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 plant-eating 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.