When dinosaur bones were first found, scientists thought
that they belonged to giant lizards. The word "dinosaur"
comes from the Greek words dinos, meaning terrible, and
saros, meaning lizard. Dinosaurs were not lizards, but the
biggest flesh-eating dinosaurs were indeed "terrible". They
stood as much as 20 feet high and had skulls 4 feet long,
with huge, daggerlike teeth. The biggest plant eaters must
have shaken the earth when they moved. Some were 90 feet
long, and some weighed 85 tons. But not all dinosaurs were
giants. One kind was only 2 feet long.
Scientists learn about dinosaurs from fossils, the records
and remains of ancient living things. They study them and
try to reconstruct the appearance and habits of the
animals. This work requires a thorough knowledge of the
body structure of living animals and of the relation of
body structure to habits.
Dinosaurs were as varied in appearance and habits as are
land animals of today. Some dinosaurs were large, some
small. Some walked on two feet, some on four. Some ate
plants, others ate meat. Some had smooth skin, others had
scaly skin. Still others had skin studded with bony plates.
But all dinosaurs had tiny brains. The dinosaurs, like
other reptiles, laid their eggs on land. Scientists believe
that large dinosaurs lived to be more than 100 years old.
When dinosaurs fist appeared, North America was a low-lying
continent with a great sea bordered by swamps where the
Rocky Mountains now rise. The climate was warm, and forests
of pines, and redwoods grew on land. The lush vegetation of
the swamps furnished food for these animals.
Scientist have suggested many theories explaining why the
dinosaurs died out. Probably the basic cause was the rise
of mountain ranges during the Cretaccous period. When the
mountain ranges formed, the great seaways drained from the
continents and the vast swamplands that were the homes of
the dinosaurs dried up. This caused tremendous changes in
climate and food supply. But the dinosaurs could not adjust
and they eventually became extinct.