Whales and dolphins are among the most fascinating animals on the planet. That they descended from four-legged land animals makes their story compelling.
That story has become easier to tell given the excellent fossil record accrued in the past 50 years.
Being entirely aquatic it seems odd that whales are mammals. Other mammals such as seals, sea lions and walrusess spend a good part of their lives in the ocean, but they are clearly tetrapods (four-legged). But whales don’t have any leg-like appendages. So what gives?
First let’s be certain about why whales are mammals and not fish, and what unique characteristics they have.
By definition, mammals are warm blooded vertebrates. They give birth to live young that nurse from their mother. Mammals typically have significant body hair, and though adult whales have none, their fetuses do, and some adults have sensory bristles around their mouth.
Unlike fish, whales have thicker, more rounded bodies. They propel themselves through the water with horizontal flukes and their dorsal fins are not supported by bony structures.
Whale skeletons have numerous unique features; several are noteworthy. Some whale species have vestigial pelvises. Amazingly modern whales have the dormant genes required to develop legs. In rare instances these genes will get expressed and a whale develops miniature legs.
The structure of whale vertebrae and the spaces between them facilitate up and down rather than side to side motion. The up-down spinal flexion is characteristic of mammals while the side to side is characteristic of fish.
Whale skulls are different from most mammals in several ways. They have long heads with their lower jaw extending forward while the upper jaw also extends rearward. This moves the nasal opening (blow hole) to the top of the skull, to a more rearward location. It also places the brain and auditory apparatus at the rear of their heads.
The brief descriptions given below demonstrate a transition in body structure from creatures living a primarily terrestrial to ever more aquatic existence. They illustrate a portion of the taxonomic tree of life with many branches. Though some branches were undoubtedly dead ends, others are representative of the lineage of modern whales.
Whale’s closest living relative is the hippopotamus. That is to say, there was an extinct common ancestor of hippos and whales some 60 million years ago (mya). As is the case for all creatures, neither modern whales nor hippos emerged fully formed from their ancient common ancestor. Modern hippos don’t even appear until about 15 mya.
The first creature we can identify with the branch that eventually yields whales dates from about 60 mya and is called sinonyx. The structure of this wolf-sized animal’s skull shows it to have been a fish hunter. As a consequence, its skull shows the beginnings of the elongation that becomes enhanced in later descendants.
Fossils of pakicetus appear in geologic strata dated at about 52 mya. Though still a tetrapod, its elongated skull has many of the features of cetaceans, the order in which whales are classified. Oxygen isotope analysis indicates these animals roamed freshwater shallows, and like crocodiles they fed on fish and land animals coming to drink. The structure of its middle ear indicates it could not dive to great depths.
Ambulocetus fossils from around 50 mya are dramatically more aquatic in appearance. The structure of its hind and fore limbs indicates it waddled about on land somewhat like modern sea lions. Its hide limbs are oriented for forward thrusts to propel it in the water. Its skull is further elongated and has many transitional characteristics showing adaptation to an aquatic life.
Rodhocetus, at about 47 mya, had a spine demonstrating it had musculature to create powerful thrusts with a tail that clearly was more fluke-like. However, its spine and small pelvis indicates it could, with difficulty, still get around on land. Again, its skull has many more transitional characteristics, including nostrils located in a more rearward position.
Basilosaurus is representative of the next phase. Its fossils are dated between 35 and 45 mya. Its pelvis is no longer attached to the spine so that it could not bear weight. Its hind limbs were still external to its body, but they were ineffective, being merely 2 feet long on an animal 50 feet long. As one would expect of a mammal living in a marine environment, its single nostril is further back on the skull.
Dorudon lived at the same time as basilosaurus but was only about 15 feet in length. Its anatomy is more consistent with being in the direct line of descent on modern whales than is basilosaurus. It too had very short hindlimbs barely projecting beyond the body.
Space does not permit a more thorough description. But, in these examples I’ve highlighted a few changes in the skull, the size and position of the hindlimbs, and anatomy of the tail and spine which enabled swimming at the cost of land mobility.
I could elaborate on any number of features of the skull such as those relating to its teeth and hearing. For instance, the skulls of squalodons, which first appear about 33 mya, show the first evidence for echolocation. As with hindlimbs, the forelimbs transitioned from appendages accommodating life on land, to shallow water and eventually exclusively aquatic locomotion. There will undoubtedly be more discoveries. But, looking at what exist today is impressive. For some, there will never be enough. Their cynicism stems more from misunderstandings and preconceived ideas than the reality of the fossil record.
Steve Luckstead is a medical physicist of Walla Walla. Contact him at firstname.lastname@example.org.