Between the time dinosaurs went went extinct 65 million years ago, and the beginning of the Ice Age, some 2 million years ago, is a time called the Age of Mammals. During this period of time the western third of Montana experienced a variety of geological activities, some of which raised mountains and lowered valleys. The valleys became the homes of a wide variety of interesting and bizarre animals. Many of these have since gone extinct or today live in much warmer climates. During the early part of this period, about 35 million years ago, Montana was home to animals like the double-horned rhinoceros-like brontotheres, and primitive little three-toed horse, camels, and deer. In the later part of the period, 20 million years ago, the valleys hosted bizarre-looking ancestors of our modern elephants, camels, deer and pigs. These included elephants with shovel-like tusks, deer-like animals with forked horns over their noses, and pigs the size of hippos. Montana was such and subtropical. The extinction of these groups of animals would coincide with the coming of the Ice Age, and the arrival of humans less than 50,000 years ago.