Basilosaurus is thought to be lived between 40 million years ago in Eocene.
The fossils were initially believed to be reptile, hence the suffix -"saurus", Greek for ‘king lizard,’, but later found to be a marine mammal.
 “Basilosaurus existed at a time when baleen-bearing mysticetes [modern baleen whales] are known to have existed, and echolocating odontocetes [toothed whales] are presumed to have existed,” whale evolution expert Dr. Lawrence Barnes of the Natural History Museum of Los Angeles, quoted in Evolution: The Grand Experiment by Dr. Carl Werner, page 144. New Leaf Press, 2007

Size compared to a human.

There is unparalleled degree of elongation compared with modern whales. The Basilosaurus is 18 meters (60 feet) long (10 times as long as Ambulocetus) with 0.6 meter (2 ft) hind limbs. They were probably used to clasping during copulation.
Gingerich said,
It seems to me that they could only have been some kind of sexual and reproductive clasper.’ Gingerich, P. D., B. H. Smith, and E. L. Simons. 1990. Hind limbs of Eocene Basilosaurus isis: evidence of feet in whales. Science, 249: 154-157

Barbara Stahl, a vertebrate paleontologist and evolutionist, points out:
"The serpentine form of the body and the peculiar shape of the cheek teeth make it plain that these archaeocetes [like Basilosaurus] could not possibly have been the ancestor of modern whales."

The jawbone of an ancient whale found in Antarctica may be the oldest fully aquatic whale yet discovered.

This jawbone, in contrast, belongs to the Basilosauridae group of fully aquatic whales, said Reguero, who leads research for the Argentine
Antarctic Institute.With this new fossil find, however, dating to 49 million years ago (bear in mind that Pakicetus lived around 53 million years ago), this means that the first fully aquatic whales now date to around the time when the alleged walking whales (Ambulocetus) first appear.

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