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Basilosaurus is a genus of large, predatory, prehistoric archaeocete whale from the late Eocene, approximately 41.3 to 33.9 million years ago (mya). First described in 1834, it was the first archaeocete and prehistoric whale known to science. Fossils attributed to the type species B. cetoides were discovered in the United States. They were originally thought to be of a giant reptile, hence the suffix "-saurus", Ancient Greek for "lizard". The animal was later found to be an early marine mammal, prompting attempts at renaming the creature, which failed as the rules of zoological nomenclature dictate using the original name given. Fossils were later found of the second species, B. isis, in 1904 in Egypt, Western Sahara, Morocco, Jordan, Tunisia, and Pakistan. Fossils have also been unearthed in the southeastern United States and Peru. Basilosaurus is thought to have been common in the Tethys Ocean.
It was one of the largest animals of the Paleogene. It was the top predator of its environment, preying on sharks, large fish and other marine mammals, namely the dolphin-like Dorudon, which seems to have been their predominant food source. Based on the localities where its fossils are discovered, Basilosaurus would have preferred to live in the shallows, specifically in the middle to outer neritic zones of the inland sea. Basilosaurus was at one point a wastebasket taxon before the genus slowly started getting reevaluated, with many species of different Eocene cetacean being assigned to the genus in the past. However, most are invalid or have been reclassified under a new or different genus, leaving only 2 confirmed species.
Basilosaurus may have been one of the first fully aquatic cetaceans, sometimes referred to as the Pelagiceti. Basilosaurus, unlike modern cetaceans, had various types of teeth–such as canines and molars–in its mouth (heterodonty), and it probably was able to chew its food, in contrast to modern cetaceans which swallow their food whole. Despite there being whales of a similar size and lifestyle to Basilosaurus, Basilosaurus was still the apex predator of the Eocene seas.