The dinosaurs ruled the world for about 150 million years. The first dinosaur came on the face of the Earth more than 200 million years ago. The last one died about 63 million years ago. Dinosaurs were a special group of prehistoric reptiles and their closest living relations are crocodiles.
All dinosaurs lived on land and almost all of them were big. The longest were the length of ten cars parked nose to tail, the tallest could have looked over the roof of a three-story building. At the other extreme there were dinosaurs no bigger than a turkey. In between there were all shapes and sizes. And many of them could be considered the weirdest creatures that ever appeared on planet Earth.
There were about 150 different dinosaur species, and on these pages, you'll find about a lot of them.
Sometimes there were more names for the same species. That's mostly because these species were found in different parts of the world, and even more because of the rival scientists in the nineteenth century who competed who will find more fossils. For example, a scientist in the late 19th century named the huge "thunder reptile" Bronrosaurus. But later it was found that a rival scientist had discovered it before him, and he named it Apatosaurus. So, now we use the name Apatosaurus because it was first given.
The dinosaurs lived in Mesozoic era, but they did not live all in the same time. Some of them lived in the same time and place together, as a fauna. If we follow the sequences through time, we can see how faunas evolved. The three main periods in the Mesozoic era are Triassic, Jurassic and Cratecous.
So, then, what is NOT a dinosaur?
Since dinosaurs lived on land only, many prehistoric animals cannot be considered dinosaurs. In the air there were Pterosaurs, a separate group of gliding reptiles which ranged in size from a crow to an airplane (with a wingspan up to 33ft. (10m) across). Two large groups of reptiles ruled the sea during the age of dinosaurs: the ichthyosaurs and the plesiosaurs. These animals were neither closely related to dinosaurs nor to each other.
There were also many reptiles that lived before dinosaurs, such as "sail lizards", and also some mammal-like reptiles. Extinct mammals (like mammoth) are also not dinosaurs. We will not talk about them.