Dolphins belong to the family of cetaceans (aquatic mammals) which includes whales, dolphins and porpoises (Save The Whales, 2003). Two types of cetaceans can be identified: those which have teeth and those which have baleen. The Odontoceti include dolphins, porpoises and some species of whales such as sperm whales, beluga whales, beaked whales and narwhals. The second type are Mysticeti, or moustached whales, which includes baleen whales such as the blue, gray, humpback and minke whales. These have baleen, keratinous projections from the roof of the mouth, which allows the whales to be filter feeders.
The primitive cetaceans are believed to have evolved during the Paleocene period, around 50 million years ago, in the area which is now the Mediterranean Sea and the Arabian Gulf (Hyperion, 2003; Save the Whales, 2003). There is a lack of fossil evidence between the first cetaceans and their ancestors, so there may have been intermediary species. They are believed to have evolved from mammals and are classified under the suborder Archaeceti, which were mainly aquatic. Their hindlimbs were reduced, they had long snouts, and their forelimbs became paddle-like flippers or pectoral fins with hand and finger bones, so were adapted to the shallow coastal fringes of the seas Hyperion, 2003; Save the Whales, 2003).
During the Paleocene and Eocene periods, a large amount of speciation took place, and natural selection favored adaptations to capture fast-moving fish, rather than the fresh-water molluscs their predecessors fed on (Hyperion, 2003). Their bodies were elongated, with nostrils on top of the snout, long palates, and heavy bones around the ears. They were limited to warm waters, and could only make short dives. In the Oligocene period from 38 to 25 million years ago, Archaeocetes were replaced by at least four families: the Agorophilidae and Squalodentidae, which were both primitive odontocetes, and the Aetiocetidae ...