Basilosaurus

Basilosaurus (meaning "king lizard") 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 southeastern 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. The second species named in 1904, B. isis, lived in the region currently known as the Mediterranean Sea, with fossils found in North Africa and Jordan.

Basilosaurus is thought to have been one of the largest animals of the Paleogene, with the type species B. cetoides measuring around 17–20 metres (56–66 ft) long and weighing up to 15 metric tons (17 short tons). It was the top predator of its environment in the shallows of the inland sea, preying on sharks, large fish and other marine mammals. The smaller dolphin-like relative Dorudon seems to have been the predominant food source of Basilosaurus based on its stomach contents and the bite marks preserved in the skull of Dorudon.

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 two 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.