Basilosaurus, extinct genus of primitive whales of the family Basilosauridae (suborder Archaeoceti) found in Middle and Late Eocene rocks in North America and northern Africa (the Eocene Epoch lasted from 55.8 million to 33.9 million years ago). Basilosaurus had primitive dentition and skull architecture; the rest of the slender, elongated skeleton was well adapted to aquatic life. It attained a length of about 21 metres (about 70 feet), with the skull alone as much as 1.5 metres (5 feet) long. Basilosaurus was common throughout late Eocene seas.