It should be obvious that cetaceans-whales, porpoises, and dolphins-are mammals.
They breathe throughlungs, not through gills, and give birth to live young. Their
streamlined bodies, the absence of hind legs, and the presence of a fluke1 and
blowhole2 cannot disguise their affinities with land dwelling mammals. However,
unlike the cases of sea otters and pinnipeds (seals, sea lions, and walruses, whose
limbs are functional both on land and at sea), it is not easy to envision what the first
whales looked like. Extinct but already fully marine cetaceans are known from the fossil
record. How was the gap between a walking mammal and a swimming whale bridged?
Missing until recently were fossils clearly intermediate, or transitional, between land
mammals and cetaceans.
Very exciting discoveries have finally allowed scientists to reconstruct the most likely
origins of cetaceans. In 1979, a team looking for fossils in northern Pakistan found what
proved to be the oldest fossil whale. The fossil was officially named Pakicetus in honor
of the country where the discovery was made. Pakicetus was found embedded in rocks
formed from river deposits that were 52 million years old. The river that formed these
deposits was actually not far from an ancient ocean known as the Tethys Sea.
The fossil consists of a complete skull of an archaeocyte, an extinct group of
ancestors of modern cetaceans. Although limited to a skull, the Pakicetus fossil
provides precious details on the origins of cetaceans. The skull is cetacean-like
but its jawbones lack the enlarged space that is filled with fat or oil and used for receiving
underwater sound in modern whales. Pakicetus probably detected sound through the
ear opening as in land mammals. The skull also lacks a blowhole, another cetacean
adaptation for diving. Other features, however, show experts that Pakicetus is a
transitional form between a group of extinct flesh-eating mammals, the mesonychids,
and cetaceans. It has been suggested that Pakicetus fed on fish in shallow water and
was not yet adapted for life in the open ocean. It probably bred and gave birth on land.
Another major discovery was made in Egypt in 1989. Several skeletons of another early
whale, Basilosaurus, were found in sediments left by the Tethys Sea and now exposed
in the Sahara desert. This whale lived around 40 million years ago, 12 million years after
Pakicetus. Many incomplete skeletons were found but they included, for the first time in
an archaeocyte, a complete hind leg that features a foot with three tiny toes. Such legs
would have been far too small to have supported the 50-foot-long Basilosaurus on land.
Basilosaurus was undoubtedly a fully marine whale with possibly nonfunctional, or
vestigial, hind legs.