Dinosaurs evolved from reptiles.However,they were different from reptiles in that their body was supported by their legs.
Early reptiles had legs that stuck out sideways from their bodies.They dragged their bodies along the ground,and raised it only for brief periods.The dinosaur's body,on the other hand,was raised off the ground,and supported by legs that were beneath the body.
As dinosaurs multiplied,they evolved in many ways.Some of them became mammal-like.They were called synapsids.One of them,known as the Dimetro-don,had a sail like structure on its back.By turning the sail to face the sun, these cold blooded reptils could absorb heat.Gradually, some dinosaurs became warm blooded, and evolved into true mammals.
There were more than 700 different types of dinosaurs.They are often classified as either meat-eaters(carnivores) like Tyrannosaurus Rex, or plant-eaters(herbivores) like Triceratops.
All dinosaurs looked different.Some walked on two legs and other walked on four.Some were very fast like Velociraptor but Some were slow and lumbering, like Ankylosaurus.
Some had horns and spikes, or bumpy skin.Some had sail-like structures on their backs.Our knowledge of dinosaurs is limited by the fact that dinosaur fossils are quite rare.
Some dinosaurs were huge,other were small.The smallest dinosaurs were only about the size of a chicken,or even smaller. The biggest was thought to be the Brachiosaurus.It was about 30 meter long, and must have weighed over 130 tonnes,making it the heaviest land animal to have ever existed.
The tallest dinosaur was the Sauropoeidon. It had a very long neck and a huge tail, and was three times as tall as giraffe !
