Name
Date
Class ______________
Eras of Earth’s History
Understanding Main Ideas
Fill in the blank with the correct era or period to complete each statement.
1. Until the
, only one-celled organisms lived
on land, but during this period, plants became abundant.
2. At the start of the
, many organisms
evolved with hard parts including shells and outer skeletons.
3. During the
widely on land.
4. At the end of the
Earth died.
animals began to spread
, most species of life on
5. Reptiles were so successful that the
often called the Age of Reptiles.
6. At the close of the
ago, a second mass extinction occurred.
is
, about 65 million years
7. The first dinosaurs appeared during the
.
8. During the
, mammals evolved to live in
many different environments—on land, in water, and even in the air.
9. The fossil record suggests that humans migrated to most continents during the
.
Building Vocabulary
Match each definition with its term by writing the letter of the correct term in the right
column on the line beside the definition in the left column.
10.
animal with a backbone
a. reptile
11.
animal without a backbone
b. mammal
12.
animal that evolved from lungfish
c. invertebrate
13.
animal with strong legs and eggs with thick shells
d. mass extinction
14.
an event in which many types of living things die out at once
e. amphibian
15.
animal that feeds its young with milk
f. vertebrate
Name
Date
Class _____________
Eras of Earth’s History
Read the passages below. Then answer the questions that follow on a separate sheet
of paper.
The End of an Era
What happened at the close of the Mesozoic Era to cause the extinction of dinosaurs and many other
types of organisms? There is more than one theory. Each uses evidence from fossils and from analysis of
rock layers.
Theory 1: An Asteroid Impact
About 65 million years ago, an asteroid about 10 kilometers in diameter collided with Earth. The
impact had the force of millions of atomic bombs. Sediment and particles of the asteroid blasted into
the atmosphere. A dust cloud blocked the sun for months or longer. Plants and other organisms that
performed photosynthesis died, and many animals that depended on those plants for food died, too.
Blocking the sun also dropped Earth’s temperature, changing the climate. Animals that couldn’t adapt
became extinct. Evidence for this theory includes the fossil record, which shows many organisms died
out suddenly at the end of the Cretaceous Period. Scientists also discovered an impact crater in the Gulf
of Mexico. Other evidence includes a thin layer of rock around Earth enriched with the rare element
iridium—65 million years old. Asteroids contain iridium.
Theory 2: Volcanic Eruptions
About 65 millions years ago, huge volcanic eruptions occurred on the Indian subcontinent of south
Asia. Movements of Earth’s continents caused these eruptions. Lava poured over a large area, equal
in size to about half of Europe. Volcanic dust spewed into the atmosphere, and dark clouds encircled
Earth. For months or longer, the volcanic dust blocked the sun’s energy from reaching Earth’s surface,
causing plants, and then animals, to die. Evidence for this theory includes a fossil record, which shows
many organisms died suddenly at the end of the Cretaceous Period. Geologists identified a huge area of
volcanic rock 65 million years old on the Indian subcontinent. Also a thin rock layer enriched with iridium
around Earth dates back 65 million years. Eruptions produce dust-containing iridium.
1. What is the evidence for the asteroid-impact theory?
2. What is the evidence for the volcanic-eruptions theory?
3. In each theory, what causes the extinction of plants and animals?
4. What evidence would be needed to prove one of the theories true?
5. Which theory makes the most sense to you? Give reasons.