Scientists find remains of a whale inside bigger whale

The stomach contents of a 15-metre-long ancient whale strengthens existing evidence of whale-on-whale predation around 35 million years ago.

Scientists have discovered Basilosaurus' favourite meal was probably a smaller whale. (AAP)

Killer whales may be the modern day "wolves of the sea" but 35 million years ago it was the 15-meter-long ancient whale Basilosaurus isis taking a bite out of the title.

Scientists studying skeletal remains of the extinct predator discovered one of its preferred meals was, in fact, other whales.

Iyad Zalmout, a Jordanian palaeontologist points to the teeth of a 40 million year old carnivorous whale. Source: AAP

The remains were discovered in the so-called Valley of Whales in Cairo, Egypt which was once a shallow sea during the late Eocene period.

Most revealing were the whale's stomach contents where scientists found the remains of sharks and large bony fish but predominantly the bones of the Dorudon atrox, a smaller species of ancient whale.

Scientists discovered bite marks on prey skulls meaning the monster shark attacked its meals by the head and hunted its prey live.

The study published in the journal PLOS ONE compares the ancient monster to the modern-day killer whale or Orca which often feeds on smaller whales and humpback whale calves.

Scientists believe the shallow sea in Cairo may have been where the smaller whales gave birth, making it a prime hunting site for the ancient predator.