For over 150 million years, the Earth was ruled by some of the most extraordinary creatures, the giant reptiles, popularly known as the dinosaurs. The age of the dinosaurs was the Mesozoic Era which lasted from about 225 million years ago to 65 million years ago. The term dinosaur was coined in 1842 by the English zoologist Richard Owens, from two Greek words deinos and sauros meaning ‘monstrous lizard’ or ‘terrible lizard.
Dinosaurs were short-legged animals; most of them had gigantic bodies. Most of the giant reptiles were vegetarians, but the few flesh-eaters were indeed terrible. They were able to live both on land and in water. Some walked on two feet, others on four. Many dinosaurs had smooth skin while others had scaly skins and still others were armoured with large bony plates. Their mental development was quite slow and hence they had small brains.
Dinosaurs are divided into two classes based on the difference between the structures of the hip bone: the saurischians or lizard hipped and the ornithischians or bird hipped.
The Saurischians: The lizard hipped saurischians are further subdivided into two groups: the sauropods or plant eaters and the theropods or meat eaters.
The sauropods (plant eating quadrupeds) were the largest dinosaurs. They include the massive brontosaurus. This huge creature grew to lengths of 24 m and weighed about 32 metric tons. The brontosaurs probably spent much of its time eating in the huge swamps that existed then. All of the sauropods had long necks and tails and walked on all four legs. Even larger than the brontosaurs were the brachiosaurs, largest of all the dinosaurs. The brachiosaurs weighed 77 metric tons and was 21 m long.
The meat-eating theropods walked on their hind legs. They used their very short front legs for grabbing and tearing food. The tall and terrible theropods hunted the plant eating sauropods. The tyrannosaur was the largest meat eating animal ever to live on the Earth. It was 6 m tall and had teeth that were 15 cm long. Its head was larger than that of most dinosaurs and its front legs were too small to be of any use.
The Ornithischians: The bird hipped ornithischians included both the armour plated dinosaurs and the duck billed dinosaurs. They were divided into four groups: ornithopods which walked on two legs and included some with bills like a duck’s; stegosaurs – which had great bony plates for protection; ankylosaurs – which were even more heavily armoured and ceratopians or horned dinosaurs.
The disappearance of dinosaurs 65 million years ago remains a mystery. Several theories have been put forward to explain this.
One theory suggests that dinosaurs became extinct due to the changing climate and want of food. Another theory suggests that the development of egg-eating mammals may have been the chief cause. Some scientists even think that certain harmful bacteria may have evolved and wiped out most of the dinosaurs. Only the first of these theories could receive general acceptance.
