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11.24: Life of the Phanerozoic Eon

Difficulty Level: Basic Created by: CK-12
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Does this look like the Earth you know?

It's not hard to take a field trip to see some ancient life. Horseshoe crabs live along the east coast of the United States and elsewhere. These ancient creatures have been more-or-less unchanged for the past 450 million years. This is indicated by fossils. That doesn't get them quite back to the beginning of the Paleozoic but close. So hundreds of millions of generations of horseshoe crabs have seen most of the Phanerozoic go by.

Phanerozoic Eon

The Phanerozoic Eon is divided into three eras — the Paleozoic, the Mesozoic, and the Cenozoic (below). They span from about 540 million years ago to the present. We live now in the Cenozoic Era.

Earth’s climate changed numerous times during the Phanerozoic Eon. At the end of the Precambrian, much of the planet was covered with glaciers. At the start of the Phanerozoic Eon, the climate became warm and humid. Since then Earth’s climate has gone through four cycles of frigid glaciers and warm tropical seas. Some organisms survive changes in the climate. Others go extinct when the climate changes too much.

Phanerozoic Time Line

Table below shows some of the life forms that developed during the Phanerozoic Eon. Life gradually became more diverse and new species appeared. Most modern organisms evolved from species that are now extinct.

Development of Life During the Phanerozoic Eon
Era Millions of Years Ago Major Forms of Life
Cenozoic 0.2 (200,000 years ago) First humans
35 First grasses; grasslands begin to dominate the land
Mesozoic 130 First plants with flowers
150 First birds on Earth
200 First mammals on Earth
251 Age of dinosaurs begins
Paleozoic 300 First reptiles on Earth
360 First amphibians on Earth
400 First insects on Earth
475 First plants and fungi begin growing on land
500 First fish on Earth


  • The Phanerozoic has spanned from 540 million years ago to present.
  • The Phanerozoic Eon is made up of the Paleozoic, Mesozoic and Cenozoic Eras.
  • The quantity and diversity of life has exploded since the beginning of the Phanerozoic.


Use this resource to answer the questions that follow.


  1. What is the time period of the Phanerozoic Eon?
  2. What does "Phanerozoic" mean?
  3. What eras make up the Phanerozoic Eon?
  4. How many extinctions have been confirmed during the Phanerozoic?
  5. What was the most recent extinction event?


  1. When did the Phanerozoic begin? When did it end?
  2. What are some of the major events that have occurred in the Phanerozoic?
  3. What eras make up the Phanerozoic?

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    Cenozoic Era

    The most recent era of the Phanerozoic; the age of mammals.

    Mesozoic Era

    The middle of the Phanerozoic; the age of reptiles.

    Paleozoic Era

    The first era of the Phanerozoic; when life exploded in type and quantity.

    Phanerozoic Eon

    The most recent eon, spanning 540 million years.

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