Dinosaurs were one of several kinds of prehistoric reptiles that lived  for over 165 million years during the Mesozoic Era. The reptiles that could fly is called pterosaurs and those who lived in the water is known as plesiosaurs. Dinosaur literally means "terrifying lizard", probably because the first fossils found was from large dinosaurs. Since 1818 dinosaur fossils have been discovered all over the world and today we know about 500 dinosaur genera.

Most dinosaurs were plant-eaters (herbivores) and they range from chicken-size to being over 100 feet (30 m) long and 50 feet (15 m) tall. It is very difficult to figure out how the dinosaurs behaved, sounded and mated. And it is impossible to tell what color or pattern the dinosaurs had. But some had primitive feathers, and the dinosaurs probably live on today as birds.

The dinosaurs went extinct 65 million years ago. The most widely accepted theory is that an asteroid impact brought about high volcanic and tectonic activity. This caused major environmental changes that the big dinosaurs couldn't adapt to.

Mesozoic Era:
Mesozoic literally means "middle animals" and this era is divided into three time periods: the Triassic, the Jurassic and the Cretaceous. During this period the fauna changed drastically from that which had been seen in the Paleozoic. The dinosaurs evolved in the Triassic, but were not very diverse until the Jurassic. 

Triassic (245 to 208 million years ago):
The Triassic followed the Perm-Triassic extinction. After this, the largest extinction event in the history of life, the survivors and new groups spread to dominate the Mesozoic era. Conifers first appeared in the early Triassic. At this time Pangaea existed, which changed the ocean circulation thereby altering the global climate.
The oldest known dinosaur is from 228 million years ago.

Jurassic (208 to 146 million years ago):
The Jurassic has become a household word with the success of the Jurassic Park movies. This is despite the fact that most of the dinosaurs from the films as well as the Dino Island Subtheme comes from the other Mesozoic time periods. In this period giant plant-eating dinosaurs roamed the earth, being stalked by smaller carnivores. At the same time pterosaurs and the first birds first took to the air. The sea levels were high and the extensive water moderated the seasonality and created a warm and moist climate.

Cretaceous (146 to 65 million years ago):
The Cretaceous is the last period of the Mesozoic, but that does not mean that new dinosaurs did not appear. It is during the Cretaceous that Triceratops appeared. Also during this time many insects, modern mammals and birds, and the first flowering plants began to appear. The end of the Cretaceous brought an end to the dinosaurs and laid open the stage for the mammals...