From the first Visitors to museums in many cities walk through exhibits that recall life on earth long ago. Many people pause at displays that show huge animals. Some of these animals look like animals of today, but they are much larger. These giants lived during a period of time called the Age of Mammals. This age followed a period called the Age of Reptiles, and it started long before the beginning of human history. It included the Ice Age when great ice sheets covered many parts of the world.