Cetaceans is the official name for the whale family. When you hear the word 'whale', you usually picture a giant animal. However, porpoises which are only 1.5 meters long also fall under cetaceans. The predecessors of the cetaceans lived on land. During the Tertiary period (around 50 million years ago), a group of predator-like mammals moved to coastal regions. They slowly adapted to swimming in the sea. They were animals without back legs, but had fins and a wide flattened tail.