Main Idea and Details

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Lesson 26

Genre: Expository Nonfiction

Genre: Narrative Poe try


Focus Skill

Main Idea and Details

The main idea is the most important idea in a text. Details are pieces of information that support the main idea. Often the author states the main idea in a topic sentence. This sentence may appear near the beginning of a nonfiction passage. Identifying the main idea and supporting details in a passage will help you better understand what you read.




Main Idea


Read the paragraph below. The main idea is stated in the topic sentence. Then look at the graphic organizer. It shows the main idea and three details that support it.

Detail Some dinosaurs ate plants.

Detail Some dinosaurs ate animals

Detail Carnivorous dinosaurs had pointed teeth, sharp claws, and strong jaws.

Main Idea Dinosaurs can be classified by the type of food they ate.


Vocabulary Build Robust









A Look at the Past

What if it were possible to build a contraption that would let you take photos of Earth in the distant past? What might you see? You would probably see that most of Earth’s surface was submerged under a single ocean. Some scientists believe that all of the continents were once joined as a giant supercontinent. An enormous ocean covered the rest of Earth.

Scientists have named the massive, prehistoric supercontinent Pangaea.


You might find it eerie to look in on such a different Earth. However, paleontologists and other scientists would be delighted to be able to see the prehistoric landscape and the creatures that roamed across it.

Modern scientists face obstacles when they try to make models of extinct animals. They cannot be certain about details such as color, shape, and skin type. Perhaps some of these creatures looked quite elegant. With a camera to look back across time, scientists would have a simple way to find answers to complicated questions about prehistoric times. Sadly, such a camera exists only in the imagination.


Dragons and Dinosaurs



How do we know we are right when we imagine the past? Well, we really don’t. We weren’t there. When it comes to dinosaurs, scientists have only bones and fossils—and now computers—to help them understand how these ancient beasts might have looked.

Until very recently, people didn’t know what the world was like before humans. They didn’t understand that entire species could disappear from the earth, or go extinct.

No human ever saw a living dinosaur, but our ancestors did fi nd—and wonder about—dinosaur bones.

Prehistory Mystery

Ancient people had their own ways of explaining the strange bones they found. They thought the bones were the remains of giants, dragons, sea monsters, and other creatures you read about today in fairy tales and myths. Consider the legend of the griffin, a fantastic cross between a lion and an eagle once believed to live in the deserts of China. People all over the ancient world—Asia, the Middle East, and Greece and Rome—told stories of these griffins. Each culture imagined them differently, but most ancient art shows them with gigantic beaks, claws, wings, and catlike bodies.

Now take a look at the skull of a small dinosaur we call Protoceratops.Scientists have found many of these skulls in the Gobi desert in Mongolia (a region of China), where Protoceratopslived about 80 million years ago.


The real Protoceratops? Scientists think so, but even if the shape is right, we may never know what color it was. We can only guess based on modern reptiles.

Protoceratopshad a huge, birdlike beak designed to chop up the plants it ate and to defend itself. A large “frill” on its head and bony bumps on its cheeks (probably both a form of protective armor) give its skull a monstrous look. Ancient people who saw these skulls guessed that they were the remains of winged, lionlike birds that guarded gold in the desert mountains. They thought they were griffins.

When you look at its skull, it’s easy to see why people once thought Protoceratopswas a fantastic birdlike creature.


The ancient people were not far from the truth. These were the bones of amazing creatures, though not human giants, nor dragons, nor griffins. In the 1800s, scientists began to recognize that ancient bones and fossils belonged to long-extinct animals, which no one, of course, had ever seen. The mystery deepened.

What did these animals really look like? The task of arranging the bones in the proper order and accurately imagining the flesh and muscle that covered them was quite a challenge.

The Therrible Lizards

Pretend for a moment that you have never seen a bicycle before. You have no idea what it does or how it is used.

Now suppose you stumble upon a funny-looking triangular leather thing sticking out of the earth. Next to it lies a curved metal bar with plastic ends. Farther along, a piece of rubber.

What could these things be? Are they parts of a single contraption? Are they related to anything you know about? How do they fit together? How many, and what, pieces are missing?

This gives you an idea of the questions scientists ask when they find dinosaur bones. Just over a hundred years ago, the first scientists to study them thought these bones belonged to gigantic, ferocious lizards that once roamed the earth.

A brilliant English scientist named Richard Owen recognized that these creatures were different enough from modern reptiles to deserve their own name. He called them dinosaurs, from the Greek for “terrible lizard.” Though Owen was correct to group them under one name, he didn’t realize that some dinosaurs were small and harmless. And though dinosaurs belong to the reptile family, they are very different from lizards.

“It’s a lizard—so its legs must be on the side.”

“ Sounds good to me.”


Today scientists believe Iguanodonheld its tail straight behind it and walked mostly on all fours.

Iguanodonwas among the first dinosaurs to be scientifically described. Its discoverer, Gideon Mantell, believed that the very large teeth and bones he found must have belonged to a huge iguana-like creature whose legs sprawled out to the side like those of a lizard. Mantell thought that the large pointed object his wife discovered was the creature’s nose horn. Over the years, scientists changed their minds many times about the appearance of Iguanodon. Today we know that it had four straight legs, like a mammal, and could walk either on two hind legs or on all fours. That spiky object mistaken for a horn was really a long, sharp toenail.

“Those guys had it all wrong—the legs go underneath.”

“It walked on its hind legs. And this wasn’t a horn—it was a toenail!”

“Sounds good to me.”


The Case of the Deceptive Dinosaur

Today we know of more than 1,000 species of dinosaurs, and others are being discovered every year. Paleontologists, scientists who study bones and fossils to learn about the past, have a tough job keeping up with all the bones. When bones are first discovered, it is sometimes difficult to know what animal they belong to.

Is it a new dinosaur or one we already know about? And when the bones of more than one creature are jumbled together, scientists must be careful to separate and identify each animal.

Take the case of poor old Apatosaurus(whose name means “deceptive lizard”). Now there’s a dinosaur with an identity crisis. This huge 30-ton creature was first discovered and named in 1877. Two years later, the same scientist examined another set of Apatosaurusbones and thought they belonged to a different type of dinosaur, which he named Brontosaurus(or “thunder lizard”). With its long graceful neck and whiplike tail, Brontosaurusbecame a familiar favorite among dinosaur lovers. But, in 1974, scientists decided that Brontosauruswas really Apatosaurus—that there was no Brontosaurusafter all.

Everyone was excited about the discovery of Apatosaurus. One newspaper published this picture of the huge creature looking into an 11th-story window.


To make matters worse, Apatosauruslost its head. Honestly. Because the original skeleton was found without a head, no one was sure what Apatosaurus’sskull looked like. For nearly 100 years, models and drawings in museums all over the world showed it with no head or the wrong head. Finally, the skull was located in a museum basement, and Apatosauruscould once again hold its head up proudly.

Do you ever feel that way?

“I feel as if I’m missing something.”

Absolutely not!


Or maybe not. Recent evidence indicates that Apatosaurusand other dinosaurs with long, elegant necks may not have been able to lift their heads far off the ground. For many years, experts believed that these vegetarian dinosaurs reached their necks into the treetops to dine, like giraffes.

Recently, however, computer modeling has revealed that the positioning of their neck bones would have prevented them from raising their heads any higher than about 12 feet. While it may have been able to rear up on its hind legs to snag treetop treats, more often Apatosaurusprobably reached its long neck across creeks or other obstacles as it grazed for food along the ground.

We finally have a complete picture of Apatosaurus, but there’s a lot more to learn.

Computer Images

To create these images of Apatosaurusand its neck movements, scientists entered information about the size, number, and positioning of the animal’s neck bones into a computer, which fi gured out just how far the dino could move its neck up and down and sideways.


Noisy Tails?

What would you do with a 50- foot, 3,500-pound tail? Scratch your own nose? Go fi shing? Apatosaurusmay have had a better idea. Computer modeling shows that these dinosaurs could have cracked their tails like a whip. Three feet thick at the body, the tail thinned to a narrow tip about the width of a human thumb. As the dino lashed it, a wave of energy would have traveled down to the tip, arriving at a speed faster than that of sound—more than 750 miles per hour. When something moves that fast it “breaks the sound barrier, “ creating a loud noise. A cracking dino tail would have made a huge, thundering clap—loud enough, perhaps, to scare off an enemy. But some scientists think that such supersonic tail wagging is unlikely because it would have hurt or damaged the dino’s tail. After all, they remind us, just because the computer says it was possible doesn’t mean it actually happened that way.


Snorkel? Weapon? Noisemaker?

Parasaurolophus’scurious crest has scientists guessing.

Maybe it’s his thinking cup?

Listening to the Bones

Computers have become a valuable tool to help paleontologists picture dinosaurs better. Scientists can now even hear what dinosaurs might have sounded like. Consider Parasaurolophus, an odd-looking fellow. A large, bony crest extended from the top of its head back over its shoulders. Handsome, perhaps, but what did the crest do? Some scientists think it may have been a weapon or even a snorkel, but no evidence supports these ideas.

How about a trombone? When a nearly complete Parasaurolophusskull was found in New Mexico, computer scientists and paleontologists joined forces to see if the head crest might actually have been a noisemaker.

“Sounds like a tuba. No, a trombone, definitely a trombone.”


Snorkeling Brachiosaurus

Apatosaurusand its cousins—the “long-necks”—weighed more than any other creature that ever lived. For a long time, scientists thought that the only way these huge animals could move their massive bodies must have been in the water. Their long necks would keep their heads up so that they could breathe as they walked along a lake or river bottom, grazing on soft aquatic plants.

But modern scientists wondered: wouldn’t the water’s pressure collapse the great creature’s lungs and disable its heart? Probably so, which made scientists imagine that the long-necks may have wandered on land after all.

New research reveals yet another possibility. Sometimes, along “trackways” where dinosaur footprints have been preserved, scientists find only front footprints. In ancient times, these trackways may have been underwater. Could the long-necks actually fl oat, touching the bottom with only their front feet? That way, they could live in water without their giant bodies being deeply submerged.

Computer modeling shows that this may have been possible, especially since we now know that the creatures had tiny air sacs along their spines that would help them stay afloat. The catch? If they lost contact with the bottom, they would probably tip over.


They took scans of the inside of the crest with special imaging equipment so that they wouldn’t have to break it open. Then they used the scans to model a three-dimensional image of the crest. They found that it was filled with a complicated series of hollow tubes and chambers. When the computer created the sounds made by blowing air through the crest, the scientists heard an eerie, deep rumbling tone. Were the scientists hearing a sound not made on earth since the dinosaurs lived here? Who can know?

This computer image shows the complicated tubes and chambers in the fossil of the Parasaurolophus’s crest.


Think Critically

1 What did ancient people believe about the dinosaur bones they found? Why did they believe what they did? MAIN IDEA AND DETAILS

2 Does the author think that paleontologists have an easy job? Explain your answer with details from the selection. AUTHOR’S PERSPECTIVE

3 Think back to a time when you came across an object you had never seen before. What did you do to figure out what it was? MAKE CONNECTIONS

4 Do you think that what scientists today believe about dinosaurs will be the same many years from now? Explain. SPECULATE

5 WRITE What are some ways in which computers have helped paleontologists learn about dinosaurs? Use details and information from the selection to support your answer SHORT RESPONSE


Saturday Night at the Dinosaur Stomp


Narrative Poetry

Word went out ‘cross the prehistoric slime:

“Hey, dinosaurs, it’s rock ‘n roll time!

Slick back your scales and get ready to romp

On Saturday night at the Dinosaur Stomp!”

By the lava beds and the tar pit shore,

On the mountaintop and the rain forest floor,

Dinosaurs scrubbed their necks and nails.

They brushed their teeth and curled their tails.

Then—ready, set, go—they trampled and tromped,

Making dinosaur tracks for the Dinosaur Stomp.


Plesiosaurus paddled up with a splash,

A pterodactyl family flew in for the bash.

Protoceratops brought along her eggs,

Diplodocus plodded on big fat legs.

A batch of bouncing babies followed Mama Maiasaur.

The last time she counted, she had twenty-four.

The old ones gathered in a gossiping bunch,

Sitting and sipping sweet Swampwater Punch.

Dinosaurs giggled and shuffled and stared,

Ready to party, but a little bit scared.

Then Iguanodon shouted, “One, two, three!”

Started up the band by waving a tree.

Brachio-, Super-, and Ultrasaurus

Sang, “Doo-bop-a-loo-bop,” all in a chorus.

Ankylosaurus drummed on his hard-shelled back,

Boomalacka boomalacka!Whack! Whack! Whack!


Pentaceratops stood up to perform

And blasted a tune on his favorite horn.

They played in rhythm, they sang in rhyme,

Dinosaur music in dinosaur time!

Duckbill thought he’d take a chance:

Asked Allosaurus if she’d like to dance.

Tarchia winked at a stegosaur she liked.

They danced together, spike to spike.

The Triassic Twist and the Brontosaurus Bump,

The Raptor Rap and Jurassic Jump.

Tyrannosaurus Rex led a conga line.

Carnosaurs capered close behind.

They rocked and rolled, they twirled and tromped.

There never was a party like the Dinosaur Stomp.


The nighttime sky began to glow.

Volcanoes put on a fireworks show.

The ground was rocking—it started to shake.

Those dinosaurs danced up the first earthquake!

The party went on—it was so outrageous,

They stayed up well past the late Cretaceous.

When the Cenozoic dawned they were tired and beat.

They yawned big yawns and put up their feet.

And they’restill asleep, snoring deep in the swamp.

But they’ll be back … next Dinosaur Stomp!



Comparing Texts

1. Do you think a paleontologist has an important job? Explain.

2. Compare the information in “Dragons and Dinosaurs” and the poem “Saturday Night at the Dinosaur Stomp.” What is the author’s purpose in each text?

3. What did you know about how scientists study dinosaur bones before you read “Dragons and Dinosaurs”? What did you learn from the article?



Reading-Writing Connection

Analyze Writer’s Craft: Expository Nonfiction

Expository nonfiction gives facts about a topic. It may include headings, diagrams, photographs,and captions. When you write expository nonfiction, you can use the works of authors such as Meg Moss as writing models. The passage below is from “Dragons and Dinosaurs.” As you read it, look for ways to improve your own writing.


Research Report

A research report is a type of expository nonfiction. It gives information about a topic. A research report includes a list of the books, articles, and websites the writer used to find the information. As you read Alex’s research report, notice the text features and text organization he used.

Student Writing Model


Now look at what Alex did to prepare to write his research report.


Alex used note cards to write down information from a variety of sources. He copied the complete website addresses of online sources onto his note cards. He wrote down each book’s title, authors, publisher, city, and year. He also copied the name of the author and the title of each magazine article, the magazine’s name, and the issue in which the article appeared.



Then Alex created an outline to organize the information for his report. The outline shows the order in which Alex plans to include ideas in his introduction, three subtopics, and conclusion.

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