The dinosaurs first appeared in the Triassic, some 200 million years ago. At the beginning they were not the giants one thinks of, but were mostly small, some no bigger than a chicken. Like a chicken, they were bipedal, walking or hopping about on their two hind legs. Though later some showed for-legged adaptations, this two-leggedness was from the start an unmistakable and typical sign of the dinosaur line. Towards the end of the Triassic, two great orders of dinosaurs began to take shape. Their main difference was in the structure of the pelvis. The first to develop, the Saurischia eventually came to include four-legged as well as two-legged members, and herbivores as well as carnivores. The other, the Ornithischia, likewise had some four-legged forms, but all ornithischians fed on plants.
The biggest dinosaurs were the giant amphibious saurischians, such as Brontosaurus, Diplodocus and Brachiosaurus. The last named weighed some 50 tons and was the biggest land vertebrate that ever lived. In the skeletons of such animals the crushing point of bone seems to have been approached and they must have spent most of their lives partly supported by water. The most terrible carnivore was also of this order the predaceous Tyrannosaurus, which reached a length of nearly 50 feet and stood, in a bipedal crouch, some 19 feet high.
During the ensuing 50 million years of Jurassic period it was the saurischians that flourished and predominated. In the Cretaceous period that followed it, however, great changes came over both the landscape and the dinosaur fauna in it. There was widespread mountain-building and the mild tropical weather in most regions gave way to more temperate climates. Much of the tropical and sub-tropical vegetation was killed off, the flowering plants evolved and spread, and by the end of the age forest and swamp were essentially modern. In this new Cretaceous landscape the herbivorous Ornithischia flourished. If the Saurischia included the most ponderous land animals the world has known and the most terrible carnivores, the Ornithischia were surely the most bizarre. The peak of dinosaur evolution came in the Cretaceous, in the duckbill, horned and armoured dinosaurs, and the iguanodons and plated dinosaurs. Some of these browsed in woods fundamentally similar to those we know today.
To be continued...
Go To Page: 1
