The best way we have of knowing what creatures were alive on this earth millions of years ago is from studying fossils. Fossils are animal and plant remains buried in the rocks. These remains, which have in most cases petrified (turned to stone), may be of shells, insects, leaves, bones, whole skeletons, or simply the tracks made by ancient animals on the shores of swamps.

From such evidence, scientists believe that dinosaurs roamed the earth about 180,000,000 years ago, and that they died out about 60,000,000 years ago. What fossils have been found that make these scientists believe this?The most common ones are bones, teeth, and claws. From these, skeletons can be reconstructed so we can tell how the body was built. In other cases, there have been trails and footprints.

In the Gobi Desert of Asia, nests and huge bones of dinosaurs have been found. Scientists have even found potato-shaped fossil eggs laid by dinosaurs with dinosaur “chicks” almost ready to hatch! In fact, dinosaur fossils have been found on all the continents, including North America. So we must conclude that the dinosaurs lived in practically every part of the world millions of years ago.

Certain conditions were necessary, however, for the dinosaurs to exist The dinosaurs were reptiles, which means they were cold-blooded. Their body temperature changed with the air temperature. Many of them lived in swamps, where the luxuriant vegetation provided them with food.

But the climate of the world underwent a great change about 60 million years ago. Swamps dried up. Lowlands became mountains. The climate became colder and drier. So the dinosaurs not only lost their homes—the swamps and lowlands—but the cooling climate made them sluggish. It also killed off much of the vegetation they lived on.

In time, the dinosaurs, who were unable to adjust to the new conditions, died out and became extinct. They were replaced by warmblooded animals with larger brains and the capacity to get along under the new and changing conditions.