Early day Cetaceans were known as Archaeocetes.  Within this group include: Mesonychids, Pakicetus, Ambulocetus, Dalanistes, Rodhocetus, Takracetus, Gaviocetus, Dorudon, Basilosaurus, and our present day whales, Mysticetes and Odontocetes.  Archaeocetes lived approximately fifteen to fifty million years ago within the Eocene to the Oligocene eras.  The earliest member of Archaeocetes was Mesonychids, a small, hoofed animal which lived about fifty million years ago.  Mesonychids hunted in the shallow waters of the Tethys Sea, presently known as the Mediterranean Sea.  Pakicetus also lived approximately fifty million years ago in the Eocene era.  Pakicetus' fossils were discovered in present day Pakistan and India and was described as resembling a fox or a wolf crossed with a rodent.  Pakicetus resided around shallow waters and survived on a diet of fish and carrion.  Ambulocetus lived about forty-five to fifty million years ago.  As did Pakicetus, Ambulocetus resided and hunted in shallow waters as well.  It resembled a crocodile with fur, having a wide, flat body with a large head and lots of teeth.  Ambulocetus was approximately nine feet long and weighed about four hundred pounds.  It was a predator which hunted and fed similar to a crocodile.  Dalanistes lived about forty-five million years ago.  It had folded hind legs, short front legs, and had the beginning development of a tail fin.  The tail fin made it more receptive to water, but still having legs allowed it to live on land as well. 

Over the next five to ten million years, approximately, cetaceans front limbs began to develop into front fins and their hind limbs began to disappear.  Their nostrils started developing into blow holes above their heads, preparing them for a more aquatic life.  Rodhocetus and Takracetus had webbed-like feet on their short front and hind limbs, along with a more developed tail fin.  Gaviocetus still had webbed-like feet on their front and hind limbs, but the hind limbs appeared more tucked away.  Dorudon started developing more fin-like front limbs and little, short hind limbs.  Its tail became longer and the tail fin became wider.  Basilosaurus lived about thirty-eight to forty million years ago.  Its fossils were first discovered in present day North America in 1840.  Basilosaurus was, approximately, fifty to seventy feet in length and was fully aquatic.  Its body was described as resembling a snake rather than a whale.  It had completely developed front fins, but still had very small hind legs.  Basilosaurus is also known as Zeuglodon.  Mysticetes lived as far back as about fifteen million years to present day.  The group Mysticetes consists of baleen whales, which the name baleen, comes from the fringed filters they use to feed rather than teeth.   Baleen whales filter in tiny organisms such as plankton and krill for feeding.  Mysticetes have front fins, no hind limbs, and developed a dorsal fin.  Some Baleen whale species include:  the Blue whale, Gray whale, Humpback whale, Minke whale, and Fin whale.  Odontocetes lived from about twenty million years ago to present day.  Odontocetes are toothed whales, with front fins, no hind limbs, and also developed a dorsal fin.  They hunt and feed on fish and smaller mammals such as seal and penguins.  Some of these toothed whales include:  Dolphins, Porpoises, Orcas (Killer whales), Sperm whales, and Belugas (White whales).