Basilosaurus ("king lizard") is a genus of large, predatory primitive whales that lived from approximately 41.3 million years ago until about 33.9 million years ago, during the Eocene epoch. First described in 1834, Basilosaurus was the first prehistoric whale ever discovered. Fossils attributed to the type species Basilosaurus cetoides were first uncovered in the United States and were initially believed to belong to a giant reptile similar to Mosasaurus—which is why the name bears the suffix "-saurus", meaning "lizard" in Ancient Greek. Later, fossils of a second species, Basilosaurus isis, were discovered in 1904 across Egypt, Western Sahara, Morocco, Jordan, Tunisia, and Pakistan. Fossils of Basilosaurus isis have also been found in the southeastern United States and Peru.
It is believed that Basilosaurus was a common predator in the ancient Tethys Ocean. It was one of the largest animals of the Paleogene period and the apex predator of its environment, reaching lengths between 15 and 20 meters. Basilosaurus fed on sharks, large fish, and other marine mammals such as Dorudon, a dolphin-like creature that appears to have been its primary food source.
