It is generally agreed that the evolution of whales (cetaceans) began 60 million years ago with Sinonyx (Mesonychids) a carnivorous undulate (hoofed animal) the size of a wolf. Key features of this fossil are its triangular shaped teeth and skull. The next step in the development of the whale was Pakicetus, 52 million years ago (Early Eocene), an animal with long tail and thick neck and hooves. Amulocetus, 50 million years ago, developed a body better adapted for swimming than walking on land. It is thought that ambulocetus filled a nitch similar to alligators and crocodiles.
Rodhecetus, 46 million years ago, had large limbs that could support the body on land, and swim in the oceans. It is likely that they were amphibious mammals. Basilosaurids and dorudontids, 35 million years ago, possessed the physical characteristics recognized as whales.
Squalodon, 33 to 15 million years ago, developed characteristics similar to odontocetes (toothed whales) and Cetotherium, 30 to 14 million years ago,developed characteristics similar to mysticetes (baleen whales).
Whales size provides them with biological advantages, protection from environmental factors such as predation, temperature and pressure. The open ocean provides the nutrients to maintain a large body mass.
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