=== Cranium ===

[[File:Basilosaurus-1070368.jpg|thumb|Skull from the [[Field Museum of Natural History]]]]

[[File:Gidley-1913-Basilosaurus-skeleton-reconstruction.JPEG|thumb|Skeletal restoration by {{Harvnb|Gidley|1913}}]]

The [[dental formula]] for ''B. isis'' is {{DentalFormula|upper=3.1.4.2|lower=3.1.4.3}}. The upper and lower molars and second to fourth premolars are double-rooted and high-crowned.<ref>{{Harvnb|Fahlke|2012|p=6}}</ref>

The head of ''Basilosaurus'' did not have room for a [[Melon (whale)|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.

{{Harvnb|Fahlke|Gingerich|Welsh|Wood|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).)<ref>{{Harvnb|Fahlke|Gingerich|Welsh|Wood|2011}}</ref>

In the basilosaur skull, the inner and middle ear are enclosed by a dense [[tympanic bulla]].<ref name="GinUhe-1998">{{Harvnb|Gingerich|Uhen|1998|p=4}}</ref> The [[Synapomorphy|synapomorphic]] cetacean air [[Sinus (anatomy)|sinus]] system is partially present in basilosaurids, including the pterygoid, peribullary, maxillary, and frontal sinuses.<ref>{{Harvnb|Racicot|Berta|2013|p=50}}</ref> 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 (anatomy)|fenestra]]. These features enabled basilosaurs to hear directionally in water.<ref name="GinUhe-1998" />

The ear of basilosaurids is more derived than those in earlier archaeocetes, such as [[Remingtonocetidae|remingtonocetids]] and [[Protocetidae|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 [[Ear canal|external auditory meatus]], strongly reduced in modern cetaceans, but though this was probably functional, it can have been of little use under water.<ref>{{Harvnb|Nummela|Thewissen|Bajpai|Hussain|2004|p=776}}</ref>

=== Hind limbs ===

[[File:Basilosaurus isis hindlimb.JPG|thumb|150px|''B. isis'' hind limb]]

A {{Convert|16|m|ft|adj=mid|sp=us}} individual of ''B. isis'' had {{Convert|35|cm|in|adj=mid|-long|sp=us}} 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.<ref>{{Harvnb|Bejder|Hall|2002|p=448}}</ref>

Analysis has shown that the reduced limbs could rapidly adduct between only two positions.{{citation needed|date=August 2017}}

=== Spine and movement ===

A complete ''Basilosaurus'' skeleton was found in 2015, and several attempts have been made to reconstruct the vertebral column from partial skeletons. {{Harvnb|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.<ref>{{Harvnb|Zalmout|Mustafa|Gingerich|2000|loc=Discussion, p. 202}}</ref>

''Basilosaurus'' has an anguilliform ([[eel]]-like) body shape because of the elongation of the [[Body of vertebra|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. ''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.<ref>{{Harvnb|Gingerich|1998|pp=433, 435}}</ref> 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.{{citation needed|date=August 2017}}

[[File:Basilosaurus cropped.png|thumb|left|''Basilosaurus'' reconstruction]]

Similarly sized thoracic, lumbar, sacral, and caudal vertebrae imply that it moved in an [[Fish locomotion#Swimming|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]].{{citation needed|date=August 2017}}

=== Feeding ===

[[File:Basilosaurus isis muscles.jpg|thumb|''B. isis'' jaw muscles]]

The cheek teeth of ''Basilosaurus'' retain a complex morphology and functional [[Occlusion (dentistry)|occlusion]]. Heavy wear on the teeth reveals that food was first chewed then swallowed.<ref name="GinUhe-1998" /> Scientists were able to estimate the [[bite force]] of ''Basilosaurus'' by analyzing the scarred skull bones of another species of prehistoric whale, ''Dorudon'', and concluded they could bite with a force of {{convert|3600| psi|kPa}}.<ref>{{cite journal|last=Snively|first=E.|last2=Fahlke|first2=J. M.|last3=Welsh|first3=R. C.|year=2015|title=Bone-Breaking Bite Force of ''Basilosaurus isis'' (Mammalia, Cetacea) from the Late Eocene of Egypt Estimated by Finite Element Analysis|journal=PLoS ONE|volume=10|issue=2|pages=e0118380|doi=10.1371/journal.pone.0118380|pmid=25714832|pmc=4340796}}</ref>

Analyses of the stomach contents of ''B. cetoides'' has shown that this species fed exclusively on fish 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 whale]]s.<ref>{{Harvnb|Fahlke|2012|p=14}}</ref> It was probably an active predator rather than a scavenger.<ref>{{cite journal|last1=Snively|first1=Eric|last2=Fahlke|first2=Julia M.|last3=Welsh|first3=Robert C.|title=Bone-Breaking Bite Force of Basilosaurus isis (Mammalia, Cetacea) from the Late Eocene of Egypt Estimated by Finite Element Analysis|journal=PLOS ONE|date=25 February 2015|volume=10|issue=2|page=e0118380|doi=10.1371/journal.pone.0118380|pmid=25714832|pmc=4340796}}</ref>

== Taxonomic history ==

===Etymology===

[[File:Owen-Basilosaurus-vertebra.JPEG|thumb|left|Caudal vertebra from {{Harvnb|Owen|1839}}]]

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.<ref>{{Harvnb|Zalmout|Mustafa|Gingerich|2000}}</ref><ref name="bbc">{{cite web | title = Basilosaurus | publisher = BBC Nature| url = http://www.bbc.co.uk/science/seamonsters/factfiles/basilosaurus.shtml | accessdate = 24 August 2013}}</ref> 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.<ref>{{Harvnb|Zimmer|1998|p=141}}</ref> Vertebrae were sent to the [[American Philosophical Society]] by a Judge Bry of [[Arkansas]] and Judge John Creagh of [[Clarke County, Alabama|Clarke County]], [[Alabama]]. Both fossils ended up in the hands of the anatomist [[Richard Harlan]], who requested more examples from Creagh.<ref name=Switek>{{cite web| last = Switek | first = Brian| title = The Legacy of the ''Basilosaurus'' | publisher = ScienceBlogs| url = http://scienceblogs.com/laelaps/2008/09/21/the-legacy-of-the-basilosaurus/| date = September 21, 2008 | accessdate = 14 September 2013}}</ref><ref>{{cite web|url=http://www.smithsonianmag.com/science-nature/how-did-whales-evolve-73276956/?all |title=How Did Whales Evolve? |author=Brian Switek |work=[[Smithsonian (magazine)|Smithsonian]] |date=December 2010 }}</ref> 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 [[andiron]]s 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.<ref>{{Harvnb|Harlan|1834|p=400}}</ref>

Harlan identified the tooth as a [[Pinna (genus)|wedge-shaped shell]] and instead focused on "a vertebra of enormous dimensions" which he assumed belonged to the order "[[Enalio Sauri|Enalio-Sauri]] of [[William Conybeare (geologist)|Conybeare]]", "found only in the sub-cretaceous series."<ref>{{Harvnb|Harlan|1834|p=401}}</ref> 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 {{Convert|80|–|100|ft|m|abbr=on}} long—and therefore suggested the name ''Basilosaurus'', meaning “king lizard”.<ref>{{Harvnb|Harlan|1834|pp=402–403}}</ref>

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 [[sirenia]]ns.<ref>{{Harvnb|Owen|1839|pp=72–73}}</ref> Consequently, Owen proposed renaming the find ''Zeuglodon cetoides'' (“whale-like [[yoke]] teeth” in reference to the double-rooted teeth) and Harlan agreed.<ref name="Owen-1939-p75">{{Harvnb|Owen|1839|p=75}}</ref> ''Zeuglodon'', now a junior synonym, is considered by many to be a more fitting name, but the first-published name always takes precedence.

===Wadi El Hitan===

{{Main|Wadi El Hitan}}

[[File:Wadi Al-Hitan.jpg|thumb|Skeleton of ''B. isis'' at [[Wadi El Hitan]]]]

Wadi El Hitan, otherwise known as Whale Valley, is an Egyptian sandstone formation where many early-whale skeletons were discovered.<ref>{{cite web|url=http://www.worldheritagesite.org/list/id/1186|title=Wadi Al-Hitan| publisher=World Heritage Site|accessdate=17 August 2017}}</ref> German botanist [[Georg August Schweinfurth]] discovered the first archaeocete whale in Egypt (''Zeuglodon osiris'', now ''[[Saghacetus|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 Dames|Wilhelm Barnim Dames]] described the material, including the type specimen of ''Z. osiris'', a well-preserved dentary.<ref>{{Harvnb|Gingerich|2007|pp=110–112}}</ref>

Hugh Beadnell, head of the Geological Survey of Egypt 1896–1906,<ref>{{Harvnb|Gingerich|2007|p=113}}</ref> named and described ''Zeuglodon isis'' in {{Harvnb|Andrews|1904}} based on a partial mandible and several vertebrae from Wadi El Hitan in Egypt.<ref>{{Harvnb|Andrews|1904|pp=214–215}}</ref> {{Harvnb|Andrews|1906}}<ref>{{Harvnb|Andrews|1906|pp=255}}</ref> described a skull and some vertebrae of a smaller archaeocete and named it ''Prozeuglodon atrox'', now known today as ''[[Dorudon|Dorudon atrox]]''. {{Harvnb|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.<ref>{{Harvnb|Kellogg|1936|p=81}}</ref><ref>{{Harvnb|Gingerich|2007|p=114}}</ref><ref>{{Harvnb|Uhen|2004|p=11}}</ref>

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.<ref name="Gin-2007-117">{{Harvnb|Gingerich|2007|pp=117–119}}</ref> {{Harvnb|Gingerich|Smith|Simons|1990}} described additional fossils including foot bones and speculated that the reduced hind limbs were used as copulatory guides.<ref>{{Harvnb|Gingerich|Smith|Simons|1990|loc=Abstract}}</ref>

In 2016, a complete skeleton, the first-ever found for ''Basilosaurus'', was uncovered in Wadi El Hitan, preserved with the remains of another whale (which was either a last meal or an unborn foetus) inside its ribcage. The whale's skeleton also shows signs of scavenging or predation by large sharks.<ref>{{cite web|last=Griffiths |first=Sarah |url=http://www.dailymail.co.uk/sciencetech/article-3110718/Fossils-reveal-whale-inside-whale-eaten-shark-Grave-40-million-year-old-marine-creatures-Egypt.html |title=Egyptian fossils reveal a whale inside a whale, eaten by a shark &#124; Daily Mail Online |publisher=Dailymail.co.uk |date=2015-06-04 |accessdate=2018-06-05}}</ref>

===''Nomina dubia''===

[[Image:Hydrarchos.jpg|thumb|Albert Koch's "Hydrarchos" fossil skeleton from 1845]]

* ''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''.<ref>{{Paleodb|63087|Basilosauridae}}: Taxonomic history. Retrieved August 2013.</ref>

* ''Zeuglodon vredense'' or ''vredensis'' was named in the 19th century based on a single, isolated tooth without any kind of accompanying description, and {{Harvnb|Kellogg|1936}} therefore declared it a ''nomen nudum''.<ref>{{Harvnb|Kellogg|1936|p=264}}</ref><ref>{{Paleodb|63141|Zeuglodon vredense (nomen nudum)}}. Retrieved August 2013.</ref>

* ''Zeuglodon puschi[i]'' from Poland was named by {{Harvnb|Brandt|1873}}. {{Harvnb|Kellogg|1936}} noted that the species is based on an incomplete vertebra of indeterminable position and, therefore, that the species is invalid.<ref>{{Harvnb|Kellogg|1936|p=263}}</ref><ref>{{Paleodb|63140|Zeuglodon puschii (nomen dubium)}}. Retrieved August 2013.</ref>

* ''Zeuglodon brachyspondylus'' was described by {{Harvnb|Müller|1849}} based on some vertebrae from ''Zeuglodon hydrarchus'', better known as Dr Albert Koch's "Hydrarchos". {{Harvnb|Kellogg|1936}}, synonymized it with ''[[Pontogeneus|Pontogeneus priscus]]'', which {{Harvnb|Uhen|2005}} declared a ''[[nomen dubium]]''.

===Reassigned species===

* ''Basilosaurus drazindai'' was named by {{Harvnb|Gingerich|Arif|Bhatti|Anwar|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]]''.<ref>{{Paleodb|53163|Basilosaurus drazindai}}. Retrieved August 2013.</ref><ref name=GZ2015>{{cite journal | last1 = Gingerich | first1 = Philip D. | last2 = Zouhri | first2 = Samir | year = 2015 | title = New fauna of archaeocete whales (Mammalia, Cetacea) from the Bartonian middle Eocene of southern Morocco | url = | journal = Journal of African Earth Sciences | volume = 111| issue = | pages = 273–286| doi = 10.1016/j.jafrearsci.2015.08.006 }}</ref>

* ''Zeuglodon elliotsmithii'', ''Z. sensitivius'', ''Z. sensitivus'', and ''Z. zitteli'' were synonymized and grouped under the genus ''[[Saghacetus]]'' by {{Harvnb|Gingerich|1992}}.

* ''Zeuglodon paulsoni'' from Ukraine was named by {{Harvnb|Brandt|1873}}. It was synonymized with ''[[Platyosphys]]'' but is now considered ''nomen dubium''. Gingerich and Zouhri (in press), however, maintain ''Platyosphys as valid''.<ref>{{Harvnb|Gol'din|Zvonok|2013|loc=Abstract}}</ref><ref name=GZ2015/>

== See also ==

{{Portal|Paleontology}}

* [[Evolution of cetaceans]]

== References ==

=== Notes ===

{{Reflist|30em}}

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| title = At the Water's Edge: Macroevolution and the Transformation of Life

| year = 1998 | publisher = Free Press

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{{Refend}}

== External links ==

{{Commons category|Basilosaurus}}

{{Archaeoceti}}

{{Taxonbar|from=Q134884}}

[[Category:Eocene mammals of North America]]

[[Category:Eocene mammals of Africa]]

[[Category:Basilosauridae]]

[[Category:Transitional fossils]]