Dinosaurs are the largest animals that have ever lived on land. They roamed the Earth for about 160 million years, but died out 65 million years ago. The first people did not appear on Earth for another 64 million years and have, so far, lived for only a fraction of the time that dinosaurs did.

Scientists knowledge of dinosaurs comes mainly from the study of fossils - hardened remains of plants or animals preserved in rock.

The history of Earth, since it began about 4,500 million years ago, is divided into eras. The age of of the dinosaurs spans most of the Mesozoic, or middle, era. This era is itself divided into three periods - the Triassic (256-208 million years ago), the Jurrasic (208-146 million years ago) and the Crettaceous (146-65 million years ago)(see Geological Time). Reptiles already existed on Earth before dinosaurs appeared. During the early part of the Triassic period many new kinds were evolving, such as the fast-running cynodont ("dog-toothed") reptiles, which preyed on slow-moving herds of plant-eaters. Most early reptiles held their legs out at an angle from the sides of their bodies, much like today's lizards.

Eventually these were replaced by archosaurs ("ruling reptiles"). One group had a different body shape and could hold their legs directly below their bodies. This seems to have been the start of the successful body design found in their descendants, the dinosaurs ("terrible lizards").

By the end of the Triassic period, the first true dinosaurs roamed the world. But the dinosaurs' heyday was during the Cretaceous period, when the greatest number and variety were found. Over 1,000 species of dinosaurs have been recorded so far. These can be divided into two dinstinct groups: herbivores (plant-eaters) and carnivores (meat-eaters).

Of all the dinosaurs that have been discovered, the most popular ones are explored in more detail below and on the next page.

The largest dinosaur was Brachiosaurus. It was nearly 24m (79ft) long and its head reached 17m (56ft) above the ground to munch the leaves on which it fed. It could have looked over the top of a house and, if it had stood in the gardens, its tail would have been four houses away from its nose. Brachiosaurus would certainly have crushed a few flowers. It weighed up to 51 tonnes - as heavy as 250 Sumo wrestlers.

Brachiosaurus walked on four legs and, like the other Brachiosaurids and unlike most dinosaurs, its front legs were longer than its hind legs.

Diplodocus was even longer than Brachiosaurus. It measured 27m (89ft) from the tip of its nose to the end of its tail, nearly as long as three buses, bumper to bumper. Its nostrils were at the top of its head and it had peg-like teeth, but only in the front of the jaws. Its front legs were shorter than its back legs.

Diplodocus's tail was so far from it's brain that you could have stood on it's tail and run away before the message reached the dinsosaur's brain. But it would have been wiser not to try it! Diplodocus had such a small brain in its head that scientists think that it may have had a second brain down its spine to control the back part of its body.

Diplodocus was more lightly built than the other giant sauropods, and may have weighed only about 10-20 tons.

Tyrannosaurus Rex was the largest meat-eating dinosaur. It was 16m (52ft) long and stood up to 6m (20ft) high. Its huge jaws were full of dagger-like teeths 15cm (6in) long.

But Tyrannosaurus Rex may not have been as ferocious as it looked. Its front legs were so small and weak, it would not have been much of a fighter. Most likely it preyed on old or weak dinosaurs, or scavenged the left-overs of other dinosaurs' prey.

Triceratops was a rhinoceros-like dinosaur. It walked on four sturdy legs and had three horns on its face along with a large bony plate projecting from the back of its skull (a frill). One short horn above its parrot-like beak and two longer horns (over 3 feet or 1 m long) above its eyes probably provided protection from predators. The horns were possibly used in mating rivalry and rituals.

It had a large skull, up to 10 feet (3 m) long, one of the largest skulls of any land animal ever discovered. Its head was nearly one-third as long as its body. Triceratops hatched from eggs.

Stegosaurus was up to 26-30 feet long (8-9 m), about 9 feet tall (2.75 m), and weighed about 6,800 pounds (3100 kg). Its small brain was only the size of a walnut (weighing roughly 2.5 - 2.9 ounces (70 - 80 grams).

Its skull was long. pointed, and narrow; it had a toothless beak and small cheek teeth. Its head was carried close to the ground, probably no more than 3 feet (1 m) high.

Stegosaurus had 17 bony plates that were embedded in its back. The plates were probably well-nourished by blood vessels, indicating that the plates may have been used to regulate the dinosaur's temperature. They may have also been used for protection or mating display purposes.