Temporal range: Late Eocene
|B. cetoides, National Museum of Natural History, Washington DC|
Basilosaurus ("king lizard") is a genus of early whales that lived in the late Eocene. The first fossil of B. cetoides was discovered in the United States and was initially believed to be some sort of reptile, hence the suffix -"saurus", but it was later found to be a marine mammal. Richard Owen wished to rename the creature Zeuglodon ("yoked tooth"), but, per taxonomic rules, the creature's first name remained permanent. Fossils of B. isis have been found in Egypt and Jordan.
Measuring 15–18 m (49–59 ft), Basilosaurus cetoides is one of the largest known animals to exist from K/T Extinction Event 66 million years ago to around 15 million years ago when modern cetaceans began to reach enormous sizes. B. isis is slightly smaller than B. cetoides.
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 social capabilities of modern whales.
Fahlke et al. 2011 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 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 be present in Basilosaurus. This cranial torsion probably evolved in protocetids and basilosaurids together with directional underwater hearing and the sound-receiving apparatus in the mandible (the auditory fat pad and the pan bone (thin portion of mandible).)
In the basilosaur skull, the inner and middle ear are enclosed by a dense tympanic bulla. The synapomorphic cetacean air sinus system is partially present in basilosaurids, including the pterygoid, peribullary, maxillary, and frontal sinuses. The periotic bone, which surrounds the inner ear, is partially isolated. The mandibular canal is large and laterally flanked by a thin bony wall, the pan bone or acoustic fenestra. These features enabled basilosaurs to hear directionally in water.
The ear of basilosaurids is more derived than those in earlier archaeocetes, such as remingtonocetids and protocetids, in the acoustic isolation provided by the air-filled sinuses inserted between the ear and the skull. 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.
A complete Basilosaurus skeleton was found in 2015, and several attempts have been made to reconstruct the vertebral column from partial skeletons. Kellogg 1936 estimated a total of 58 vertebrae, based on two partial and nonoverlapping skeletons of B. cetoides from Alabama. More complete fossils uncovered in Egypt in the 1990s allowed a more accurate estimation: the vertebral column of B. isis has been reconstructed from three overlapping skeletons to a total of 70 vertebrae with a vertebral formula interpreted as seven cervical, 18 thoracic, 20 lumbar and sacral, and 25 caudal vertebrae. The vertebral formula of B. cetoides can be assumed to be the same.
Basilosaurus has an anguilliform (eel-like) body shape because of the elongation of the centra of the thoracic through anterior caudal vertebrae. In life, these vertebrae were filled with marrow, and because of the enlarged size, made them buoyant. From this it can be deduced that Basilosaurus swam predominantly in two dimensions at the sea surface, in contrast to the smaller Dorudon, which was probably a diving, three-dimensional swimmer. The skeletal anatomy of the tail suggests that a small fluke was probably present, which would have aided only vertical motion. Most reconstructions show a small, speculative dorsal fin similar to a rorqual's, but other reconstructions show a dorsal ridge.
A 16 m (52 ft) individual of B. isis had 35 cm (14 in) long hind limbs with fused tarsals and only three digits. The limited size of the limb and the absence of an articulation with the sacral vertebrae, makes a locomotory function unlikely. Analysis has shown that the reduced limbs could rapidly adduct between only two positions.
Basilosaurus probably relied on unusual modes of locomotion, relative to other cetaceans; 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.
The vertebrae appear to have been hollow, and likely also fluid-filled. This would imply that Basilosaurus typically functioned in only two dimensions at the ocean surface, compared with the three-dimensional habits of most other 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.
The cheek teeth of Basilosaurus retain a complex morphology and functional occlusion. Heavy wear on the teeth reveals that food was first chewed then swallowed. Scientists were able to estimate the bite force of Basilosaurus by analyzing the scarred skull bones of another species of prehistoric whale, Dorudon. By the damage caused, the aquatic predator could chomp down with a skull-crushing force of more than 3600 pounds.
Analyses of the stomach contents of B. cetoides has shown that this species fed exclusively on fishes and 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.
During the early 19th century, B. cetoides fossils were so common (and sufficiently large) that they were regularly used as furniture in the American South. Vertebrae were sent to the American Philosophical Society by a Judge Bry of Arkansas 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 four hundred feet 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, 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 to have been "no less" than 80–100 ft (24–30 m) long — and therefore suggested the name Basilosaurus, meaning “king lizard”.
Harlan brought his assembled specimens (including fragments of jaw and teeth, humerus, and fragments of rib) 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 erroneously with "herbivorous cetaceans" (all cetaceans are carnivorous) such as the dugong (a sirenian, not a cetacean). Consequently, Owen proposed renaming the find Zeuglodon cetoides (“whale-like yoke teeth” in reference to the double-rooted teeth) and Harlan agreed. Zeuglodon, now a junior synonym, is considered by many to be a more fitting name, but the first-published name always takes precedence.
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 Zeuglodon Valley with 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 Andrews 1904 based on a partial mandible and several vertebrae from Wadi Al-Hitan in Egypt. Andrews 1906 described a skull and some vertebrae of a smaller archaeocete and named it Prozeuglodon atrox (=Dorudon atrox). Kellogg 1936 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 Al-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.  Gingerich, Smith & Simons 1990 described additional fossils including foot bones and speculated that the reduced hind limbs were used as copulatory guides.
- Zeuglodon wanklyni was described in 1876 based on a skull found in the Wanklyn's Barton Cliff in the United Kingdom. This single specimen, however, quickly disappeared and has since been declared a nomen nudum or referred to as Zygorhiza wanklyni.
- Zeuglodon vredense or vredensis was named in the 19th century based on a single, isolated tooth without any kind of accompanying description, and Kellogg 1936 therefore declared it a nomen nudum.
- Zeuglodon puschi[i] from Poland was named by Brandt 1873. Kellogg 1936 noted that the species is based on an incomplete vertebra of indeterminable position and, therefore, that the species is invalid.
- Zeuglodon brachyspondylus was described by Müller 1849 based on some vertebrae from Zeuglodon hydrarchus, better known as Dr Albert Koch's "Hydrarchos". Kellogg 1936, synonymized it with Pontogeneus priscus, which Uhen 2005 declared a nomen dubium.
- Basilosaurus drazindai was named by Gingerich et al. 1997 based on a single lumbar vertebra. It was later declared a nomen dubium by Uhen (2013), but Gingerich and Zouhri (in press) re-assigned it to Eocetus.
- Zeuglodon elliotsmithii, Z. sensitivius, Z. sensitivus, and Z. zitteli were synonymized and grouped under the genus Saghacetus osiris by Gingerich 1992.
- Zeuglodon paulsoni from Ukraine was named by Brandt 1873. It was synonymized with Platyosphys but is now considered nomen dubium. Gingerich and Zouhri (in press), however, maintain Platyosphys as valid
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