Basilosaurus is considered as an approximately 18 meters (60 feet) long sea serpent who lived in the late Eocene. 30 million years before its ancesters were little ratlike mammals which hid themselves in the trees for hungry dinosaurs. In the course of millions of years the skeleton and the body adapted to a life in water. Basilosaurus had small hind legs, which show that its ancester once was a land living creature. The hind legs still had little toes. For Basilosaurus the legs had no use, and later they entirely disappeared. Although present whales still have two "rudimentary bones", little bones which are placed where in former days the hind legs were which stands not in connection with the others, an evolutionairy remainder of the kneejoint. The first skeletons of Basilosaurus were found around 1830. The people who found it thought it was a huge marine reptile, so they called it Basilosaurus (King Lizard). Later it turned out to be a whale. Richard Owen later renamed it Zeuglodon meaning ("Yoke Teeth"), but the first published name must always stick on even if it turns out to be inappropriate! Basilosaurus can be found in North-America and Egypt, we know much of this whale because many skeletons have been found. Animals which live in coast areas do always have a better chance to get fossilized. Basilosaurus was in its time the largest animal on earth, and was on top of the food chain. In a skeleton from Egypt flatulence contents have been preserved, and those contained a ball of fishbones and a small shark. In Egypt is also a place which is called "the valley of the whales", and there are indications that Basilosaurus ate also bigger pray such as seacows, turtles and smaller whales like Dorudon and Zygorhiza. Basilosaurus must have been a slow swimmer to catch its prey
Differently than the present whales Basilosaurus did not have such large brain. At least, not to whale criteria. Consequently, Basilosaurus was a solitary animal, since animals which generally show group behaviour do have large brains. And perhaps that will be the reason that Basilosaurus has died out. The contemporary whales and dolphins are no offspring of Basilosaurus, but of its group-tied relatives. Basilosaurus probably died out as a result of the huge ell Niño of the late Eocene.