The Evolution of Ancient Whales

Indohyus lived about 50 million years ago. It had long legs designed for walking on land, and a long tail. But it also had several adaptations that distinguish it from the other small mammals of the time, like a thick, bony covering over the inner ear, and thicker bones so that it could wade on the bottom of rivers.

Ambulocetus natans lived 49 million years ago. It had webbed feet, and a streamlined body plan to help it swim. It is believed that it still wasn't a very good swimmer, and that its adaptations for water dwelling inhibited its movement on land. So it is believed to be an ambush predator, like modern crocodiles.

Rodhocetus lived about 46 million years ago. It had many more adaptations for living in water, such as having its nostrils on top of its head, unfused sacral vertebrae so that it could flex its body up and down for locomotion, and webbed feet. It also had some holdovers from its landlubbing ancestors, including differentiated types of teeth, and well developed hind limbs.

Basilosaurus cetoides looked a lot like a modern whale, with an elongated, streamlined body, hindlimbs that were vestigial, a blowhole, flippers, and even a tail fluke. But on the other hand, it still did have hindlimbs, as vestigial as they were, and it also had differentiated teeth. Basilosaurus was an apex predator in its day, about 40 million years ago.

Dorudon atrox lived 40 million years ago. It had many of the same adaptations that the modern day whales do, such as a tail fluke, blowhole, and a streamlined body. But it also had vestigial hindlimbs, a strange flipper/hand combo, and differentiated teeth. However, it was these creatures, not the seemingly more advanced Basilosaurus, that would give rise to modern day whales.

Finally, below is a modern Blue Whale (Balaenoptera sp.). It has many of the adaptations handed down by its ancestors, such as a streamlined body plan, a flexible spine, a blowhole, flippers, and a tail fluke. However, it has a few things that set it and other modern whales apart from their predecessors. Its hindlimbs are completely absent, it has only a single type of dentition, and it is a filter feeder (though not all modern whales have this feeding strategy.)