With their little brains, they were not very smart, but nevertheless, the huge reptiles we call dinosaurs ruled the earth for more than 100 million years. Then suddenly, almost instantly, they disappeared.
All we know about dinosaurs comes from studying fossils. Scientists have categorized them into two basic types: the saurischians, which had hips like lizards, and the ornithischians, which had hips like birds. Many dinosaurs were plant-eaters, but some were meat-eaters. They ranged from the turkey-sized Compsognathus to the 90-foot-long Diplodocus and the 50-ton Brachiosaurus.
No one knows what caused the extinction of these "terrible lizards." One theory is that the Earth's climate became cooler and the cold-blooded dinosaurs became too sluggish to hunt for food. (Cold-blooded animals need warm weather to survive.)
But that theory does not explain the almost instant disappearance of dinosaur bones from the fossil record.
Recent evidence supports another theory, that a large meteor or asteroid may have struck the earth and caused a huge dust cloud that blocked so much sunlight that almost all of the plants that fed the dinosaurs died. And when the plants they ate died, so did the dinosaurs.
