HOW MANY TYPES OF DINOSAURS WERE THERE?

Dinosaurs roamed the earth during the Mesozoic era, a period of time that ran from about 225 million years ago to 65 million years ago. When the dinosaurs first appeared on earth, scientists say North America was a low¬lying continent with a great sea bordered by swamps where the Rocky Mountains now stand. The weather was warm and there were many tropical forests.

Dinosaur is a common name for two types of reptiles that lived in prehistoric times. The word comes from Greek words dinos, which means terrible, and sauros, which means lizard.

There were two groups or orders of dinosaurs. These groups differ in the structure of the reptiles' hip bone or pelvis. The Saurischia had hips similar to those of modern lizards. The Ornithischia had birdlike pelvises.

The lizard hipped dinosaurs in turn were also divided into two groups: the giant plant eaters which were called the sauropods and the meat eaters which were called theropods.

There were two groups or orders of dinosaurs. These groups differ in the structure of the reptiles' hip bone or pelvis. The Saurischia had hips similar to those of modern lizards. The Ornithischia had birdlike pelvises.

The lizard hipped dinosaurs in turn were also divided into two groups: the giant plant eaters which were called the sauropods and the meat eaters which were called theropods.

Sauropods were the largest of all land animals. Included in this group was the Brontosaurus, a swamp resident who grew up to 80 feet in length. All the plant ¬eating sauropods had long necks and long tails. Their heads were out of scale: they were only about two feet long.

The meat eating theopods walked on their hind legs rather than on all four. They had big heads and large, sharp teeth.

The bird hipped dinosaurs were divided into four groups: the stegosaurs and the ankylosaurs, two types of armored dinosaurs; the ornithopods or duck billed dinosaurs and the ceratopsians, or horned dinosaurs.

Why did the dinosaurs disappear? Scientists say it was probably because the mountain ranges were formed and the great seaways drained from the swamplands. New plants appeared and old ones died out. The plant eating dinosaurs could not live on the new plants and as a result they too died out. The meat eating dinosaurs that depended on the plant eaters also died out. This dying out process was a long, slow time that actually took between 10 and 20 million years.

Some scientists say that the development of mammals also ranked as an important reason why the dinosaurs became extinct. Mammals had better brains and were better protected against the cold. Also, the developing mammals probably ate lots of dinosaur eggs and young dinosaurs which contributed to the extinction of the prehistoric reptiles.