Читать книгу Encyclopedia: Dinosaurs - Daniel Coenn - Страница 2

Dinosaurs were one of the most successful types of animal ever to have lived. They are a type of reptile that first appeared on Earth about 230 million years ago. The word dinosaur means "terrible lizard", but this is misleading because the dinosaurs were not actually lizards. They belonged to a group of archosaurs (reptiles), which also included primitive crocodiles and giant flying creatures called pterosaurs. The dinosaurs roamed the planet for over 160 million years. They lived during three different geological periods - the Triassic, Jurassic and Cretaceous - which are often known as The Age of Reptiles.

Scientists have so far identified more than 500 types of dinosaur, but there are probably many more that remain undiscovered. The dinosaurs ranged in size from chicken-sizes species to giants such as Argentinosaurus, which weighed up to 100 tonnes and was largest land animals ever to walk the earth. There were two main groups of dinosaur - the Saurischia, which had hipbones arranged like a lizard's, and the Ornitischia, which had hipbones arranged like a bird's. Each species of dinosaur had a different diet and lifestyle. Some were vegetarian, while others were hunters or scavengers.

Meat-eating dinosaurs had strong jaws and numbers of big, backward-curving teeth. The teeth were often saw-edged, back and front, to help slice through flesh. Meat-eaters skulls were large but had big gaps between some bones to make them lightweight and allow space for the biting muscles to work. The meat-eaters always walked on two legs. Their front legs had sharp claws for grasping prey.

Huge meat-eating dinosaurs, such as Allosaurs and Ceratosaurs, were especially common from 150 to 130 million years ago. This was the time when the really big plant-eaters like Brachiosaurus and Diplodocus were abundant. This suggests that these were the food of the giant meat-eaters. But some people argue that Allosaurus and its kind would have been too slow and weak to overpower the giant plant-eaters, and were scavengers, feeding on dinosaurs already dead.

Tyrannosaurus rex, at 5-6 m tall, was the biggest meat-eater ever to walk the land. It could open its mouth very wide, helped by a lower jaw that bent in the middle, and use its steak-knife teeth on its prey. The teeth of Tyrannosaurus grew to 18 cm in length. An adult Tyrannosaurus weighed up to 7 tonnes. Although Tyrannosaurus is usually shown with two fingers on each hand; nobody knows how many it had really. We do know that its relative Albertosaurus had two.

An animal jaws and teeth can tell us what it ate. Meat-eating dinosaurs mostly had dagger-shaped teeth to size their prey. They probably tore off chunks of meat and swallowed them, rather than chewing. The plant-eaters often had teeth which chopped or crushed their food. Some dinosaurs had (like birds) a bag called a gizzard between mouth and stomach. It contained stones and had muscular walls. It ground up plant food.

Many dinosaurs may have been able to swim or wade, just like mammals such as tigers and elephants nowadays. But dinosaurs were mainly land animals. While they ruled the land, two other groups of reptile took the sea and became good swimmers. These were the fish-shaped, tail-propelled ichthyosaurs and the barrel-shaped plesiosaurs.

The first animals we can recognize as dinosaurs lived about 225 million years ago. To begin with dinosaurs were small animals about a meter long, running on their hind legs. Procompsognathus was one of them. It ate smaller animals. Plateosaurus was one of the first large dinosaurs. It grew to 6 meters long and fed on plants. Crocodiles had the same ancestors as dinosaurs and so are the closest living relatives of birds.

Early dinosaurs were among the first animals to evolve legs tucked right underneath the body. This is the best place for supporting great weight or moving fast to catch prey or escape enemies. It may well have given them an advantage over other animals around.

But the dinosaurs died out in just a few million years. By 65 million years ago they were extinct. At about this time the climate became much cooler and plant-life changed. For animals adapted to warm conditions, like dinosaurs, life became difficult. Some scientist think a sudden climatic change killed them off. This could have happened when a giant meteorite hit the Earth and threw up dust, blocking out sunlight.

There were mammals around before the first dinosaurs, but as dinosaurs came to rile they did not evolve or multiply greatly. Most of them fed on small animals such as insects. As the dinosaurs declined, the mammals grew in number. Perhaps their furry bodies made them better suited to a cooler climate. Once the dinosaurs died out, the mammals took over.

Within a few million years, large mammals arose, including many plant-eaters. Some evolved into early types of elephants, camels and horses. They took over the ways of life of the big dinosaurs.