Basilosaurus was a species of primitive cetacean that lived some 38 million years ago. When its fossilized remains were first discovered in 1840, it was initially mistaken for an aquatic lizard, hence the suffix "-saurus". Individuals were 18m long on average, and had a fusiform (spindle-shaped) body that generally resembled that of modern whales.

Basilosaurs lived at around the same time as dorudons , which were similar in appearance but only 5m long on average. The two species shared many physiological attributes that distinguish them from modern whales. Both species had a bulbous head and well-defined neck. Its rear limbs, though withered and useless, had yet to become fully vestigial. Perhaps most importantly, both species had smaller brains and lacked the "melon organ" that modern whales use to sing and ultrasound. This suggests that basilosaurs and dorudons were solitary creatures that lacked the complex social structure of today's whales.

See also

• Evolution of cetaceans