Cetaceans, including whales, dolphins, and porpoises, are a group of aquatic mammals that evolved from terrestrial ancestors. The discovery of proto-land whales, the pakicetids, in Pakistan in 1983 provided evidence of their evolution. Pakicetids had three characteristics unique to whales and were at least semi-aquatic. Ambulocetids, also found in Pakistan, were fully aquatic and had special ear adaptations for underwater hearing. Protocetids were a diverse group of large, stumpy-footed cetaceans found in Asia, Europe, Africa, and North America. Basilosaurids, including Basilosaurus and Dorudon, were the first fully marine cetaceans and eventually evolved into modern whales.
Cetacea, derived from the Latin word for whale, “cetus”, is the order of mammals that includes whales, dolphins and porpoises. Cetaceans are one of four groups of mammals adapted to aquatic life, the others being the sirenids (dugongs and manatees), the pinnipeds (seals and walruses) and an aquatic subfamily of mustelids, the otters. Like other aquatic mammals, cetaceans evolved from terrestrial ancestors.
For many years it has been a mystery how cetaceans evolved to be what they are today. This lasted until the discovery of a group of proto-land whales, the pakicetids, in Pakistan in 1983. The pakicetids are the earliest known cetaceans, living during the early Eocene, about 53 million years ago. Their fossils were unearthed in an area of Pakistan that was coastal to the ancient Tethys Sea, a body of water connected to the World Ocean and most directly analogous to today’s Indian Ocean.
Pakicetids are considered cetaceans because of three telltale characteristics that only whales have: a specific placement of the ear bones within the skull, a bend in the middle ear bone, and the way the cusps are arranged on the molars of the animal. These features may seem like minor details, but they prove that pakicetids were ancestors of whales. Carnivorous terrestrial animals, pakicetids looked more like wolves than anything we would recognize as cetaceans today. Because their bones were found near what was once water, they are thought to have been at least semi-aquatic.
Some pakicetids are thought to have evolved into ambulocetids (“walking whales”), best exemplified by a nearly complete fossil skeleton of the species Ambulocetus natans, a remarkable 3 m (10 ft) long “mammal crocodile”, considered an excellent example of a “missing link” (transitional) fossil. Ambulocetids, also found in Pakistan, lived 50-49 million years ago. The animal was clearly very aquatic, although it had legs and could have supported its own weight on land. Analysis of the teeth showed that it could live in both freshwater and saltwater environments and possessed special ear adaptations that would have allowed it to hear well underwater. Look for a picture of ambulocetus and you will see how strange it looked.
During the same period, the protocetids emerged. The Procetids, meaning “early whales,” were a complex and diverse group. Fossils or fossil fragments of protocetids have been found in Asia, Europe, Africa and North America. Protocetids were large, with stumpy feet, and were beginning to look more like today’s cetaceans. Protocetus, one species, had a dolphin-like body. It is not known whether this family had tail flukes like modern whales, as this body part is not skeletal and does not fossilize well.
The first two groups of fully marine cetaceans were basilosaurids, including Basilosaurus — the “King Lizard,” mistaken for a reptile when discovered in 1840, hence the “saurus” in the name — and Dorudon. Basilosaurus skeletons, up to 8 m (60 ft) long, have always been considered fantastic. Their most famous feature is their 0.6 meter (2 ft) long rudimentary limbs. Basilosaurid fossils date from 40 to 34 million years ago and have been found in Egypt, Pakistan and the United States.
Basilosaurids eventually evolved into the whales as we know them today.
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