恐龙曾是地球上无可争议的主人，它们统治地球长达1.6亿年。三叠纪时（距今2.5亿到2.1亿年前的一个地质阶段），它们从原始爬行动物开始演化，在侏罗纪时占据了统治地位，直到白垩纪结束时仍雄霸一方（“恐龙”的意思就是令人恐惧的蜥蜴）。
大约6500万年前，恐龙同其它数不胜数的动、植物一起灭绝了。时至今日，世界上发现了约350个恐龙属。有的恐龙如兔子般大小，有的恐龙身长超过45米。恐龙属陆地动物（蛇颈龙和翼龙分别生活在海洋和空中，它们不属于恐龙），它们曾成功地征服过地球上的每一个角落。恐龙的灭绝，就像它们在陆地上生存的大部分时间一样，被蒙上了一层神秘的色彩。
人类对于恐龙的认识，都是从点滴的蛛丝马迹开始的。几块碎骨和几颗牙齿在不经意中为寻找那段遥远的过去点燃了一丝光亮。1822年，在英国首次发现了恐龙化石，从那时起就掀起了一股恐龙热，人们的热衷程度如此之高，使得这股恐龙热变成一种时髦。很难确定大众对恐龙如此青睐的原因，但有一点是肯定的，即这些庞然大物在人们的脑海中激发了无穷而又美好的遐想。
Imagine walking through a prehistoric forest and coming face to face with a terrifying dinosaur.This could never have happened，of course.People and dinosaurs did not live on Earth at the same time.Dinosaurs died out long before people appeared.Dinosaurs lived from about 230million years ago to about 65million years ago.
Scientists use fossils to learn about dinosaurs.Fossils can be bones or impressions in rock.These footprints were made by a dinosaur that walked through the mud millions of years ago.The mud hardened and eventually turned into rock.
Dinosaurs varied greatly in size and shape.The giant Apatosaurus grew to lengths of 80feet (24meters).The Compsognathus was a small predator about 2feet (60centimeters)tall about the size of a modern day goose.
These skulls show how the jaws and teeth of dinosaurs differed depending on what they ate.The knifelike teeth of the meat eating Allosaurus，left，helped the animal rip the flesh of other animals.The flat front teeth of the plant eating Diplodocus，center，helped it remove leaves from plants，which it swallowed whole.The small，broad teeth of Massospondylus，right，enabled the animal to chew plants before swallowing.
How Do We Know About Dinosaurs？