There be whales in those whales

Around 35 million years ago, in the late Eocene, a giant whale roamed the oceans, feasting on large fish and, the new evidence suggests, other smaller whales. Yikes.

Discovered in 2010, this 15 metre (50 foot) ancient whale specimen (Basilosaurus isis) has been linked to the nearby remains of sharks, fish, and juveniles of a smaller species of ancient whale called Dorudon atrox.

It turns out these specimens weren't all just found fossilised near each other – the smaller remains were actually B. isis' prey

....

B. isis was a fearsome creature, living between 34 and 38 million years ago, and specimens have been found that were a whopping 15- 18 metres in length (50-60 feet).


https://www.sciencealert.com/this-ancient-whale-has-been-found-devouring-other-whales