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 life-forms 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 TIMEIt 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 ancientstromatolites (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 ERABeginning 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 asEarth’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 withEarth. 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 ERAThe 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.

Which mountain ranges formed during this era?

Many mountain ranges formed during the Cenozoic Era as Earth’s plates moved and collided. These include the Alps in Europe and the Andes in South America. Many people think the growth of these

mountains helped create cooler worldwide climates.

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 well known mountain range was formed in during the

Paleozoic Era?11. What caused the end of the Paleozoic Era?12. What was happening to the continents during the Mesozoic Era?13. What was the most abundant creature during the Mesozoic Era?14. What was the first bird like creature called and what did it look

like?15. Describe the mammals of the Mesozoic?16. How did the Mesozoic Era end?17. How long ago did the Cenozoic Era begin?18. Which mountain range formed during the Cenozoic Era?19. What creatures evolved during the Cenozoic Era?20. How long ago did Humans evolve?