The sea goddess Tethys was one of the Titans who ruled the world before Zeus and his cohorts came along. This pre-Olympian deity lends her name to a prehistoric sea, the Tethys Ocean, which once divided ancient land masses. Before the Indian subcontinent slammed into Asia and crumpled up the Himalayas, the Tethys Ocean stretched from present-day Indonesia to Spain. It was warm, shallow and teeming with fish — no wonder those ancient land-roving cetaceans decided that life in the sea was the life for them!
Among those adapting to this welcoming submarine environment was a smaller version of the Basilosaurus called the Dorudon atrox. In fact, it appears that D. atrox had to keep a sharp eye out for its bigger cousin; the fossil record indicates that the Basilosaurus diet included Dorudons [source: Fahlke]. Still, at roughly 15 feet (5 meters) in length and fully equipped with razor-sharp teeth, the Dorudon was a menacing critter.
Curiously, field work uncovering the fossils of Basilosaurus, Dorudon and numerous other ancient cetaceans has established a distant but significant relationship between modern whales and hippos [source: Gingerich et al.].