Dinosaurs.

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Dinosaurs.
Imagine walking through a prehistoric forest and coming face to face with a terrifying dinosaur.
This could never have happened, of course. People and dinosaurs did not live on Earth at the
same time. Dinosaurs died out long before people appeared. Dinosaurs lived from about 230
million years ago to about 65 million years ago.

HOW DO WE KNOW ABOUT DINOSAURS?
Paleontologists (scientists who study prehistoric life) learn about dinosaurs by studying fossils of their
bones. These fossils are the remains of dead animals that have turned into rock.
People probably found fossils of dinosaur bones thousands of years ago but did not know what the bones
were. In the early 1800s, people realized that the fossil bones belonged to prehistoric animals. The first
dinosaurs that paleontologists studied were named Megalosaurus and Iguanodon. The bones looked like
those of reptiles, such as lizards. The flat teeth of Iguanodon showed that it was a plant eater. The
pointy teeth of Megalosaurus showed that it was a meat eater.
The early fossils came from England. British scientist Sir Richard Owen in 1842 named these animals
dinosaurs. The word comes from two Greek words meaning "terrible" and "lizard." Scientists now know
that dinosaurs were not lizards.

KINDS OF DINOSAURS
Paleontologists have found fossils of hundreds of different kinds of dinosaurs that lived all over the
world. Some dinosaurs were small like birds. Other dinosaurs were much bigger than an elephant.
Paleontologists divide dinosaurs into two groups, or orders, by the way their hips looked. One group had
hips that were like birds' hips. They called this order Ornithischia. The other group had hips like those of
lizards. They called this order Saurischia.
There were three basic kinds of dinosaurs in the Saurischia order: theropods, prosauropods, and
sauropods. There were five basic kinds of dinosaurs in the Ornithischia order: stegosaurs, ankylosaurs,
ornithopods, pachycephalosaurs, and ceratopsians.

THEROPODS
All dinosaurs were plant eaters except for theropods. Theropods were meat eaters that hunted planteating dinosaurs and smaller theropods. Most theropods walked on their two hind legs. One of the most
famous theropods is Tyrannosaurus rex. This dinosaur was about 39 feet (12 meters) long and weighed
5 metric tons.
Some theropods were called raptors. Velociraptor and other raptors had powerful claws, like the claws of
an eagle. These dinosaurs probably hunted in packs.

PROSAUROPODS
Prosauropods had spoon-shaped teeth and long, slender necks. These large, plant-eating dinosaurs stood
on two legs and grazed on tall bushes and trees. One type of prosauropod, Plateosaurus, was 30 feet (9
meters) long and weighed 1.8 metric tons.

SAUROPODS
Sauropods descended from prosauropods. Some of the best-known sauropods were Apatosaurus and
Diplodocus. These dinosaurs had thick legs and feet like elephants' feet, but with claws. They walked on
four legs. At one time sauropods were the biggest dinosaurs on Earth. Some sauropods may have been
more than 82 feet (25 meters) long and weighed about 90 metric tons.

STEGOSAURS
These ornithischians walked on four legs and had a row of bony plates down their back. Stegosaurus, a
large stegosaur, also had spikes on its tail. It used its spiked tail for fighting off meat-eating theropods.
Stegosaurus was about 30 feet (9 meters) long.

ANKYLOSAURS
These dinosaurs were covered with bony plates that acted as body armor to protect them against
theropods. Some had a bony plate in each eyelid and large clubs on their tails. Bony rings and spines

protected their necks. Ankylosaurus was about 33 feet (10 meters) long.

ORNITHOPODS
Ornithopods were once the most numerous plant-eating dinosaurs. There were many different kinds.
Over time, ornithopods developed broad beaks. These later ornithopods are called duck-billed dinosaurs.
Some ornithopods were small and ran fast. Others were huge. Iquanodon, for example, was 25 feet (7.5
meters) long.

PACHYCEPHALOSAURS
These dinosaurs walked on two feet. They had thick, dome-shaped skulls. Scientists think they may
have used their thick heads to butt other dinosaurs.

CERATOPSIANS
These dinosaurs ware also called horned dinosaurs. One of the best-known ceratopsians was Triceratops.
It had three horns on the top of its head: one horn on its snout and one horn above each eye. Triceratops
could reach lengths of up to 26 feet (8 meters) and weighed more than 12 metric tons.

WHAT HAPPENED TO THE DINOSAURS?
There are several theories about why dinosaurs died out. Many scientists believe that there was a
sudden, violent event. They think that an asteroid or comet collided with Earth. Fire from the impact
burned large areas. Dust blocked sunlight from reaching the surface of Earth. Acid rain fell everywhere.
Plants could not grow. There was no food for the plant-eating dinosaurs, so they died out. Dinosaurs that
ate plant-eating animals also died out.
Other scientists think that the environment changed. Maybe the climate grew colder and dinosaurs
slowly became extinct over several million years.
Many scientists think that birds descended from a small kind of dinosaur. If this is true, then dinosaurs,
in a sense, are still alive in the form of modern birds.
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