Livyatan

One of the largest predatory mammals ever to have existed, Livytitan was the Apex predator of its time, along with Carcharodon megalodon, the largest predatory shark discovered. In form, it was similar to the modern sperm whale, but while the sperm whale was adapted to hunt deep sea squid, Livyatan was adapted to go after far larger game. It was the largest toothed whale ever to have existed, and probably the most fearsome. While a full skeleton has not been found of this beast, the bones that have been found so far in the desert of Peru suggest that this animal was an active hunter, with teeth adapted to tackle prey moer solid bodied than the Cephalopods that its near relative the Sperm whale subsists on.
Whales had started to assume the role of apex predator in the worlds oceans with the Evolution of Basilosaurus, but Livyatan was a member of the modern toothed whale linege, which meant it had functioning echolocation, and may well have been adapted to deep water diving.