Download Why Did the Dinosaurs Die Out?

Name __________________________________________ Class Period ________ Date ____________
Why Did the Dinosaurs Die Out?
evidence
supercontinent
paleontologists
deadly
cause
interesting
extinction
geologists
allergic
violent
supernova
fossil
ridiculous
dating
ecologists
Directions: Fill in each blank with the word that best completes the reading comprehension.
Why did all of the remaining dinosaurs die out suddenly at the end of the Cretaceous period? There is a
short answer and a long answer to this question.
Here is the short answer. No one knows for sure.
Here is the long answer. There are a number of theories. Some are scientific, and some are
(1) _______________________ . Most scientists believe in one or two main ones, but no one knows for
sure.
The reason no one knows for sure is because the whole thing happened about 65 million years ago. No
one was around to see or hear what was going on back then, and no one was around to keep records.
What we do know comes mainly from fossil records. Scientists study the fossils themselves, and they
study the rock in which the fossils are found. Fossil records haven't given us definite proof for any one
theory. This is because there are some problems with (2) _______________________ records. For one
thing, most fossil evidence of dinosaurs has been found in North America. We don't know as much about
dinosaur activity on other parts of the earth. Another problem is that it is very hard to date fossils.
(3) _______________________ fossils means determining how old they are.
One really (4) _______________________ thing about the big dinosaur question is that scientists
from many different fields are working to solve the problem. Astronomers, physicists,
(5) _______________________ , (6) _______________________ , (7) _______________________ ,
chemists, geologists and others are all gathering (8) _______________________ , experimenting, and
researching to try to solve the puzzle.
The most popular theory is called the Alvarez Hypothesis. This hypothesis says that, 65 million years
ago, a big asteroid hit the earth. This asteroid crash scattered dust high up into the atmosphere. It blocked
much of the sunlight and caused the earth to become cooler. The crash also started huge fires, high winds,
and (9) _______________________ storms that killed some of the dinosaurs. The lack of sunlight and
warmth caused many plants to die. This shortage of plants for food caused the rest of the dinosaurs to die
out.
The second most popular theory says that activity on the earth itself caused the
(10) _______________________ . As the (11) _______________________ of Pangaea was breaking
up, movement of the earth's giant land plates caused volcanoes to erupt and the climate to change. These
changes affected the earth's atmosphere and temperature, and the dinosaurs died out from the cold or from
lack of plants to eat.
Other theories suggest that dinosaurs became so big that they couldn't move, or that there were too
many dinosaurs and they ate up all of the food.
There is a theory that, as the continents shifted, dinosaurs spread germs from one herd to another.
According to this theory, dinosaurs died out from sickness.
Another idea is that mammals ate up all of the dinosaurs' eggs.
Some people have suggested that an unusually bright star, a (12) _______________________ , killed
the dinosaurs with its (13) _______________________ radiation.
There is even a theory that dinosaurs died out from hay fever. Flowering plants began to grow during
the late Cretaceous Period. Maybe dinosaurs were very (14) _______________________ to flowers.
So, what did (15) _______________________ the dinosaurs to die out? No one knows for sure.