Prehistoric animals are animals that lived before people began to record history. Some of these creatures, such as dinosaurs, were different from any animal alive today.<NP> Scientists learn about prehistoric animals by studying fossils. The oldest animal fossils are estimated to be about 700 million years old. But most experts believe the first simple animals lived millions of years before then.<NP> All animals lived in the sea until after the first plants appeared on land about 430 million years ago. Early forms of spiders and insects were among the first animals to live on land. Dinosaurs and other reptiles dominated the earth from 240 million to 63 million years ago. Mammals have dominated since then.
