Basilosaurus. Illustration and photo-reconstruction of two Basilosauruses with open jaws swimming in the sea. Basilosaurus (king lizard) is a genus of early whale that lived 40 to 34 mya in the Late Eocene. The first fossil of B. cetoides was discovered in the United States and was initially believed to be some sort of reptile, hence the suffix saurus, but it was later found to be a marine mammal. The head of Basilosaurus did not have room for the melon found in modern day toothed whales, and the brain was smaller in comparison as well. It is believed that they therefore did not have the social capabilities of modern whales.