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).

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.

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 bones were lying in a curved line "measuring upwards of four hundred feet [122 meters] in length, with intervals which were vacant."

[12] Harlan identified the tooth as a wedge-shaped shell and instead focused on "a vertebra of enormous dimensions" which he assumed belonged to the order "Enalio-Sauri of Conybeare", "found only in the sub-cretaceous series.

Comparing his vertebra to those of large dinosaurs such as Megalosaurus and Iguanodon, Harlan concluded that his specimen was considerably larger—he estimated the animal to have been no less than 80–100 ft (24–30 m) long—and therefore suggested the name Basilosaurus, meaning "king lizard".

German paleontologist Wilhelm Barnim Dames described the material, including the type specimen of Z. osiris, a well-preserved dentary.

[21][18][22] In the 1980s, Elwyn L. Simons and Philip D. Gingerich started to excavate at Qasr el-Sagha and Wadi El Hitan with the hope of finding material that could match archaeocete fossils from Pakistan.

[24] In 2015, a complete skeleton, the first-ever such find for Basilosaurus, was uncovered in Wadi El Hitan, preserved with the remains of its prey, including a Dorudon and several species of fish.

[48] Basilosaurus is distinguished from other genera of basilosaurids by its larger body size and its more elongated posterior thoracic, lumbar, and anterior caudal vertebrae.

The mandibular canal is large and laterally flanked by a thin bony wall, the pan bone or acoustic fenestra.

The basilosaurid ear did, however, have a large external auditory meatus, strongly reduced in modern cetaceans, but, though this was probably functional, it can have been of little use under water.

Basilosaurus probably swam predominantly in two dimensions at the sea surface, in contrast to the smaller Dorudon, which was likely a diving, three-dimensional swimmer.

[58] Similarly sized thoracic, lumbar, sacral, and caudal vertebrae imply that it moved in an anguilliform fashion, but predominantly in the vertical plane.

Paleontologist Philip D. Gingerich theorized that Basilosaurus may also have moved in a very odd, horizontal anguilliform fashion to some degree, something completely unknown in modern cetaceans.

Judging from the relatively weak axial musculature and the thick bones in the limbs, Basilosaurus is not believed to have been capable of sustained swimming or deep diving, or terrestrial locomotion.

[52] Scientists were able to estimate the bite force of Basilosaurus isis by analyzing the scarred skull bones of another species of prehistoric whale, Dorudon, and concluded that it could exert a maximum bite force of at least 16,400 newtons (3,700 lbf) and could possibly exceed 20,000 newtons (4,500 lbf),[60] roughly equivalent to the range between modern alligators[61] and crocodiles.

[62] Analyses of the stomach contents of B. cetoides has shown that this species fed exclusively on fish and large sharks, while bite marks on the skulls of juvenile Dorudon have been matched with the dentition of B. isis, suggesting a dietary difference between the two species, similar to that found in different populations of modern killer whales.

The finding further cements theories that B. isis was an apex predator that may have hunted newborn and juvenile Dorudon at Wadi Al Hitan when mothers of the latter came to give birth.

[65][66][67] 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.

[68] It would have coexisted with the dolphin-like Dorudon,[69] the whales Cynthiacetus and Basiloterus,[70] the primitive sirenian Protosiren, the early elephant Moeritherium,[71][72][73] the sea turtle Puppigerus[74] and many sharks, such as Galeocerdo alabamensis,[75] Physogaleus, Otodus, Squatina prima, Striatolamia, Carcharocles sokolovi and Isurus praecursor.