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 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, which prompted 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. In 1984, *Basilosaurus* became the state fossil of Alabama.

*Basilosaurus* is one of the largest animals known to exist between the K–Pg extinction event 66 million years ago (mya) and around 15 million years ago when modern cetaceans began to reach enormous sizes. *B. cetoides* measured 17–20 m (56–66 ft) long and weighed more than 5.8 tonnes (6.4 tons), while *B. isis* measured 15–18 m (49–59 ft) long and weighed nearly 6.5 tonnes (7.2 tons). *Basilosaurus* is distinguished from other genera of basilosaurids by its larger body size and its more elongated posterior thoracic, lumbar, and anterior caudal vertebrae. *Basilosaurus* does not have the vertically oriented metapophyses seen in its closest relative the basilosaurid known as *Basiloterus*. *Basilosaurus* is considered to be the largest of archeocete whales.

*Basilosaurus* is thought to have been common in the Tethys Ocean. 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.

*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 they 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.

Basilosaurus fossil record seems to end at about 35–33.9 mya. Basilosaurus extinction coincides with the Eocene–Oligocene extinction event which happened 33.9 mya, which also resulted in the extinction of almost all other archaeocetes. The event has been attributed to volcanic activity, meteor impacts or a sudden change in climate (such as the environment getting cooler), the latter of which might have caused changes in the ocean by disrupting oceanic circulation. *Basilosaurus* went extinct leaving no descendants. Despite this new currents and deep ocean upwelling favored the diversification of modern cetaceans such as early toothed and baleen whales.