Louis J Sheehan 654358

Basilosaurus (discovered in 1840 and initially mistaken for a reptile, hence its name) and Dorudon lived around 38 million years ago, and were fully recognizable whales which lived entirely in the ocean. Basilosaurus was a monstrous creature, up to 18m long; dorudontids were within the range of modern cetacean size, about 5m long.
Reconstruction of Dorudon
Reconstruction of Dorudon

Although they look very much like modern whales, basilosaurids and dorudontids lacked the 'melon organ' that allows their descendants to sing and use ultrasound as effectively as modern whales. They had small brains; this suggests they were solitary and did not have the complex social structure of modern whales. Basilosaurus had two tiny but well-formed hind legs which were probably used as claspers when mating; they are a small reminder of the lives of their ancestors.