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Basilosaurus is a genus of prehistoric cetacean that existed during the Late Eocene, 40 to 35 million years ago. 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. Basilosaurus represents one of the earliest whales although its actually descended from terrestrial mammals like Ambulocetus.‭ ‬Evidence for this can be seen in the flippers.‭ ‬The front flippers still have an elbow joint,‭ ‬something that today is only seen in seals.‭ ‬The back flippers where the hind legs would have been in its ancestor are greatly reduced and although they may have been used to get extra grip on a mate,‭ ‬they would eventually disappear in later whales.‭ ‬The size of its flippers in comparison with the massive bulk of its body means that it was almost certainly an entirely pelagic animal.

The large jaws of Basilosaurus housed teeth suitable for catching prey that would not have been especially small,‭ ‬perhaps smaller whales or large fish that while smaller than Basilosaurus,‭ ‬but‭ ‬were still too large for most other predators to tackle.‭ ‬Superficially the fossils of Basilosaurus skulls resemble the skulls of mosasaurs that swam the oceans tens of millions of years earlier,‭ ‬disappearing from the fossil record at the time of the KT extinction‭ ‬65‭ ‬million years ago.‭ ‬It is partly for this reason that Basilosaurus was misidentified as a reptile because mosasaur remains were known,‭ ‬yet no one was aware that the early whales looked so similar.‭ ‬The skull also has a reduced area for the brain when compared to modern whales.‭ ‬Going on the basis that larger brained cetaceans are typically more social,‭ ‬Basilosaurus itself may have been a solitary predator.