Dinosaurs were all different sizes. Some
dinosaurs were smaller than a chicken.
Others weighed as much as 100 tons—
more than 10 times as much as the largest
elephants.
Dinosaurs did not have any hair. Their
skin had a bumpy or pebbly surface.
Some scientists believe that dinosaurs
were gray or green in color. These
colors would have helped the dinosaurs
blend in with their surroundings. Other
scientists think that dinosaurs were very
colorful. Bright colors would have
helped males get the attention of
females.
Scientists have divided dinosaurs into
two large groups. The groups are based
on the way a dinosaur’s hip bones were
shaped. One group is the Ornithischia,
or bird-hipped dinosaurs. Members of
this group are called ornithischians. The
other is the Saurischia, or lizard-hipped
dinosaurs. Members of this group are
called saurischians.
Some dinosaurs had hip bones that resembled those of modern reptiles. Others had hip
bones that resembled those of modern birds.
50 Dinosaur BRITANNICA STUDENT ENCYCLOPEDIA
Ornithischian Dinosaurs
The ornithischians had hips shaped like
the hips of today’s birds. Despite this
similarity, however, today’s birds are not
related to the ornithischians.
The ornithischians were plant eaters.
They walked on either two or four
thick, sturdy legs. Ornithischians generally
had flat teeth to grind tough plants.
They also had an extra bone at the tip of
the lower jaw. This bone joined the two
halves of the lower jaw together. It also
helped to form a horny beak that made
it easier for the dinosaurs to bite plants.
Many of the ornithischian dinosaurs had
special features. Some, such as Triceratops,
had horns like a modern rhinoceros.
Euoplocephalus had a huge club at
the end of its tail. Stegosaurus had large,
triangular bony plates sticking out of its
back.
Saurischian Dinosaurs
The saurischians had hips that look like
the hips of today’s reptiles. But they also
had some features in common with
today’s birds. In fact, scientists believe
that the saurischians are more closely
related to birds than to some other dinosaurs.
The earliest bird, Archaeopteryx,
was a saurischian. The saurischians are
divided into the meat-eating theropods
and the plant-eating sauropods.
Theropods
The theropods include all the meateating
dinosaurs. Among them is the
fierce Tyrannosaurus rex, which weighed
as much as 8 tons. Giganotosaurus and
Spinosaurus were even larger. Theropods
walked on their two back legs. Most of
them were built to hunt. They used their
short front arms to catch and tear prey.
Their fingers and toes had long, sharp
claws. They had powerful jaws and sharp
teeth for tearing flesh.
Theropods normally hunted alone. They
caught and ate smaller, plant-eating
dinosaurs. Sometimes they hunted in
groups and attacked larger dinosaurs.
Sauropods
Sauropods were not only the largest of
all dinosaurs. They were also the largest
Plant-eating and meat-eating dinosaurs had
different mouth features.
BRITANNICA STUDENT ENCYCLOPEDIA Dinosaur 51
land animals that ever lived. Sauropods
were plant eaters that walked on four
legs. Brachiosaurus, Diplodocus, and
Apatosaurus are examples of sauropods.
Sauropods had a very small head and
brain when compared to their huge
body. But they had a long neck, which
let them reach leaves on even the tallest
trees. They had thick, strong legs, like
those of an elephant. Many sauropods
probably could stand up on their back
legs to reach even higher into the trees.
They used their long, muscular tail to
balance themselves when standing.
Some sauropods also might have used
the tail as a whiplike weapon.
Life Cycle
Most, if not all, dinosaurs laid eggs.
Some dinosaurs built large nests to keep
their babies together. Young dinosaurs
grew very fast. Many reached full size in
seven or eight years. Large dinosaurs
might have lived almost 100 years.
Extinction
The last dinosaurs died out about 65
million years ago. Scientists still do not
agree about why this happened. Some
scientists think that dinosaurs died out
because the temperature on Earth got
too hot or too cold for them. Others
The biggest dinosaurs may have been more than 130 feet (40 meters) long.
A dinosaur sits on its eggs.
52 Dinosaur BRITANNICA STUDENT ENCYCLOPEDIA
believe that a huge asteroid collided with
Earth. The collision could have produced
dust that blocked the sun’s heat
and light for months or even years.
Plants would have stopped growing, and
plant-eating dinosaurs would have died
from lack of food. So would have the
meat eaters that hunted them.
Some animals lived through the time
when the dinosaurs disappeared. The
ancestors of today’s frogs, turtles, lizards,
and snakes found a way to survive. Birds
also survived. Scientists do not know
why some animals lived but the dinosaurs
did not.
#More to explore
Apatosaurus • Archaeopteryx • Bird
• Brachiosaurus • Euoplocephalus
• Fossil • Geology • Reptile
• Stegosaurus • Triceratops
• Tyrannosaurus Rex
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