The extinction of dinosaurs

Excavations of rocks in the late Cratecous period have shown that there were a wide variety of dinosaurs living at that time. Then, at the end of the Cratecous period, 65 million years ago, they all died out. There was no obvious slow disappearance and there were o survivors afterward. We can guess that the dinosaurs died out at the same time everywhere in the world – although this has not been proved.

Many other kinds of animals also died out 65 million years ago, but many survived, too.

We can see that nearly all the big land and sea animals died out, including the fifteen or so different families of dinosaurs that were living at the end of the Cratecous, such as alamosaurus, ankylosaurus, edmontosaurus, ornithomimus, oviraptor, pachycephalosaurus, saurornithoides, triceratops, and tyrannosaurus.

These plants and animals died out	These plants and animals survived
Forams (single-celled plankton) Some shell-fish (rudist bivalves, ammonites, belemmites) Ichtyosaurs Plesiosaurs Mosasaurs Pterosaurs Dinosaurs	Most microscopic plants Land plants Other shell-fish (bivalves gastropods) Snails Sharks, bony fish Frogs, salamanders Turtles Crocodiles Lizards Birds Mammals

There are a lot of theories about how the dinosaurs died out, but only few of them could be searched out a bit more.

One of the recent ideas is that there were worldwide changes in the temperature. In many areas, it seems that the air and sea became hotter at the end of the Cratecous. We have evidence for this from changes in the plants, and from the measurements of the chemical composition of certain kinds of rocks that were being laid down at the time. A rise in the temperature could have seriously upset the habitat of many animals, and sensitive ones might have died out. Large dinosaurs could have simply overheated.

The temperature change may have had another effect. The high temperatures were followed by a gradual cooling. This caused climate changes and the subtropical forests where the last dinosaurs lived were replaced by open pine forests.

The other theory is that radiation from the Sun or from an exploding planet broke through the protective layers of the Earth's atmosphere. This would mean that the dinosaurs have died from radiation sickness. One particular idea was that radiation from the Sun might have caused blindness in the dinosaurs. These ideas are difficult to prove.

The most popular "Disaster theory" for the extinction of the dinosaurs is that the Earth was hit by an asteroid or a comet 65 million years ago. The asteroid is thought to have weighed 4 million tons. When it hit the Earth it would have formed a crater 60 to 90 miles (100 to 150 km) wide. The asteroid would have exploded deep down in the crater and shot out up to 400 trillion tons of rock and dust. The dust would have blocked the light from the sun. The asteroid might also have heated the air and produced acid rain. The effects of this kind of impact would have killed off many forms of life. However, no crater of the right size and age has been found. The best evidence for the asteroid is found in the sedimentary rocks at the very end of the Cratecous. In several places around the world, at exactly 65 million years age, geologists have found large amounts of the metal iridium in clay bands. Iridium is a rare metal that comes only from space. We normally find only very small amounts, which come from the tiny meteorites that land on earth all the time. But there are very large amounts of iridium at the time the dinosaurs died out. Many scientists belive this is good evidence for an asteroid, but the case is not proved.