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.