Basilosaurus

Basilosaurus was a prehistoric whale from the late Eocene epoch. It was the top predator of the late Eocene oceans that ate anything that moved.

Description
Basilosaurus, just like Dorudon, was a prehistoric whale. Over four times the length of a Great White Shark, Basilosaurus was around 18-20 meters in length with a skull approximately 1.5 meters long, and weighed 60 tons. Its' post-cranial skeleton resembled that of the modern whales, but it still had nostrils rather than a blowhole, and its' ribcage indicates that it couldn't dive as deep as the modern whales can.

Confusion
Incidentially, when it was first discovered in the 1830s and according to a drawing made in 1960, Basilosaurus was thought to have been another giant sea reptile (like a Mosasaur), just like sea serpents, or even the Loch Ness Monster, and hence its name means "the regal reptile". Later though, via its teeth, Basilosaurus true identity as a primitive whale was established, but due to scientific rules, the original name had to remain.

Skull
The skulls of Basilosaurus are chilling to most people. There are no whales with skulls like the skull of Basilosaurus in modern times. Great peg–like teeth at the front for seizing prey. Once inside the mouth, the prey is sliced up by the big teeth at the back, great big cusps at the front: they are for slicing through flesh

Basilosaurus lived in the late Eocene time period, roughly 43-38 MYA, even though the Walking with Beasts episode is dated 36 MYA instead. It lived in the warm and shallow seas (like the Tethys) of the time, and it resembled the thought carnivorous marine reptiles that died during the K/T extinction event, especially in the skull, where it had saw-edged teeth used for catching fish and other slippery sea life.

Mating
Females were eagerly pressured by several males, but it was the eldest and the biggest males the females chose to mate with. Mating was not an easy task for such huge, free–floating animals, so Basilosaurus needed a little extra help. As the successful male maneuvered into position, they called upon one small legacy of their distant lend ancestors. Basilosaurus retained two tiny back legs. These were useless for walking or even swimming, but they were used to lock their long narrow bodies together during mating.

Whale Killer
A female Basilosaurus was seen attacking and killing two Physogaleus sharks. Later, the female was pursued by two males that want to mate with her. She chose the bigger male and they mated. Days later, the pregnant female swam into a mangrove swamp where it chased a Moeritherium. The Basilosaurus waited until the tide came in and attempted to attack it but got stuck on the sand. The Moeritherium was able to escape just in time. The Basilosaurus swam back into open waters and found a pod of Dorudon. The pod harassed the mother and drove her away. The mother then scrubbed the barnacles off her back and came back to the Dorudon for food. The mother managed to kill several Dorudon calves. Months later, she gave birth.

The Fourth Most Deadliest Sea
Nigel Marven used an underwater microphone/speaker to record a Basilosaurus' voice. Later, a Basilosaurus came to the ship. Nigel quickly jumped into the water and stayed under the Ancient Mariner. The Basilosaurus appeared and circled the speaker and tried to attack it. It then grabbed it in its jaws and swam away.