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There are two families of odontocetes distinguished by the shape of their teeth: the porpoises (with spade-like teeth) and dolphins (with round teeth).
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Living baleen whales (suborder Mysticeti, in the mammalian order Cetacea) include the biggest animals to have lived. They are well adapted in terms of hydrodynamic, thermal and biological strategies to life in water. But, they retain some typically mammalian attributes which indicate a distant origin on land.
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From a distance, living Dorudon probably looked very much like modern species.
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Kutchinecetus probably spent more time diving than Pakicetus. Hair was no longer needed in its aquatic environment. Blubber provided insulation and helped with body streamlining, which in turn aided swimming.
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Ambulocetus’ bone chemistry suggests it lived partly in fresh water, possibly where a river mouth met the sea. Ambulocetus looked and hunted like modern crocodiles, eating marine fish and maybe even ambushing animals that ventured too close to the water’s edge.
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It bears the title of the "first whale." Pakicetus lived along the margins of a large shallow ocean, the Tethys Sea. Although it had the body of a land animal, its head had the distinctive long skull shape of a whale’s.
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It had a condition that can be described as Osteosclerosis, and is quite common in animals that live aquatic lifestyles because the denser bones allow the body to sink into the water rather than resisting the effects of the water buoyancy pushing back. The theory for Indohyus being either an ancestor or related to the ancestors of whales remains controversial.
