EVOLUTION In the beginning...

After the mass extinction event at the K-T boundary 65 million years ago, large creatures such as the dinosaurs and many other organisms disappeared. Mammals began to dominate the landscape. Some began to fill the niches of the aquatic environments that had lost large predators like the pleisiosaur and icthyosaur . Food was abundant in the water and these mammals adapted to their new environment by making the transition from land to water.

So what is the terrestrial ancestor?...

Basilosaurus lived in the Eocene, around 35 to 40 million years ago.