GEOLOGIC TIME SCALE
GEOLOGIC TIME
A group of students is
searching for fossils. By looking in
rocks that are hundreds of millions
of years old, they hope to find
fossils of organisms called
trilobites (TRI loh bites).
Trilobites are small, hard-shelled
animals that lived in ancient seas.
Trilobites are considered to be
index fossils. Index fossils lived
over vast regions of the world
during specific periods of geologic
time. The students hope that by
studying trilobite fossils, they can
help piece together a puzzle. They
want to know what caused the
trilobites to disappear from Earth
millions of years ago.
What is the geologic time scale?
The appearance or disappearance of types of organisms throughout Earth’s
history marks important events in geologic time. Paleontologists, scientists who
study the prehistoric world, divide Earth’s history into time units based on lifeforms that existed only during certain periods. This division of Earth’s history is
known as the geologic time scale. Sometimes few fossils remain from a period.
Then paleontologists use other methods to define a division of geologic time.
What are major subdivisions of geologic time?
The fossil record is used to divide Earth’s history into geologic time periods.
The figure below shows the four major subdivisions of geologic time—eons, eras,
periods, and epochs. Eons are the longest subdivision and are based on the
abundance of certain fossils.
Eons are divided
into smaller time periods
called eras. An era is
marked by major
worldwide changes in the
types of fossils present.
For example, at the end
of the Mesozoic Era,
many kinds of
invertebrates, birds,
mammals and reptiles
became extinct.
Eras are subdivided into periods. A period is a unit of geologic time during
which certain types of life-forms existed all over the world.
Geologic periods are divided into epochs. An epoch is also characterized by
differences in life-forms, but these may vary from continent to continent. Epochs
may be given names, like those in the Cenozoic Era or may be called simply early,
middle, or late.
What limits the divisions of geologic time?
There is a limit to how finely geologic time can be subdivided. It depends on
the kind of rock record that is being studied. Sometimes it is possible to
distinguish different layers of rock that formed during a single year. In other
cases, there is little information to help scientists subdivide geologic time.
PRECAMBRIAN TIME
It may seem strange, but during
the first billion years of Earth’s
history, the land was covered with
volcanoes. Over the next 3 billion
years, simple life-forms began to live in
the oceans. Precambrian (pree KAM
bree un) time is the longest part of
Earth’s history. Precambrian time
lasted from about 4.5 billion years ago
to about 544 million years ago.
What is known about early life forms?
Little is known about the organisms that lived during Precambrian time. Most
Precambrian rocks are buried deep within Earth where they have been changed by
heat and pressure. Few fossils can survive these conditions. Most Precambrian
organisms had soft bodies. These organisms did not have hard body parts that
leave fossil imprints in rock. One clue to early history of life is found in ancient
stromatolites (stroh MA tuh lites). Stromatolites are layered mats formed by
colonies, or groups, of cyanobacteria. Cyanobacteria are blue-green algae thought
to be one of the earliest life-forms on Earth.
How did early life affect the
atmosphere?
Cyanobacteria first
appeared on Earth about 3.5
billion years ago. Cyanobacteria
contained chlorophyll and used
photosynthesis. This is important
because during photosynthesis,
they produced oxygen, which
helped change Earth’s
atmosphere. Following the
appearance of cyanobacteria, oxygen became a major gas in the atmosphere.
Also of importance was that the ozone layer in the atmosphere began to
develop, shielding Earth from ultraviolet rays. It is hypothesized that these
changes allowed species of single-celled organisms to evolve into more complex
organisms.
Near the end of Precambrian time, invertebrates (ihn VUR tuh brayts)
appeared. Invertebrates are animals without backbones. Early invertebrates had
soft bodies, so few were preserved as fossils. Because of this, many Precambrian
fossils are trace fossils. Examples of trace fossils are tracks, trails, or burrows.
Trace fossils provide information about how organisms lived and behaved.
What were other unusual life-forms?
In the late Precambrian, a group of animals lived that were similar to some animals
today. These soft-bodied animals looked like modern jellyfish and worms. The
first fossils of these earliest invertebrates were found in the Ediacara Hills of
Australia. This group of organisms became known as the Ediacaran (ee dee uh KAR
un) animals. They have been found on every continent, except Antarctica.
Ediacaran animals lived on
the bottom of Precambrian seas.
Some scientists think these
animals may have had tough outer
coverings on their bodies.
Trilobites may have outcompeted
the Ediacaran animals and caused
their extinction. However, no one
knows for sure why the
Ediacaran animals disappeared.
THE PALEOZOIC ERA
Beginning in the Paleozoic
(pay lee uh ZOH ihk) Era, animals with shells and other hard body parts began to
appear. Because hard body parts are well preserved in fossil, it is easier to find
traces of life in this era. The Paleozoic Era, or era of ancient life, began about
544 million years ago and ended about 248 million years ago.
What organisms lived during this era?
During most of the Paleozoic Era, warm,
shallow seas covered much of the planet. Many
life-forms lived in the oceans. Trilobites were
common, especially early in the Paleozoic. Other
organisms developed shells. As a result, the
fossil record of this time contains many shells.
However, invertebrates weren’t the only animals
in Paleozoic
seas.
Animals with backbones, called
vertebrates, evolved during this time. The
earliest vertebrates were fishlike creatures
without jaws. During the Devonian Period,
fish with strong jaws evolved. These huge
fish, armed with heavy protective coverings
on their bodies, could eat large sharks. By
the Devonian Period, forests began to grow
on land. Some vertebrates adapted to the
land environment.
How did early life forms move onto land?
Most fish, both ancient and modern,
breathe through gills. But in the Devonian
Period, many fish also had lungs. Because of
their lungs, they could live in water that had low
levels of oxygen and swim to the surface to
breathe air.
One kind of ancient fish with lungs also developed fins that were like legs, as
shown in the figure below. These leg like fins were used to swim and to crawl
around on the ocean floor. Paleontologists hypothesize that today’s amphibians
might have evolved from these fish. Modern amphibians, such as frogs, live both in
water and on land. All amphibians have one thing in common—they all lay their
eggs in water or in a moist place.
What adaptations allowed reptiles to remain
on land?
By the Pennsylvanian Period, some
amphibians evolved eggs that were covered by
a protective coating. The coating helped
prevent the eggs from drying out. As a result,
these animals, called reptiles, did not need to
lay their eggs in water. Reptiles also have skin
covered with hard scales. The scales prevent
water loss from their bodies. These adaptations allow reptiles to live farther
from water and in dry climates where many amphibians cannot live.
How were mountains formed?
During the Paleozoic Era, there were great changes on Earth’s surface.
Several mountain ranges formed during this time. One example is the Appalachian
Mountains in the Eastern United States. Mountain building occurred in several
stages.
First, North America moved closer to Europe and Africa. This closed the
ocean that had separated them. Several volcanic island chains that had been in the
ocean collided with the North American Plate. The collision of the plate and the
island chains created high mountains.
The next mountain-building event occurred when the African Plate crashed
into the North American Plate. This collision formed mountains on both North
America and Africa. Rock layers were folded and faulted. Some rocks that were
near the eastern coast of the North American Plate were pushed west along
faults as far as 65 km. Sediments were uplifted to form an immense mountain
belt. Then, about 200 million years ago, the North American and African plates
began to separate. The ocean between them began to open up again.
What caused the end of the Paleozoic Era?
At the end of the
Paleozoic Era, more than 90
percent of all ocean species
and 70 percent of all land
species died. Perhaps as the
supercontinent Pangaea
formed, changes in the ocean
and land caused species to die.
Another hypothesis is that
erupting volcanoes changed
Earth so much that organisms
could not survive. Perhaps an
asteroid hit the planet and
destroyed its environments.
THE MESOZOIC ERA
People have been interested in dinosaurs since their bones were first
discovered more than 150 years ago. Dinosaurs lived during the Mesozoic (meh zuh
ZOH ihk) Era. The Mesozoic Era, or era of middle life, occurred between 248 and
65 million years ago and was a time marked by rapid movement of Earth’s plates.
What changes occurred to Pangaea?
At the beginning of the Mesozoic Era, all the continents were joined and
formed one landmass called Pangaea. During a part of the Mesozoic Era called the
Triassic Period, Pangaea began to break up. The continents began to drift apart as
Earth’s plates moved away from each other. Pangaea split in two. Over time, the
continents we know today formed.
Some species, such as reptiles, survived the tremendous changes and mass
extinction that occurred at the end of the Paleozoic Era. In the early Mesozoic
Era, the climate became drier. The reptile’s scaly skin kept in moisture, so reptiles
could live in this drier climate. Reptile eggs are protected by a shell, so their
young survived as well. Reptiles became the most abundant animals of the
Mesozoic Era.
What were the first bird-like creatures?
Some paleontologists believe that modern
birds evolved from small, meat-eating dinosaurs.
The earliest bird-like dinosaur known,
Archaeopteryx, had both wings and feathers. Since
Archaeopteryx had some features different from modern birds, it is not a direct
ancestor of today’s birds.
What were the first mammals like?
The first mammals appeared in the Triassic Period.
These tiny, mouselike animals were warm-blooded and
covered with fur, as shown in the figure below. The
females produced milk to feed their young. Because of
their furry coat and milk production, mammals where able to survive many
changing environments.
What ended the Mesozoic Era?
The Mesozoic Era ended about 65 million years ago, when most land and
ocean species became extinct. The dinosaurs disappeared. Many paleontologists
hypothesize that this mass extinction was caused by an asteroid that collided with
Earth. The impact put a huge cloud of dust and smoke in the air, blocking sunlight.
Without sunlight, plants died. As a result, the animals that fed on plants died.
Some organisms managed to survive. They are the ancestors of the many species
on Earth today.
THE CENOZOIC ERA
The Cenozoic (se nuh ZOH ihk) Era
began about 65 million years ago and
continues today. During this time, mountain
ranges in North and South America formed.
In the late Cenozoic, the climate cooled and
ice ages occurred. The early Cenozoic Era is
called the Tertiary Period. The present time
is part of the Quaternary Period, which began
about 1.8 million years ago.
How have
mammals
evolved?
During the Cenozoic Era, grasslands
expanded. As a result, grazing mammals like
horses, deer, and elephants survived and grew
larger. Horses evolved from small animals with
many toes into the large, hoofed animals of
today. Some mammals evolved to live in the
sea, such as dolphins and whales.
As the continents continued to move
apart, some species became isolated. For this reason, animals like kangaroos and
koalas evolved in Australia and are not found anywhere else on Earth.
Modern humans, Homo sapiens, probably first appeared about 140,000 years
ago. The appearance of early humans may have caused the extinction of many
other mammals. Humans competed for food that other animals ate and also
hunted animals.
Geologic Time Analysis Questions:
1. What is a paleontologist?
2. What is the geologic time scale?
3. List the four major geologic subdivisions in order from longest to
shortest.
4. How are each of the subdivisions divided?
5. How long did the Precambrian Time last?
6. What kinds of life existed in the Precambrian Time?
7. What is a trace fossil and what can they tells us?
8. What kids of life existed in the Paleozoic Era?
9. What adaptations allowed reptiles to remain on land?
10. What caused the end of the Paleozoic Era?
11. What was happening to the continents during the Mesozoic Era?
12. What was the most abundant creature during the Mesozoic Era?
13. Describe the mammals of the Mesozoic?
14. How did the Mesozoic Era end?
15. How long ago did the Cenozoic Era begin?
16. Which mountain range formed during the Cenozoic Era?
17. What creatures evolved during the Cenozoic Era?
18. How long ago did Humans evolve?