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Why do cetaceans have horizontal tail flukes?

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Illustration showing various cetaceans, including baleen whales like the blue whale, fin whale, humpback whale, gray whale, sei whale, northern right whale, and bowhead whale, as well as toothed whales such as the sperm whale, narwhal, beluga, Baird's beaked whale, killer whale, dolphins, and porpoises. Each species is labeled with its scientific name and typical or maximum length, with a size scale in the corner for reference. — cetacean Whales (order Cetacea).

Cetaceans possess horizontal tail flukes as a direct result of their evolutionary history as mammals. Unlike fish, which evolved to move their bodies in a horizontal plane, cetaceans descended from four-legged terrestrial ancestors that moved their limbs and spines in a vertical plane.1

Because of this ancestral movement pattern, cetaceans utilize vertical strokes to swim, powered by large back muscles that drive the tail up and down. The horizontal flukes serve to increase the surface area for propulsion, allowing these animals to move efficiently through the water. This unique anatomical arrangement reflects their transition from land-dwelling creatures to fully aquatic cetaceans, where the spine and back muscles became the primary drivers of locomotion.12