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,
was a low-lying continent with a great sea bordered by swamps where the Rocky 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.