Tyrannosaurus Rex (American Museum of Natural History)
The first dinosaurs evolved out of earlier types of chordate reptiles about 230 million years ago, on the big land mass called Pangaea, when all of the land on earth was stuck together in one big continent. Like other reptiles, dinosaurs were generally cold-blooded (they got all their body heat from the sun), and they laid eggs and waited for them to hatch. Some dinosaurs may have been warm-blooded, or partly warm-blooded.
Some dinosaurs gradually evolved to be bigger than any animal had ever gotten before, and bigger than any mammal today. Big dinosaurs like the ultrasaurus had more than 1000 trillion cells in them, and may have weighed as much as fifty tons - that's a little more than a loaded semi truck. Other dinosaurs were much smaller, the size of chickens, and some of them could fly. Some dinosaurs may have had feathers.
Some dinosaurs ate only plants, while other dinosaurs like Tyrannosaurus Rex ate meat - mainly from other dinosaurs, but they also ate the small mammals that were around after about 200 million years ago.
Dinosaurs lasted a long time, but about 65 million years ago, at the end of the Cretaceous period, most of the dinosaurs died. Probably a big meteor hit the Earth from space, and the huge dust cloud raised by the meteor blocked so much sunlight that Pangaea got too cold for the dinosaurs. Because there was only one big continent, all the dinosaurs lived close enough together to be killed by the same dust cloud. The only kind of dinosaurs that lived through this cold period was a small kind that evolved into modern birds, turning their scales into feathers for warmth and becoming warm-blooded like mammals, though they still lay eggs like dinosaurs.
To find out more about dinosaurs, check out these books from Amazon.com or from your library: