Mammals

Mammals are warm-blooded. But the platypus and the echidna lay eggs. People are mammals too. Mammals started out as rat and mouselike small animals in the dinosaur ages. When the age of the reptiles ended, the mammals and birds took over. Tropical forests appeared, and also big plains, where indricotheres, creatures that could tower over elephants and giraffes, lived. There was Hyaenodon, a doglike predator the size of a large rhino, and bear-dogs, little doglike animals the size of housecats. Then after that, in South America, grassy plains appeared, bringing agile antelope that could go thirty meters in several seconds. But there were also huge ground sloths, and armadillos with spiked clubs at the end of their tails. One couldn't forget Smilodon, the sabertoothed cat, or Phorusrhacos, flightless birds who preyed on hippidiforms, small skittish horses. When that ended, our ancestors appeared, apelike animals that could walk upright.

Ocean Mammals
In the beginning of mammals, there were ten-foot crocodile-like animals that swam by undulating their bodies up and down, and had fur. These were the predecessors of whales. There were two types of whales. Dorudon, a sixteen-foot whale, and Basilosaurus, a much larger whale. Modern ocean mammals include dolphins, sea otters,seals,sea lions, and whales. Above all, mammals are a large and diverse group.