Life on Earth - Dinosaurs
The first dinosaurs evolved about 225mn years ago, and mammals first appeared at this time too, although their reign only really began at the extinction of the dinosaurs.

Dinosaur literally means “terrible reptile/lizard”. Classification of dinosaurs has lead to dispute. Many believe they should have a classification of their own equal in status to “mammals” and “reptiles” (one that crawls). Others believe they should be classified as reptiles. The first dinosaur named was in England in 1824 and was called “Megalosaurus” (giant reptile).Ultrasauros. Image from http://www.kidsdinos.com/images/dinosaurs/Ultrasauros1143667423.jpg


The dinosaurs were present during the Jurassic period (See Age of the Earth) which lasted roughly 70 million years. At the time of the dinosaur the continents were all joined together thereby facilitating migration over the whole surface of the earth. Dinosaur fossils have been found in all continents including Antartica. There were 300 different species of dinosaur and about 2000 species of prehistoric animals altogether. The largest creatures to ever have walked the earth are the sauropods including Ultrasauros and Supersauros.

Extinction
Dinosaurs suddenly became extinct about 65 million years ago. It is not known why the dinosaurs disappeared in the Jurassic Period. It is thought that maybe the type of food they normally ate may have changed or that small mammals were eating their eggs. It is also thought that a meteorite hit the Earth causing a layer of dust to block the sun therefore gradually killing off dinosaurs and their food supply. However, no theory has ever been proven.

There have been many mass extinctions throughout the history of the earth, but we often think about the extinction of the dinosaurs as the main one only because of how spectacular these animals were (this extinction is known as the KT boundary extinction). Other extinctions have come far more close to destroying all life on earth.

To go further
http://www.bbc.co.uk/sn/prehistoric_life/

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