Basilosaurus

A giant killer whale

In the seas of the late Eocene, about 40 million years ago, Basilosaurus ruled. When scientists first found the fossils, they assumed the bones had to belong to a dinosaur. This is how Basilosaurus got its misleading name. Contrary to what its name suggests, it was not a reptile but a mammal. Today, Basilosaurus is considered the ancestor of whales.

Basilosaurus was extremely impressive in shape and the largest mammal of its time. It could grow up to 18 metres long and, like today's whales, had a fluke, the characteristic tail fin. What it didn't have was a blowhole, so it had to stick its head out of the water to catch its breath every 20 minutes or so.

Basilosaurus hunted everything that came in front of its voracious mouth. This included sharks as well as smaller whales. Since it lived in the warm tropical seas of the Eocene, it did not develop a layer of fat, unlike today's whales.

This is exactly what doomed Basilosaurus. When the oceans cooled down considerably with the transition from the Eocene to the Oligocene about 36 million years ago, it was defenceless against the lower temperatures and became extinct.

This may be the reason why Basilosaurus is not directly related to any whale species living today. The evolution of the large, carnivorous marine mammal, which was exclusively adapted to warm waters, proved to be an evolutionary dead end. Basilosaurus' prey, on the other hand, the smaller whales, evolved over the following millions of years into today's toothed and baleen whales.