Living things have been on Earth for thousands of millions of years. During that time, there were long periods in which nothing much changed, and then short catastrophes in which changes happened quickly. Geologists call the long quiet times 'periods' and name them to match places where good fossils are found. The age of the dinosaurs and other giant reptiles began about 230 million years ago. The time is called the Triassic Period by geologists. It was a time when there was just one continent and much of it was near desert.
