The 3 Groups
Cetaceans are divided into three main groups or sub-orders, one of which is extinct: the archaeoceti (the archaeocetes or ancient whales, which are extinct), mysticeti (the mysticetes or baleen whales) and odontoceti (the odontocetes or toothed whales). The main differences between the extant (not extinct) groups are their feeding methods, number of blowholes and use of echolocation.
This group contains four families: the grey whale, the right whales, the pygmy right whale and the rorquals, which include whales of the genus Balaenoptera and the humpback whales. They are enormous filter feeders which use baleen plates (made of keratin, the same material as our fingernails or rhino horn) to strain small organisms such as plankton, krill and small fish from the water. Many baleen plates are arranged along the upper jaw of the whale hanging parallel to each other. The inside edge of each plate is frayed and these frayed strands knit together with the strands from nearby plates forming a mat of strands. When the whale is feeding it fills its mouth with water (or in the case of grey whales, sediment) then pushes the water (or mud) out with it tongue. The baleen acts like a strainer keeping the food inside the mouth while letting the water (or mud in the case of grey whales) escape.
Mysticetes also differ from odontocetes by having two blow holes instead of one and are not known to use echolocation.
Odontocetes belong to a much more diverse group and are divided into ten families: the oceanic dolphins, four river dolphin families, beaked whales, porpoises, the Kogia, sperm whales and the monodontidae family which contains the beluga and narwhale. The members of this group are active hunters which mostly feed on larger prey including fish, crustaceans, mollusks (such as squid), seabirds and other marine mammals. They generally have uniformly shaped teeth for catching and grasping their food and possess teeth in one or both jaws, although in some species of beaked whales the female’s teeth may never emerge through the gums.
All members of the group odontoceti have a single external blowhole and make use of echolocation to help learn about the world around them and to hunt for food.
When archaeocetes first appeared about 50million years ago they had hind limbs and were probably mostly terrestrial but hunted for fish in the water. Later species had vestigial hind limbs, probably looked more like sea lions and were semi-aquatic, living mostly in the water but going on land occasionally. Only the most recent archaeocetes were fully aquatic animals. The archaeocetes died out about 30 million years ago.