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. The generic name, meaning "king lizard", was given due to the initial misconception about the fossil material as that of a giant reptile. 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 long and weighing up to . 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 being 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.
Taxonomic history Etymology
The two species of Basilosaurus are B. cetoides, whose remains were discovered in the United States, and B. isis, which was discovered in Egypt. B. cetoides is the type species for the genus. The holotype of B. cetoides was found in Ouachita Parish, Louisiana. The exact formation is said to be the Yazoo Clay/formation. Vertebrae were sent to the American Philosophical Society by a Judge Henry Bry of Ouachita Parish, Louisiana and Judge John Creagh of Clarke County, Alabama. Both fossils ended up in the hands of the anatomist Richard Harlan, who requested more examples from Creagh. The first bones were unearthed when rain caused a hillside full of sea shells to slide. The bones were lying in a curved line "measuring upwards of in length, with intervals which were vacant." Many of these bones were used as andirons and destroyed; Bry saved the bones he could find, but was convinced more bones were still to be found on the location. Bry speculated that the bones must have belonged to a "sea monster" and supplied "a piece having the appearance of a tooth" to help determine which kind.
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". He noted that some parts of the vertebra were similar to those of Plesiosaurus and skull was similar to Mosasaurus, but that they were completely different in proportions. 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 "could not have been less than from long") and therefore suggested the name Basilosaurus, meaning "king lizard".
Harlan brought his assembled specimens (including fragments of jaw and teeth, humerus, and rib fragments) to the UK where he presented them to anatomist Richard Owen. Owen concluded that the molar teeth were two-rooted, a dental morphology unknown in fishes and reptiles, and more complex and varied than in any known reptile, and therefore that the specimen must be a mammal. Owen correctly associated the teeth with cetaceans, but he thought it was an herbivorous animal, similar to sirenians. Consequently, Owen proposed renaming the find Zeuglodon cetoides ("whale-like yoke teeth" in reference to the double-rooted teeth) and Harlan agreed.
Wadi El Hitan
Wādī al-Ḥītān () is an Egyptian sandstone formation where many early-whale skeletons were discovered. German botanist Georg August Schweinfurth discovered the first archaeocete whale in Egypt (Zeuglodon osiris, now Saghacetus osiris) in 1879. He visited the Qasr el Sagha Formation in 1884 and 1886 and missed the now famous Wadi El Hitan by a few kilometers. German paleontologist Wilhelm Barnim Dames described the material, including the type specimen of Z. osiris, a well-preserved dentary.
Hugh Beadnell, head of the Geological Survey of Egypt 1896–1906, named and described Zeuglodon isis in based on a partial mandible and several vertebrae from Wadi El Hitan in Egypt. described a skull and some vertebrae of a smaller archaeocete and named it Prozeuglodon atrox, now known today as Dorudon atrox. discovered deciduous teeth in this skull and it was then believed to be a juvenile [Pro]zeuglodon isis for decades before more complete fossils of mature Dorudon were discovered.
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. Since then, over 500 archaeocete skeletons have been found at these two locations, of which most are B. isis or D. atrox, several of the latter carrying bite marks assumed to be from the former. A 1990 paper described additional fossils including foot bones and speculated that the reduced hind limbs were used as copulatory guides. One thing that was noted, was that whale fossils were so common, that when a mason company looked at their newest table counter, they realized that they had created a cross section of a 40 million year old basilosaurid fossil. This find was another thing that caught the eye of Gingerich.
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. The whale's skeleton also shows signs of scavenging by large sharks such as the otodontid Otodus sokolovi, though the study considered it possible that this shark was also part of the diet of Basilosaurus.
Wastebasket taxa Many dubious species have been assigned to Basilosaurus in the past which have since been invalidated or were too incomplete to determine anything.
Nomina dubia A nomen dubium is a scientific name that is of unknown or doubtful application. There are a few documented cases of this being applied to Basilosaurus in the past.
Zeuglodon vredense or vredensis was named in the 19th century based on a single, isolated tooth without any kind of accompanying description, and therefore declared it a nomen nudum.. Retrieved August 2013. Zeuglodon puschi[i] was a species that was said to come from Poland, it was named by . noted that the species is based on an incomplete vertebra of indeterminable position and, therefore, that the species is invalid.. Retrieved August 2013. Zeuglodon brachyspondylus was described by Johannes Peter Müller based on some vertebrae from "Zeuglodon hydrarchus", better known as "Dr." Albert Koch's "Hydrarchos" (first called "H. sillimani", then "H. harlani"). , synonymized it with Pontogeneus priscus, which a 2005 study declared a nomen dubium.
Reassigned species
Basilosaurus drazindai was named by a 1997 study based on a single lumbar vertebra. Originally, the species was thought to have lived in Pakistan and the UK. It was later declared a nomen dubium by Uhen (2013), but Gingerich and Zouhri (2015) reassigned it to the genus Eocetus. This species was at one point in time concluded to be the earliest record of the genus Basilosaurus, before its reclassification.. Retrieved August 2013. Zeuglodon elliotsmithii, Z. sensitivius, and Z. zitteli were synonymized and grouped under the genus Saghacetus by a 1992 study. Zeuglodon paulsoni from Ukraine (then the Russian Empire) was named by . It was synonymized with Platyosphys but is now considered nomen dubium. Gingerich and Zouhri (2015), however, maintain Platyosphys as valid. Basilosaurus caucasicus also known as Basilosaurus caucasicum or Zeuglodon caucasicum was a species described in the Russian Empire, it gets its name from the Caucasus of where it was found in the 1890s. The fossil was reassigned to the toothed whale . Basilosaurus harwoodi was discovered in the Murray River near Wellington in South Australia. This species classification was controversial; T. S. Hall (1911) placed Basilosaurus harwoodi (or Zeuglodon harwoodi) in the genus . In 1906, German naturalist Othenio Abel thought fossils from the Eocene of Alabama, previously described in 1900 as being a Basilosaurus hip bone by American zoologist Frederic Augustus Lucas, represented the shoulder of a large bird similar to Gastornis, and named it Alabamornis gigantea. Lucas later countered his conclusion in 1908 as he reassigned the fossil specimens to the original conclusion of a Basilosaurus hip bone. In 1876, Harry Seeley described a skull found in the Wanklyn's Barton Cliff in the United Kingdom as a new species of Zeuglodon, Z. wanklyni. This specimen, however, has since been lost and been referred to as Zygorhiza wanklyni by or to as a nomen dubium. In 2022, Gingerich and colleagues suggested that the lost skull possibly belongs to Pachycetus. In 2026, Henk Jan van Vliet and colleagues redescribed the taxon as a new genus of dorudontine basilosaurid, Bartoncetus, on the basis of the proposed neotype (or possibly syntype) postcranial specimen that may or may not be from the same individual of the lost skull.
Description
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 long and B. isis measured long. A 1998 study estimated that B. cetoides weighed more than and B. isis weighed nearly , while the 2025 study estimated that a long B. cetoides weighed . 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.
Cranium
The dental formula for B. isis is . The upper and lower molars and second to fourth premolars are double-rooted and high-crowned.
The head of Basilosaurus did not have room for a melon like modern toothed whales, and the brain was smaller in comparison, as well. They are not believed to have had the echolocation capabilities nor the social dynamics of extant cetaceans.
A 2011 study concluded that the skull of Basilosaurus is asymmetrical like in modern toothed whales, and not, as previously assumed, symmetrical like in baleen whales and artiodactyls (which are closely related to cetaceans). In modern toothed whales, this asymmetry is associated with high-frequency sound production and echolocation, neither of which is thought to have been present in Basilosaurus. This probably evolved to detect sound underwater, with a fatty sound-receiving pad in the mandible.