The first dinosaurs evolved out of earlier types of chordate reptiles about 230 million years ago, on the big land mass called Pangaea. At that time, 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). They laid eggs and waited for them to hatch. Some dinosaurs were 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. They 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 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. The huge dust cloud raised by the meteor blocked so much sunlight that Pangaea got too cold for the dinosaurs. The only kind of dinosaurs that lived through this cold period was a small kind with feathers that evolved into modern birds, becoming warm-blooded like mammals, and developing beaks instead of teeth. But birds still lay eggs like dinosaurs.