Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
The Evolution of the Whale
This is a story of an ancient land mammal that reversed the evolutionary pattern and returned to sea. As it made this transition, it lost its legs and developed a body tailored to marine life. For many years this hypotheses was underdeveloped, until fossil discoveries in Afghanistan made researches nearly certain of its accuracy. A look at these ancient creatures will bring us to understand the physical changes that resulted in the whale.
Evolution of Whales
The best approach to understanding the evolution of the whale is to
understand the whale itself. A whale is a Cetacean, an order of mammals that
are extremely smart, and have very advanced methods of communication. There are
two groups of whales; the Mysticeti, which are baleen whales and Odontoceti,
which are toothed whales. All whales are mammals (warm blooded vertebrates),
they give birth to live young which are nursed by their mothers. Adult whales do
not have fur, yet while in the womb the fetus has body hair temporarily. The
whale’s vertebrae are very flexible so the movements of the tail are very
Whales have many exceptional characteristics that are not shared by other
land or aquatic mammals. The head of the whale is
long and has an extended jaw. The top jaw bone extends up and often covers the
skull. This is because the nasal opening has moved to the top of the skull,
creating a blow hole and moving the brain to the back of the skull. Because of
the design of their jaw, whales are unable to chew their food; rather the food
is ground in their fore stomach. Whales do not have skin and tear glands, nor do
they have external ear openings, though they have exceptional hearing. This is
because of a shell like bone called a melon organ in their skull that picks up
Pakicetus is believed to be the whale’s oldest ancestor. These
animals lived around 52 million years ago, in the early Eocene. The first
evidence of Pakicetus was found in Pakistan, which is how it got its name.
Pakicetus was a mammal that looked much like a dog with hoofs. They had a long
bushy tail and their ear region was crossed between a land and aquatic animal.
Pakicetus is believed to have been an aquatic predator that spent much of its
life on land but hunted in the water. The tooth and ear structures of Pakicetus
resembled that of a modern whale.
Ambulocetus (meaning walking whale) is believed to be the next step of the evolutionary pattern. Discovered in Pakistan, this animal lived about 50 million years ago, in the early to middle Eocene. Ambulocetus was equipped for both land and sea, it is believed to have moved between fresh and salt water. Its front feet had fingers with small hooves adapted to walk on land, while its back feet were more adapted for water propulsion. Ambulocetus looked similar to a hairy alligator, its hunting patterns also resembled that of an alligator. They went after large pray, their jaw and tooth structure lead us to believe that they drowned large pray by holding it underwater. Ambulocetus had an ear structure similar to that of a modern whale and a special nose that allowed it to swallow food underwater.
Rodhocetus lived in the Middle Eocene 46-7 million years ago, and was still able to walk on land though not very well. The first fossils were discovered in Pakistan but remains have also been found in Asia, Europe, Africa, and North America. This creature was believed to have a very powerful swimming tail. Its vertebrae and muscles were farther apart than that of earlier ancestors. The spine was fused expect for 4 vertebrae allowing more flexibility. It had a hefty dorsal spine much like whales today. Rodhocetus had a larger head and tail than its predecessors, yet the limbs were much smaller and less flexible. The ankle and ear bones are very similar to that of the modern whale.
Dorudon lived during the Eocene about 40 to 36 million years ago. The remains of this animal have been discovered in Egypt and North America. Dorudon reached about 5 meters in length and was a carnivore. The overall look of Dorudon was very similar to the modern whale. Dorudon did not have the melon organ used for communication among modern whales. Dorudon lacked the ability to go on land. It had tiny hind legs that barely protruded from the body. Dorudon was a carnivore that ate mostly small fish and mollusks.
Basilosaurus (meaning king lizard/reptile) lived at the same time as Dorudon in the Eocene 40 to 37 million years ago. Its remains were discovered in abundance in the southern United States and have also been found in Australia, and Egypt. This creature was about 60 to 85 feet long and had a pair of very small hind legs, about 2 feet long. Its jaw was around 5 feet long and was equipped with cone shaped teeth in the front and triangular shaped teeth in the back. Bailosaurus is believed to have had eel like movements. It is considered to be the closest the whale has ever come to a snake. Basilosaurus was not equipped for deep sea swimming. It was a carnivore whose diet was believed to consist of fish and squid. Like Dorudon, Basilosaurus lacked the melon organ that allowed it to sing and use ultrasound like modern whales.
After a run through of the whale’s ancestry, it is much easier to distinguish the evolutionary patterns. Evolution is a response to opportunities and challenges within an environment. Because of the mass extinction during the Cretaceous; mammals were able thrive and alter to fit their preferred environment. This was all possible because they were no longer threatened of the previously predominate Reptiles. The location of these creatures is a contributing factor to the evolutionary theory. The early land dwelling animals were limited to certain locations. As they became more adapted to sea life these areas began to expand. The more aquatic they became the farther away their remains could be found. The fossils themselves also tell a story, the early whales were freshwater animals, so they must have made a transition to sea. The sea has a different chemical balance than fresh water. The early whales had to adapt to the change, which can be detected in the bone composition. The bone structure of the modern whale has evidence as well. The pelvic bone has odd seminaries to that of walking mammals. Also the tiny hind legs found in some modern whales provides better evidence that at one time the now worthless limbs were used for walking. Whale embryos develop hair for a short time while in the womb but do not posses hair as adults; this is a sign of the ancestor’s body hair.