* Livyatan melvillei, a giant sperm whale that lived about 13.6 Mya. Livyatan had teeth up to 36 inches long, and it is thought to have been about the same size as the megalodon.
* Basilosaurus isis, a giant sea serpent that lived about 40 Mya. Basilosaurus could grow up to about 180 feet long, and it is thought to have been a powerful predator.
* Orcas (Orcinus orca), a highly social species of toothed whale known to hunt in groups. Orcas are apex predators, often preying on seals, sea lions, and dolphins. Given their intelligence, adaptability, and group hunting strategy, it's feasible that a group of orcas could have posed a threat to even a megalodon.