Basilosaurus isis was a whale. A 55-foot, serpentine, multi-toothed, apex predator of a whale that looked nothing like what the word whale currently brings to mind. It lived 34 to 40 million years ago during the Eocene Period and represented an early, deeply unsettling chapter in cetacean evolution. The modern whale is what happened after nature had several million years to reconsider.
Fossil evidence from Egypt's Wadi Al-Hitan — the Valley of the Whales — puts it at the top of the Eocene food chain, hunting fish and smaller whales with a body type that evolution would spend the next 34 million years quietly reconsidering.