Skeleton of a mesonychian. There is, however, disagreement among scientists what these ancestors looked like. Whatever the real ancestor looked like, whale origins took very little time. In about 8 million years whales evolved from quadrupedal terrestrial mammals to obligate marine swimmers. This transition did not only affect the locomotor system, but also the hearing organ and the means in which these animals fed.
Some scientists think that the land relatives of whales are a group of mammals called mesonychians.
Mesonychians are long extinct, they lived from the Paleocene through Oligocene in North America, Asia, and Europe. They varied in size from a weasel to a grizzly bear and had teeth that suggest that they ate meat or carrion. Other scientists think that whales are closely related to the modern hippopotamus and that they had an ancestor in common. Hippo ancestors can be traced back to a nimble even-toed ungulate that lived in the Eocene of Europe and was only as large as a cat. All pictures on these pages are public access, although the source must be identified in publication.