By Chelsea Thomas
Dolphins are cetacean mammals closely related to whales and orpoises. There are almost forty species of dolphin in 17 genera. They vary in size from 1.2m and 40kg, up to 9.5m and 10 tones.
Their terrestrial origins are indicated by their need to breathe air from the watee surface, the bones of their fins which resemble the limbs of land mammals. And the vertical movement of their spines.
Around 50 million years ago, certain four legged land creatures started spending more and more of their time in water. (An animal that there are fossil records of from around this time named Pakicetus may represent this initial stage). For some reason, this change must have suited them, as they gradually evolved, and their bodies changed form, so that they eventually lost the ability to move onto the land at all. An interim stage in this development is represented by the Protocetids and examples of the first exclusively aquatic dwelling form are the Durudonand the 60 foot/18 metre long Basilosaurus. They lived some 38 million years ago.
The cetaceans (whales, dolphins and porpoises) are descendants of land-living mammals, and remnants of their terrestrial origins can be found in the fact that they must breathe air from the surface; in the bones of their fins, which look like huge, jointed hands; and in the vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish.
Pakicetus was the first whale. Odd as it seems, a four footed land mammal named Pakicetus, living some 50 million years ago in what we know as Pakistan today.