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Life first appeared on Earth about 3.8 billion years ago. From the first simple life forms that lived in the sea, life gradually moved on the land, and into the sky. When dinosaurs were alive, Earth was very different from the world we know today.The scene above shows a landscape from about 100 million years ago. It was a time when the first flowering plants appeared, and when oak, maple, walnut and beech trees grew alongside conifers, cycads and ferns. It lasted for about 185 million years, starting 250 million years ago and ending 65 million years ago.One of the most popular ideas about the death of the dinosaurs is the Asteroid idea. A giant block of rock from space smashed into Earth. The impact sent huge clouds of dust and debris into the air, to spread on the wind. The skies darkened for days, perhaps years, cutting out the Sun's light and warmth. In the cold gloom of this "asteroid winter" plants withered and died.With no food to eat, plant-eating animals starved and the meat-eaters followed, as their food also disappeared.
Another idea about the death of the dinosaurs is Invisible Rays from deep space. As a huge star reaches the end of its life, it may explode in a cosmic fireball. The blast sends out rays and radiation at the speed of light. Dangerous radiation bathed the Earth, killing animals which could not hide.
Another mass extinction theory involves widespread, long-lasting volcanic eruptions. They poured poisonous, choking fumes into the air and darkened the skies with dust. There is some evidence for this idea. The Deccan Traps are a vast area of volcanic rocks in what is now southern India. Parts of these uplands date back to the end of the Cretaceous Period. The eruptions could have been set off by rock movements and drifting continents( or by the impact of an asteroid!).

( The World of the Dinosaurs by John Malam & Steve Parker)