You may know that land mammals evolved from sea creatures that eventually were able to grow legs and lungs, but what about whales, sea mammals? Were they in the ocean before land mammals existed?
Interestingly, whales came from land mammals that adapted to the water. This means that whales went through a full circle: originating in the ocean, moving to land, and then moving back to the ocean.
Land mammals evolved at around the time of the dinosaurs, 178 million years ago. They were small and were dominated by the other huge land animals existing around their time. However, the meteor that caused the extinction of the dinosaurs (65 million years ago) helped mammals become more prominent, and it was after the mass extinction that whales started to evolve.
Timeline to the Basilosaurus
It took many years for land mammals to be able to evolve into a whale, and between those years there were many intermediate species that have now become extinct. Scientists have only discovered these intermediate species in the past century. The oldest living “whale” known is a creature called pakicetus, a wolf-like animal, and it was discovered in 1983. It had an ear position that was between a land animal’s ear position and an aquatic animal’s ear position. After pakicetus came ambulocetus, an animal similar to pakicetus but its hind feet were better adapted to swimming. Ambulocetus was discovered in 1994 and also had a tail. Then, there was rodhocetus, which had a shorter neck vertebrae for a more stable and short neck, similar to whales. Basilosaurus was the last intermediate species that only split into one species. It had sturdy flippers and a flexible body, and even though it had legs, it could not walk. It looked similar to whales these days, except it had a much more snake-like body.
Above: a photo of what basilosaurus could have looked like
Around the time basilosaurus was alive (33.9- 55.8 million years ago), the continent of Antarctica was in the process of moving away from South America. As it moved farther away, the currents on the other side of South America were able to cool and the basilosaurus wasn’t able to survive the changing temperatures. The extinction of basilosaurus split whales into 2 main groups: odontoceti, toothed whales who could use echolocation, and mysticeti, large whales who had baleen. Some examples of odontoceti are dolphins, porpoises, killer whales, and sperm whales, and some examples of mysticeti are blue whales and humpback whales.
Odontoceti and Mysticeti
To become the whales we are able to see today, odontoceti had to develop echolocation, and mysticeti (baleen whales) had to become larger and replace teeth with baleen. Baleen whales first went through a period of sucking prey into their mouth and then chewing before transitioning into having neither teeth nor baleen. They relied on their cheeks to digest food. All of this happened 20 million years ago, before the ice age, when whales were smaller but also much more diverse than they are nowadays. The megalodon, an extremely large shark, was roaming the ocean and it could prey on whales.
Scientists are not sure why baleen whales evolved to be so large, but one theory is that during the ice age, prey was harder to find, and a larger body would help them travel longer distances. They also had a chance to get bigger because krill would crowd in one area. This also explains the extinction of the megalodon because as whales grew in size, the megalodon had fewer prey to hunt, and would eventually die of starvation.
Hippos and Whales
Because many intermediate species have become extinct, the whale’s closest living relative is now the hippo. Both whales and hippos are mammals well adapted to water, and both have blubber, multi chamber stomachs, thick bones, and do not have hair or glands. Studies show that there was once a 4 legged-mammal well adapted to water that was the common ancestor of whales and hippos. However, scientists have been unable to find intermediate species linking that ancestor to the hippo.