sea.
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Middle Paleozoic
The Early Paleozoic ended with a mass extinction event,
but many invertebrates survived. Huge coral reefs along the
edges of the continents were home to new forms of life
during the Middle Paleozoic. The Middle Paleozoic consists
of the Silurian (suh LOOR ee un) period, 441–416 mya, and
the Devonian (dih VOH nee un) period, 416–359 mya.
Soon, vertebrates evolved. Vertebrates are animals with
backbones.
The Age of Fishes
Some of the earliest vertebrates were fishes. So many
types of fishes lived during the Middle Paleozoic that this
time is often called the age of fishes. Bony armor covered
the bodies of some fishes, such as the Dunkleosteus. This fish
was a top Devonian predator.
On land, cockroaches, dragonflies, and other insects
evolved. Earth’s first plants grew. These early plants were
small and lived in water.
Geology of the Middle Paleozoic
Middle Paleozoic rocks contain evidence of major
collisions between moving continents. These collisions
created mountain ranges. When several landmasses collided
with the eastern coast of North America, the Appalachian
(ap uh LAY chun) Mountains began to form. By the end of
the Paleozoic era, these mountains were probably as high as
the Himalayas are today.
Late Paleozoic
Like the Early Paleozoic, the Middle Paleozoic ended with
a mass extinction event. Many of the invertebrates that lived
in the seas and some land animals disappeared.
The Age of Amphibians
In the Late Paleozoic, some fishlike organisms spent part
of their lives on land. Amphibians had lungs and could
breathe air. Tiktaalik (tihk TAH lihk) was one of the earliest
of amphibians. Amphibians were so common in the Late
Paleozoic that this time is known as the age of amphibians.
Amphibians were adapted to land in several ways. They
had lungs and could breathe air. They had thick skin, which
kept them from drying out too quickly. They also had strong
limbs, which helped them move around on land. These early
amphibians, like amphibians today, had to return to the
water to mate and lay eggs.
Coal Swamps
During the Late Paleozoic, dense, tropical forests grew in
swamps along shallow inland seas. When trees and other
plants died, they sank into the swamps. A coal swamp is an
oxygen-poor environment where, over time, plant material changes into
coal. The coal swamps of the Carboniferous (car buhn IF er us)
period, 359–299 mya, and the Permian period, 299–251 mya,
are the major sources of coal that we use today.
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Reptile species evolved toward the end of the Paleozoic
era. Reptiles were the first animals that did not need water
for reproduction. Like the eggs of reptiles today, their eggs
had tough, leathery shells that kept them from drying out.
Because of this, the entire life of a reptile could be spent on
land.
Formation of Pangaea
Geologic evidence shows that many continental
collisions occurred during the Late Paleozoic. As continents
moved closer together, new mountain ranges formed. By the
end of the Paleozoic era, Earth’s continents had formed a
giant supercontinent called Pangaea. A supercontinent is an
ancient landmass that separated into present-day continents. Pangaea
formed from land masses that came together close to Earth’s
equator, as shown below. As Pangaea formed, coal swamps
dried up. Earth’s climate became cooler and drier.
P a n
g a
e
a
The Permian Mass Extinction
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The largest mass extinction in Earth’s history occurred at
the end of the Paleozoic era. Fossil evidence indicates that
95 percent of life-forms in the oceans and 70 percent of all
life on land became extinct. This extinction event is called
the Permian mass extinction.
Scientists debate what caused this mass extinction. The
formation of Pangaea likely decreased the amount of space
where ocean organisms could live. The formation of Pangaea
also would have changed ocean currents, making the center
part of Pangaea drier. But Pangaea formed over many millions
of years. The extinction event occurred more suddenly.
Some scientists hypothesize that the impact of a large
meteorite caused an extreme climate change. Other scientists
propose that huge volcanic eruptions changed Earth’s
climate. Both a meteorite impact and erupting volcanoes
would have caused large amounts of ash, dust, and rock to
enter the atmosphere. This debris would have blocked
sunlight, reducing temperatures and destroying food webs.
Whatever the cause, Earth had fewer species after the
Permian mass extinction. Only a few species adapted to the
changes and survived this mass extinction.