Basic Dolphin Evolution
All dolphins, whales, and porpoises are related. Fossil evidence
from the early Eocene epoch, indicate that the early dolphins
or Protocetidae were already aquatic 45 to 50 million years
ago. However, they bore little resemblance to the dolphins
and whales we know today.
Fossil dolphins can be traced through time by the changes
in their teeth. About 30 million years ago the early dolphins
split into the two main divisions known today: the toothed
whales, and the baleen whales. Delphie, the group from which
modern dolphins arose, began to appear in the early Miocene
about 25 million years ago.
Because the dolphins evolutionary line diverged some 50 million
years ago, their physical structure had time to develop independently
from terrestrial mammals. For instance, the fossil record
indicates that there was a stage when the animals were amphibious.
However, this record also shows that once the early dolphins
decided to be aquatic, it didn't take long for the bones of
the hind limbs to disappear altogether.
The obvious streamlining of a dolphin's bodies caused by
living in water has produced several other modifications to
the animal's physiology. The thickened body and raised head
has caused the vertebrae of the cetacean neck to become fused.
Instead of the major blood supply to the brain running up
the outside of the neck, as in land animals, it passes through
an artery within the fused vertebrae. This insures a constant
supply of blood when the dolphin dives to extreme depths in
the ocean. (Dolphins and man...equals?)
What are cetaceans?
Cetaceans is a collective term for whales, dolphins, and porpoises.
It contains two suborders Mysticeti and Odontoceti. The baleen
whales are members of the Mysticeti suborder, while the toothed
whales, dolphins and porpoises make up the Odontoceti suborder.
Within these two suborders contain eighty-one known species,
separated into thirteen different families. (Cetacea)
Dolphins are mammals, not fish. They are warm-blooded like
man, and give birth to one baby or calf at a time.