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[[File:Cetaceans.svg|thumb|right|320px|Species of the infraorder Cetacea]]
[[File:Evolutionary pathway.jpg|thumb|right|320px|A [[phylogenetic tree]] showing the relationships among cetacean families.<ref name=radiations>{{cite journal
 | last=Thewissen
 | first=J. G. M.|author2=Williams, E. M.
 | title=The Early Radiation of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations
 | journal=Annual Review of Ecology and Systematics
 | date=1 November 2002
| volume=33
 | issue=1
 | pages=73–90
 | doi=10.1146/annurev.ecolsys.33.020602.095426| bibcode=2002AnRES..33...73T}}</ref> ]]

The '''evolution of cetaceans''' is thought to have begun in the [[Indian subcontinent]] from [[even-toed ungulates]] (Artiodactyla) 50 million years ago (mya) and to have proceeded over a period of at least 15 million years.<ref>Davis, R. W. 2019. Return to the Sea: The Evolution of Marine Mammals. Pages 7–27 in R. W. Davis, ed. Marine Mammals: Adaptations for an Aquatic Life. Springer International Publishing, New York.</ref> [[Cetacean]]s are fully aquatic [[mammal]]s belonging to the order [[Artiodactyla]] and branched off from other artiodactyls around 50&nbsp;[[Year#Abbreviations yr and ya|mya]]. Cetaceans are thought to have evolved during the [[Eocene]] (56-34 mya), the second epoch of the present-extending Cenozoic Era. Molecular and morphological analyses suggest Cetacea share a relatively recent closest [[common ancestor]] with [[hippopotamus|hippopotamuses]] and that they are sister groups.<ref>Geisler, J. H., and M. D. Uhen. 2005. Phylogenetic relationships of extinct Cetartiodactyls: results of simultaneous analyses of molecular, morphological, and stratigraphic data. Journal of Mammalian Evolution 12:145–160.</ref> 

Being mammals, they surface to breathe air; they have five [[finger bones]] (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors.<ref>Berta, A., J. L. Sumich, and K. M. Kovacs. 2005. Marine Mammals: Evolutionary Biology. Elsevier, Amsterdam.</ref><ref name=":3">Thewissen, J. G. M., L. N. Cooper, J. C. George, and S. Bajpai. 2009. From land to water: the origin of whales, dolphins, and porpoises. Evolution: Education and Outreach 2:272–288.</ref> Research conducted in the late 1970s in [[Pakistan]] revealed several stages in the transition of cetaceans from land to sea.

The two modern [[parvorder]]s of cetaceans – [[Mysticeti]] (baleen whales) and [[Odontoceti]] (toothed whales) – are thought to have separated from each other around 28–33 mya in a second cetacean [[evolutionary radiation|radiation]], the first occurring with the [[archaeocetes]].<ref>{{Cite journal|last1=Nikaido|first1=M.|last2=Matsuno|first2=F.|title=Retroposon analysis of major cetacean lineages: The monophyly of toothed whales and the paraphyly of river dolphins|journal=Proceedings of the National Academy of Sciences|volume=98|issue=13|pages=7384–9|bibcode=2001PNAS...98.7384N|year=2001|doi=10.1073/pnas.121139198|pmid=11416211|pmc=34678|doi-access=free}}</ref> The adaptation of [[animal echolocation]] in toothed whales distinguishes them from fully aquatic archaeocetes and [[mammalodontidae|early baleen whales]]. The presence of [[baleen]] in baleen whales occurred gradually, with earlier varieties having very little baleen, and their size is linked to baleen dependence (and subsequent increase in filter feeding).

==Early evolution==
{{Main|Archaeoceti}}
[[Image:Cladogram of Cetacea within Artiodactyla.png|thumb|upright|Cladogram showing the position of Cetacea within [[Artiodactylamorpha]]]]
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 | image1    = Indohyus BW.jpg
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 | caption1  = ''[[Indohyus]]'' ([[Raoellidae]] cca 49–48 Ma)
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 | caption2  = ''[[Pakicetus]]'' ([[Pakicetidae]] cca 49–48 Ma)
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 | caption3  = ''[[Ambulocetus]]'' ([[Ambulocetidae]] cca 49–48 Ma)
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 | caption4  = ''[[Kutchicetus]]'' ([[Remingtonocetidae]] cca 48 Ma)
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 | caption5  = ''[[Remingtonocetus]]'' ([[Remingtonocetidae]] cca 48 Ma)
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 | caption6  = ''[[Maiacetus]]'' ([[Protocetidae]] cca 47.5 Ma)
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 | caption7  = ''[[Rodhocetus]]'' ([[Protocetidae]] cca 45 Ma)
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 | caption8  = ''[[Protocetus]]'' ([[Protocetidae]] cca 45 Ma)
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 | caption9  = ''[[Dorudon]]'' ([[Basilosauridae]] cca 35 Ma)
 | image10    = Squalodon BW.jpg
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 | caption10  = ''[[Squalodon]]'' ([[Squalodontidae]], Odontoceti cca 25 Ma)
 | image11   = Kentriodon BW.jpg
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 | caption11 = ''[[Kentriodon]]'' ([[Kentriodontidae]]. Odontoceti cca 20 Ma)
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 | caption12 = ''[[Aulophyseter]]'' (Physeteridae, Odontoceti cca 20 Ma)
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 | caption14 = ''[[Aetiocetus]]'' ([[Aetiocetidae]], Mysticeti cca 27 Ma)
 | image15   = Janjucetus BW (mirrored).jpg
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 | caption15 = ''[[Janjucetus]]'' ([[Mammalodontidae]], Mysticeti cca 25 Ma)
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 | caption16 = ''[[Cetotherium]]'' ([[Cetotheriidae]], Mysticeti cca 18 Ma)
}}The aquatic lifestyle of cetaceans first began in the [[Indian subcontinent]] from [[even-toed ungulates]] 50 million years ago, with this initial stage lasting approximately 4-15 million years.<ref>{{cite newsgroup|url=http://news.nationalgeographic.com/news/2011/11/111116-antarctica-whales-oldest-evolution-animals-science/|archive-url=https://web.archive.org/web/20111118050846/http://news.nationalgeographic.com/news/2011/11/111116-antarctica-whales-oldest-evolution-animals-science/|archive-date=18 November 2011|title=Oldest Antarctic Whale Found; Shows Fast Evolution|last=Roach|first=John|year=2011|access-date=3 January 2018}}</ref> Archaeoceti is an [[extinct]] parvorder of Cetacea containing ancient whales. The traditional hypothesis of cetacean evolution, first proposed by Van Valen in 1966,<ref>{{cite journal|url=http://www.ivpp.cas.cn/cbw/gjzdwxb/xbwzxz/201504/P020150420393427001650.pdf|first1=Gao|last1=Hong-Yan|first2=Ni|last2=Xi-Jun|year=2015|title=Diverse stem cetaceans and their phylogenetic relationships with mesonychids and artiodactyls|journal=Vertebrata PalAsiatica |volume=53|issue=2|page=165}}</ref> was that whales were related to the [[mesonychia]]ns, an extinct order of [[carnivorous]] [[ungulate]]s (hoofed animals) that resembled wolves with hooves and were a [[sister group]] of the [[artiodactyls]] (even-toed ungulates). This hypothesis was proposed due to similarities between the unusual triangular teeth of the mesonychians and those of early whales. However, [[molecular phylogeny]] data indicates that whales are very closely related to the artiodactyls, with [[hippopotamuses]] as their closest living relative. Because of this observation, cetaceans and hippopotamuses are placed in the same [[suborder]], [[Whippomorpha]].<ref name="science_news_3">{{cite newsgroup |url=https://www.sciencedaily.com/releases/2005/02/050205103109.htm |title=UC Berkeley, French Scientists Find Missing Link Between The Whale And Its Closest Relative, The Hippo |access-date=21 December 2007 |author=University of California, Berkeley (7 February 2005)}}</ref> [[Cetartiodactyla]] (formed from the words Cetacea and Artiodactyla) is a proposed name for an [[order (biology)|order]] that includes both cetaceans and artiodactyls. However, the earliest [[anthracothere]]s, the ancestors of hippos, do not appear in the fossil record until the Middle Eocene, millions of years after ''[[Pakicetids|Pakicetus]]'', whereas the first known whale ancestor appeared during the Early Eocene; this difference in timing implies that the two groups diverged well before the Eocene. Molecular analysis identifies artiodactyls as being very closely related to cetaceans, so mesonychians are probably an offshoot from Artiodactyla, and cetaceans did not derive directly from mesonychians, but the two groups may share a common ancestor.<ref name=Carroll/>

The molecular data are supported by the discovery of ''Pakicetus'', the earliest [[Archaeoceti|archaeocete]]. The skeletons of ''Pakicetus'' show that whales did not derive directly from mesonychians. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychians. Archaeocetes retained aspects of their mesonychian ancestry (such as the triangular teeth) which modern artiodactyls, and modern whales, have lost. The earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today's artiodactyls and [[perissodactyl]]s became herbivores later in their evolution. Whales, however, retained their carnivorous diet because prey was more available and they needed higher [[Food energy|caloric content]] in order to live as marine [[endotherm]]s (warm-blooded). Mesonychians also became specialized carnivores, but this was likely a disadvantage because large prey was uncommon. This may be why they were out-competed by better-adapted animals like the [[hyaenodontid]]s and later [[Carnivora]].<ref name=Carroll>{{cite book|chapter-url={{Google books|plainurl=yes|id=ntSspji0LYC|page=547}}|first1=Carroll|last1=L. Fenton|first2=Pat|last2=V. Rich|first3=Mildred|last3=A. Fenton|first4=Thomas|last4=H. V. Rich|year=1996|title=The Fossil Book: A Record of Prehistoric Life|chapter=The Early Mammals|publisher=Dover Publishing|pages=[https://archive.org/details/fossilbookrecor00rich/page/547 547–548]|isbn=978-0-486-29371-4|url=https://archive.org/details/fossilbookrecor00rich/page/547}}</ref>

=== Raoellidae ===
{{Main|Raoellidae}}Raoellidae ([[Help:IPA/English|/reɪoʊˈɛlɪdeɪ/]]) is a family of extinct [[semiaquatic]] [[Artiodactyl|artiodactyls]] from the middle [[Eocene]] (about 48 million years ago) closely related to [[Cetacea|cetaceans]].<ref name=":23">{{Cite web |title=Whales originated from aquatic artiodactyls in the Eocene Epoch of India |url=https://www.researchgate.net/publication/5752497 |url-status=live |archive-url=https://web.archive.org/web/20230602044623/https://www.researchgate.net/publication/5752497_Whales_originated_from_aquatic_artiodactyls_in_the_Eocene_Epoch_of_India |archive-date=2023-06-02 |access-date=2025-05-24 |website=ResearchGate |language=en}}</ref> They are known primarily from northern India and Pakistan, although potential remains of the raoellid ''[[Khirtharia]]'' have been found in northern China.<ref name=":32">{{Cite web |last1=Orliac |first1=Maeva |last2=Ducrocq |first2=Stephane |date=2011 |title=Eocene raoellids (Mammalia, Cetartiodactyla) outside the Indian Subcontinent: Palaeogeographical implications |url=https://www.researchgate.net/publication/259008437 |website=ResearchGate}}</ref> They are relatively small animals; on average, raoellids were the size of a [[red fox]].<ref name=":24">{{Cite web |title=Whales originated from aquatic artiodactyls in the Eocene Epoch of India |url=https://www.researchgate.net/publication/5752497 |url-status=live |archive-url=https://web.archive.org/web/20230602044623/https://www.researchgate.net/publication/5752497_Whales_originated_from_aquatic_artiodactyls_in_the_Eocene_Epoch_of_India |archive-date=2023-06-02 |access-date=2025-05-24 |website=ResearchGate |language=en}}</ref> However, ''Khirtharia major'', at about twice the size of an average-sized raoellid, would have been approximately the size of a coyote.<ref name=":52">{{Cite web |last1=Thewissen |first1=J.G.M. |last2=Gingerich |first2=P.D. |last3=Russell |first3=D.E. |date=1987 |title=Artiodactyla and Perissodactyla (Mammalia) from the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan) |url=https://deepblue.lib.umich.edu/bitstream/handle/2027.42/48532/ID385.pdf}}</ref> Meanwhile, the smallest raoellid, ''[[Metkatius]]'', was roughly the size of a house cat. Raoellidae is of particular importance to the understanding of cetacean evolution due to representing a [[Transitional fossil|transitional]] form between fully-terrestrial artiodactyls and the semi-aquatic [[Pakicetidae|pakicetids]]. The raoellid that has been most thoroughly studied and most heavily contributed to the knowledge of Raoellidae is ''[[Indohyus]].'' It showed signs of adaptations to aquatic life, including dense limb bones that reduce buoyancy so that they could stay underwater, which are similar to the adaptations found in modern [[aquatic mammal]]s such as the hippopotamus.<ref name="science_news_3" /><ref name="science_news_1">{{cite newsgroup |url=https://www.sciencedaily.com/releases/2001/09/010920072245.htm |title=New Fossils Suggest Whales And Hippos Are Close Kin |access-date=21 December 2007 |author=University of Michigan |year=2001}}</ref> This suggests a similar survival strategy to the African chevrotain or [[water chevrotain]] which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.<ref name="GuardianIndohyus">{{cite newsgroup |url=https://www.theguardian.com/science/2007/dec/19/whale.deer?gusrc=rss&feed=networkfront |title=Whales may be descended from a small deer-like animal |access-date=21 December 2007 |author=Ian Sample |year=2007}}</ref>

[[File:Raoellids&cetaceans.jpg|thumb|left|300px|Possible relationships between cetaceans and other ungulate groups.<ref name="radiations" />
<ref name="indohyus">{{cite journal |last=[[Hans Thewissen|Thewissen]] |first=J. G. M. |author2=Cooper, Lisa Noelle |author3=Clementz, Mark T. |author4=Bajpai, Sunil |author5=Tiwari, B. N. |date=20 December 2007 |title=Whales originated from aquatic artiodactyls in the Eocene epoch of India |journal=Nature |volume=450 |issue=7173 |pages=1190–1194 |bibcode=2007Natur.450.1190T |doi=10.1038/nature06343 |pmid=18097400 |s2cid=4416444}}</ref>]]<br>






===Pakicetidae===
{{Main|Pakicetidae}}
The pakicetids were [[digitigrade]] hoofed mammals that are thought to be the earliest known cetaceans, with Raoellidae being the closest sister group.<ref name=science_news_2>{{cite newsgroup| url = https://www.sciencedaily.com/releases/2007/12/071220220241.htm | title = Whales Descended From Tiny Deer-like Ancestors | access-date = 21 December 2007 | author = Northeastern Ohio Universities Colleges of Medicine and Pharmacy |year=2007}}</ref><ref>{{cite journal|first1=Philip D.|last1=Gingerich|first2=D. E.|last2=Russell|year = 1981| title = ''Pakicetus inachus'', a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan)| journal = Museum of Paleontology|volume = 25|pages = 235–246}}</ref> They lived in the early [[Eocene]], around 50 million years ago. Their fossils were first discovered in North Pakistan in 1979, located at a river not far from the shores of the former [[Tethys Sea]].<ref name=marine>{{cite book|last1=Castro|first1=Peter|last2=Huber|first2=Michael E.|title=Marine Biology (4 ed)|url=https://archive.org/details/marinebiology00pete|url-access=registration|year=2003|publisher=McGraw-Hill}}</ref> After the initial discovery, more fossils were found, mainly in the early Eocene [[fluvial]] deposits in northern Pakistan and northwestern India. Based on this discovery, pakicetids most likely lived in an arid environment with [[ephemeral stream]]s and moderately developed floodplains millions of years ago.<ref name=radiations/> By using stable [[oxygen isotope]]s analysis, they were shown to drink fresh water, implying that they lived around freshwater bodies.<ref name=poster>{{cite journal|last1=Thewissen|first1=J. G. M.|last2=Bajpai|first2=Sunhil|title=Whale Origins as a Poster Child for Macroevolution|journal=BioScience|year=2001|volume=51|issue=12|page=1037|doi=10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2|issn=0006-3568|doi-access=free|author1-link=Hans Thewissen}}</ref> Their diet probably included land animals that approached water for drinking or some freshwater aquatic organisms that lived in the river.<ref name=radiations/> The elongated [[cervical vertebrae]] and the four, fused [[sacral vertebrae]] are consistent with artiodactyls, making ''Pakicetus'' one of the earliest fossils to be recovered from the period following the Cetacea/Artiodactyla [[Genetic divergence|divergence event]].<ref name=":4">{{cite journal|last=Uhen|first=Mark|year=2010|title=The Origin(s) of Whales|journal=Annual Review of Earth and Planetary Sciences|doi=10.1146/annurev-earth-040809-152453|volume=38|issue=1|pages=189–219|bibcode=2010AREPS..38..189U}}</ref>

Pakicetids are classified as cetaceans mainly due to the structure of the [[auditory bulla]] (ear bone), which is formed only from the [[ectotympanic|ectotympanic bone]]. The shape of the ear region in pakicetids is highly unusual and the skull is cetacean-like, although a blowhole is still absent at this stage. The jawbone of pakicetids also lacks the enlarged space ([[mandibular foramen]]) that is filled with fat or oil, which is used in receiving underwater sound in modern cetaceans.<ref name=hearing>{{cite journal|last=Nummela|first=Sirpa|author2=Thewissen, J. G. M. |author3=Bajpai, Sunil |author4=Hussain, S. Taseer |author5=Kumar, Kishor |title=Eocene evolution of whale hearing|journal=Nature|date=11 August 2004|volume=430|issue=7001|pages=776–778|doi=10.1038/nature02720|pmid=15306808|bibcode = 2004Natur.430..776N |s2cid=4372872}}</ref> They have dorsal [[Orbit (anatomy)|orbits]] (eye sockets facing up), which are similar to crocodiles. This eye placement helps submerged predators observe potential prey above the water.<ref name=poster/> According to a 2009 study, the teeth of pakicetids also resemble the teeth of fossil whales, being less like a dog's [[incisors]], and having serrated triangular teeth, which is another link to more modern cetaceans.<ref>{{cite journal|first1=Lisa N.|author2-link=Hans Thewissen|last1=Cooper|first2=J.G.M. |last2=Thewissen|first3=S.T.|last3=Hussain|year=2009|title=New Middle Eocene Archaeocetes (Cetacea:Mammalia) from the Kuldana Formation of Northern Pakistan|journal=Journal of Vertebrate Paleontology|volume=29|issue=4|pages=1289–1299 |doi=10.1671/039.029.0423|bibcode=2009JVPal..29.1289C |s2cid=84127292}}</ref>
It was initially thought that the ears of pakicetids were adapted for underwater hearing, but, as would be expected from the anatomy of the rest of this creature, the ears of pakicetids are specialized for hearing on land.<ref name=skeleton>{{cite journal
|author1=J. G. M. [[Hans Thewissen|Thewissen]]|author2=E. M. Williams |author3=L. J. Roe |author4=S. T. Hussain | title = Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls|journal = Nature|volume = 413| pages = 277–281| year = 2001| doi = 10.1038/35095005| pmid = 11565023| issue = 6853|bibcode = 2001Natur.413..277T |s2cid=4416684 }}</ref> However, pakicetids were able to listen underwater by using enhanced bone conduction, rather than depending on the [[tympanic membrane]] like other land mammals. This method of hearing did not give directional hearing underwater.<ref name=hearing/>

Pakicetids have long thin legs, with relatively short hands and feet which suggest that they were poor swimmers.<ref name=radiations/> To compensate for that, their bones are unusually thick ([[osteosclerotic]]), which is probably an adaptation to make the animal heavier to counteract the buoyancy of the water.<ref name="indohyus" /> According to a 2001 morphological analysis by [[Hans Thewissen|Thewissen]] et al., pakicetids display no aquatic skeletal adaptation; instead they display adaptations for running and jumping. Hence pakicetids were most likely aquatic waders.<ref name=skeletons>{{cite journal|last=[[Hans Thewissen|Thewissen]]|first=J. G. M.|author2=Williams, E. M. |author3=Roe, L. J. |author4=Hussain, S. T. |title=Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls|journal=Nature|year=2001|volume=413|issue=6853|pages=277–281|doi=10.1038/35095005|pmid=11565023|bibcode = 2001Natur.413..277T |s2cid=4416684}}</ref>

===Ambulocetidae===
{{Main|Ambulocetidae}}
''[[Ambulocetus]]'', which lived about 49 million years ago, was discovered in Pakistan in 1994. They were vaguely [[crocodile]]-like mammals, possessing large brevirostrine jaws. In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Sea in northern Pakistan. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and [[littoral zone|littoral mollusks]].<ref name=radiations/> Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed a range of water with different degrees of salinity, with some specimens having no evidence of sea water consumption and others that did not ingest fresh water at the time when their teeth were fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations. Hence, ambulocetids represent a transition phase of cetacean ancestors between fresh water and marine habitat.<ref name=poster/>

The [[mandibular foramen]] in ambulocetids had increased in size, which indicates that a fat pad was likely to be housed in the lower jaw. In modern toothed whales, this fat pad in the mandibular foramen extends posteriorly to the middle ear. This allows sounds to be received in the lower jaw, and then transmitted through the fat pad to the middle ear. Similar to pakicetids, the orbits of ambulocetids are on the top of the skull, but they face more laterally than in pakicetids.<ref name=poster/>

Ambulocetids had relatively long limbs with particularly strong hind legs, and they retained a tail with no sign of a [[Fin#Evolution of fins|fluke]].<ref name="marine" /> The hindlimb structure of Ambulocetids shows that their ability to engage in terrestrial locomotion was significantly limited compared to that of contemporary terrestrial mammals, and likely did not come to land at all. The skeletal structures of the knee and ankle indicates that the motion of the hindlimbs was restricted into one plane. This suggests that, on land, propulsion of the hindlimbs was powered by the extension of dorsal muscles.<ref>{{cite journal |last1=Thewissen|first1=J.G.M. |last2=Hussain |first2=S. T. |last3=Alif |first3=M. |title=Fossil Evidence for the Origin of Aquatic Locomotion in Archaeocete Whales |journal=Science |year=1994 |volume=263 |pages=210–212 |doi=10.1126/science.263.5144.210 |pmid=17839179 |issue=5144 |bibcode=1994Sci...263..210T|s2cid=20604393 |author1-link=Hans Thewissen }}</ref> They probably [[aquatic locomotion|swam]] by pelvic paddling (a way of swimming which mainly utilizes their hind limbs to generate propulsion in water) and caudal undulation (a way of swimming which uses the [[Undulatory locomotion|undulations]] of the vertebral column to generate force for movements), as [[otter]]s, [[pinniped|seals]] and modern cetaceans do.<ref>{{cite journal |last=Thewissen|first=J. G. M |author2=F.E.Fish |title=Locomotor Evolution in the Earliest Cetaceans: Functional Model, Modern Analogues, and Paleontological Evidence |journal=Paleobiology |date=August 1997 |volume=23 |issue=4 |pages=482–490|doi=10.1017/S0094837300019850 |bibcode=1997Pbio...23..482T |s2cid=51685113 }}</ref> This is an intermediate stage in the evolution of cetacean locomotion, as modern cetaceans swim by caudal oscillation (a way of swimming similar to caudal undulation, but is more energy efficient).<ref name="poster" />

A recent study suggests that ambulocetids were fully aquatic like modern cetaceans, possessing a similar thoracic morphology and being unable to support their weight on land. This suggests that complete abandonment of the land evolved much earlier among cetaceans than previously thought. However the scientists involved in the study cautioned that the study was limited by a lack of information on the exact density of the bone, the location of the centre of mass, and the reliance of false ribs for thoracic support.<ref>{{cite journal | doi = 10.1111/joa.12518 | volume=229 | title=Farewell to life on land – thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals | year=2016 | journal=Journal of Anatomy | pages=768–777 | last1 = Ando | first1 = Konami | issue=6 | pmid=27396988 | pmc=5108153}}</ref>

===Remingtonocetidae===
{{Main|Remingtonocetidae}}
Remingtonocetids lived in the Middle-Eocene in [[South Asia]], about 49 to 43 million years ago.<ref name=fordyce>{{cite journal|last=Fordyce|first=R E|author2=Barnes, L G|title=The Evolutionary History of Whales and Dolphins|journal=Annual Review of Earth and Planetary Sciences|date=30 April 1994|volume=22|issue=1|pages=419–455|doi=10.1146/annurev.ea.22.050194.002223|bibcode = 1994AREPS..22..419F }}</ref> Compared to family Pakicetidae and Ambulocetidae, Remingtonocetidae was a diverse family found in north and central Pakistan and western India. Remingtonocetids were also found in shallow marine deposits, but they were obviously more aquatic than ambulocetidae. This is demonstrated by the recovery of their fossils from a variety of coastal marine environments, including near-shore and lagoonal deposits.<ref name=radiations/> According to stable oxygen isotopes analysis, most remingtonocetids did not ingest fresh water, and had hence lost their dependency on fresh water relatively soon after their origin.<ref name=poster/>

The orbits of remingtonocetids faced laterally and were small. This suggests that vision was not an important sense for them. The nasal opening, which eventually becomes the blowhole in modern cetaceans, was located near the tip of the [[snout]]. The position of the nasal opening had remained unchanged since pakicetids.<ref name=poster/> One of the notable features in remingtonocetids is that the [[semicircular canal]]s, which are important for balancing in land mammals, had decreased in size.<ref name=vestibular>{{cite journal|last=Spoor|first=F.|author2=Bajpai, S. |author3=Hussain, S. T. |author4=Kumar, K. |author5=[[Hans Thewissen|Thewissen]], J. G. M. |title=Vestibular evidence for the evolution of aquatic behaviour in early cetaceans|journal=Nature|date=8 May 2002|volume=417|issue=6885|pages=163–166|doi=10.1038/417163a|pmid=12000957|bibcode = 2002Natur.417..163S |s2cid=4331789}}</ref> This reduction in size had closely accompanied the cetacean [[evolutionary radiation|radiation]] into marine environments. According to a 2002 study done by Spoor et al., this modification of the semicircular canal system may represent a crucial 'point of no return' event in early cetacean evolution, which excluded a prolonged semi-aquatic phase.<ref name=vestibular/>

Compared to ambulocetids, remingtonocetids had relatively short limbs.<ref name=poster/> Based on their skeletal remains, remingtonocetids were probably amphibious cetaceans that were well adapted to swimming, and likely to swim by caudal undulation only.<ref name=radiations/>

===Protocetidae===
{{Main|Protocetidae}}
The protocetids form a diverse and heterogeneous group known from Asia, Europe, Africa, and North America. They lived in the Eocene, approximately 48 to 35 million years ago. The fossil remains of protocetids were uncovered from coastal and lagoonal [[facies]] in South Asia; unlike previous cetacean families, their fossils uncovered from Africa (e.g., ''[[Phiomicetus]]'', ''[[Protocetus]]'', ''[[Aegyptocetus]]'', ''[[Togocetus]]'')<ref>{{cite journal |last1=Gohar |first1=AS |last2=Antar |first2=MS |last3=Boessenecker |first3=RW |last4=Sabry |first4=DA |last5=El-Sayed |first5=S |last6=Seiffert |first6=ER |last7=Zalmout |first7=IS |last8=Sallam |first8=HM |title=A new protocetid whale offers clues to biogeography and feeding ecology in early cetacean evolution. |journal=Proceedings. Biological Sciences |date=25 August 2021 |volume=288 |issue=1957 |article-number=20211368 |doi=10.1098/rspb.2021.1368 |pmid=34428967|pmc=8385364 }}</ref><ref>{{Cite journal|last1=Berger |first1=E. |last2=Amson |first2=E. |last3=Peri |first3=E. |last4=Gohar |first4=A. S. |last5=Sallam |first5=H. M. |last6=Ferreira |first6=G. S. |last7=Chowdhury |first7=R. R. |last8=Martinez |first8=Q. |title=The endocranial anatomy of protocetids and its implications for early whale evolution |year=2025 |journal=Evolution |doi=10.1093/evolut/qpaf109 |pmid=40372400 |doi-access=free }}</ref> and North America (e.g., ''[[Georgiacetus]]'') also include open marine forms.<ref name=radiations/> They were probably amphibious, but more aquatic compared to remingtonocetids.<ref name=fordyce/> Protocetids were the first cetaceans to leave the [[Indian subcontinent]] and disperse to all shallow subtropical oceans of the world.<ref name=poster/>
There were many genera among the family Protocetidae. There were different degrees of aquatic adaptations in this group, with some able to support their weight on land, and others not.<ref name=radiations/> The discovery of the southeastern Pacific ''[[Peregocetus]]'' indicates they crossed the [[Atlantic]] and achieved circumstances-equatorial distribution by 40 mya.<ref>{{cite journal|first1=O.|last1=Lambert|first2=G.|last2=Bianucci|first3=R.|last3=Salas-Gismondi|first4=C.|last4=Di Celma|first5=E.|last5=Steurbaut|first6=M.|last6=Urbina|first7=M.|last7=de Muizon|year=2019|title=An Amphibious Whale from the Middle Eocene of Peru Reveals Early South Pacific Dispersal of Quadrupedal Cetaceans|journal=Current Biology|volume=29|issue=8|pages=1352–1359.e3|doi=10.1016/j.cub.2019.02.050|url=https://www.researchgate.net/publication/332217452|doi-access=free|pmid=30955933|bibcode=2019CBio...29E1352L |hdl=11581/425570|hdl-access=free}}</ref> Their amphibious nature is supported by the discovery of a pregnant ''[[Maiacetus]]'', in which the fossilised fetus was positioned for a head-first delivery, suggesting that ''Maiacetus'' gave birth on land. If they gave birth in the water, the fetus would be positioned for a tail-first delivery to avoid drowning during birth.<ref name=plos_one_1>{{cite journal| title = New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism|last1=Gingerich|first1=P. D.|last2=ul-Haq|first2=M.|last3=von Koenigswald|first3=W|last4=Sanders|first4=W. J.|last5=Smith|first5=B. H.| journal = PLOS ONE| volume = 4| issue = 2| article-number = e4366|doi=10.1371/journal.pone.0004366| pmid = 19194487|year=2009| bibcode = 2009PLoSO...4.4366G| pmc=2629576|doi-access=free}}</ref>

Unlike remingtonocetids and ambulocetids, protocetids have large orbits which are oriented laterally. Increasingly lateral-facing eyes might be used to observe underwater prey, and are similar to the eyes of modern cetaceans. Furthermore, the nasal openings were large and were halfway up the snout. The great variety of teeth suggests diverse feeding modes in protocetids.<ref name=fordyce/>
In both remingtonocetids and protocetids, the size of the mandibular foramen had increased.<ref name=poster/> The large mandibular foramen indicates that the mandibular fat pad was present. However the air-filled sinuses that are present in modern cetaceans, which function to isolate the ear acoustically to enable better underwater hearing, were still not present.<ref name=hearing/> The external auditory meatus ([[ear canal]]), which is absent in modern cetaceans, was also present. Hence, the method of sound transmission that were present in them combines aspects of pakicetids and modern [[odontocetes]] (toothed whales).<ref name=hearing/> At this intermediate stage of hearing development, the transmission of airborne sound was poor due to the modifications of the ear for underwater hearing while directional underwater hearing was also poor compared to modern cetaceans.<ref name=hearing/>

Some protocetids had short, wide fore- and hindlimbs that were likely to have been used in swimming, but the limbs gave a slow and cumbersome locomotion on land.<ref name=poster/> It is possible that some protocetids had flukes. However, it is clear that they were adapted even further to an aquatic life-style. In ''Rodhocetus'', for example, the [[sacrum]] (a bone that, in land-mammals, is a fusion of five vertebrae that connects the [[pelvis]] with the rest of the [[vertebral column]]) was divided into loose vertebrae. However, the pelvis was still connected to one of the sacral vertebrae. The ungulate ancestry of these archaeocetes is still underlined by characteristics like the presence of hooves at the ends of the toes in ''Rodhocetus''.<ref>{{cite book |last1=Madar |first1=Sandra I. |editor-first=J.G.M |editor-last=[[Hans Thewissen|Thewissen]]|year=1998 |title=The Emergence of Whales |chapter=Structural Adaptations of Early Archaeocete Long Bones|series=Advances in Vertebrate Paleobiology |volume=1 |pages=353–378 |doi=10.1007/978-1-4899-0159-0_12|isbn=978-1-4899-0161-3 }}</ref>

The foot structure of ''Rodhocetus'' shows that protocetids were predominantly aquatic. A 2001 study done by Gingerich et al. hypothesized that ''Rodhocetus'' locomoted in the oceanic environment similarly to how ambulocetids pelvic paddling, which was supplemented by caudal undulation. Terrestrial locomotion of Rodhocetus was very limited due to their hindlimb structure. It is thought that they moved in a way similar to how [[eared seal]]s move on land, by rotating their hind flippers forward and underneath their body.<ref name=gingerich2001>{{cite journal|last=Gingerich|first=Philip D|author2=Munir ul Haq|author3=Lyad S. Zalmout|author4=Intizar Hyssain Khan|author5=M. sadiq Malkani|title=Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from Pakistan|journal=Science|date=21 September 2001|volume=293|issue=5538|pages=2239–42|doi=10.1126/science.1063902|bibcode=2001Sci...293.2239G|pmid=11567134|s2cid=21441797|doi-access=free}}</ref>

===Basilosauridae===
{{Main|Basilosauridae|Dorudontinae}}
[[File:Dorudon atrox skull 03 - Smithsonian.JPG|thumb|left|Archaeocetes (like this ''Basilosaurus'') had a [[heterodont]] dentition]]
Basilosaurids and [[Dorudontinae|dorudontines]] lived together in the late Eocene around 41 to 33.9 million years ago, and are the oldest known obligate aquatic cetaceans.<ref name=skeleton/> They were fully recognizable whales which lived entirely in the ocean. This is supported by their fossils usually found in deposits indicative of fully marine environments, lacking any freshwater influx.<ref name=radiations/> They were probably distributed throughout the tropical and subtropical seas of the world. Basilosaurids are commonly found in association with dorudontines, and were closely related to one another.<ref name=poster/> The fossilised stomach contents in one basilosaurid indicates that it ate fish.<ref name=radiations/>

Although they look very much like modern cetaceans, basilosaurids lacked the '[[Melon (whale)|melon organ]]' that allows toothed whales to use [[Animal echolocation|echolocation]]. They had small brains; this suggests they were solitary and did not have the complex social structures of some modern cetaceans. The mandibular foramen of basilosaurids covered the entire depth of the lower jaw as in modern cetaceans. Their orbits faced laterally, and the nasal opening had moved even higher up the snout, closer to the position of the blowhole in modern cetaceans.<ref name=poster/> Furthermore, their ear structures were functionally modern, with the insertion of air-filled sinuses between ear and skull.<ref name=hearing/> Unlike modern cetaceans, basilosaurids retained a large external auditory meatus.<ref name=hearing/>

Both basilosaurids have skeletons that are immediately recognizable as cetaceans. A basilosaurid was as big as the larger modern whales, with genera like ''[[Basilosaurus]]'' reaching lengths of up to {{convert|60|ft|m|abbr=on}} long; dorudontines were smaller, with genera like ''[[Dorudon]]'' reaching about {{convert|15|ft|m|abbr=on}} long. The smallest basilosaurid whale is ''[[Tutcetus]]'' and measures {{convert|8.2|ft|m|abbr=on}} long.<ref>{{cite journal |last1=Antar |first1=Mohammed S. |last2=Gohar |first2=Abdullah S. |last3=El-Desouky |first3=Heba |last4=Seiffert |first4=Erik R. |last5=El-Sayed |first5=Sanaa |last6=Claxton |first6=Alexander G. |last7=Sallam |first7=Hesham M. |title=A diminutive new basilosaurid whale reveals the trajectory of the cetacean life histories during the Eocene |journal=Communications Biology |date=10 August 2023 |volume=6 |issue=1 |page=707 |doi=10.1038/s42003-023-04986-w |pmid=37563270 |language=en |issn=2399-3642|pmc=10415296 }}</ref> The large size of basilosaurids is due to the extreme elongation of their lumbar vertebrae. They had a tail fluke, but their body proportions suggest that they swam by caudal undulation and that the fluke was not used for propulsion.<ref name=radiations/><ref>{{cite journal|first1=Alexandra|last1= Houssaye|first2=Paul|last2=Tafforeau|first3=Christian|last3=de Muizon|first4=Philip D.|last4=Gingerich|year=2015|title=Transition of Eocene Whales from Land to Sea: Evidence from Bone Microstructure|journal=PLOS ONE|volume=10 |issue=2|article-number=e0118409|doi=10.1371/journal.pone.0118409|pmid=25714394|pmc=4340927|bibcode=2015PLoSO..1018409H|doi-access= free}}</ref> In contrast, dorudontines had a shorter but powerful vertebral column. They too had a fluke and, unlike basilosaurids, they probably swam similarly to modern cetaceans, by using caudal oscillations.<ref name=poster/> The forelimbs of basilosaurids were probably flipper-shaped, and the external hind limbs were tiny and were certainly not involved in locomotion.<ref name=radiations/> Their fingers, however, retained the mobile joints of their ambulocetid relatives. The two tiny but well-formed hind legs of basilosaurids were probably used as [[clasper]]s when mating. The pelvic bones associated with these hind limbs were not connected to the vertebral column as they were in protocetids. Essentially, any sacral vertebrae can no longer be clearly distinguished from the other vertebrae.<ref name=radiations/><ref>{{cite newsgroup|url=https://www.nytimes.com/1990/07/12/us/whales-hind-feet-show-up-in-fossils.html|last=N. Wilford|first=John|year=1990|title=Whales' hind feet show up in fossils|access-date=1 March 2016}}</ref>

Both basilosaurids and dorudontines are relatively closely related to modern cetaceans, which belong to parvorders [[Odontoceti]] and [[Mysticeti]]. However, according to a 1994 study done by Fordyce and Barnes, the large size and elongated vertebral body of basilosaurids preclude them from being ancestral to extant forms. As for dorudontines, there are some species within the family that do not have elongated vertebral bodies, which might be the immediate ancestors of Odontoceti and Mysticeti. The other basilosaurids became extinct.<ref name=fordyce/>

==Evolution of modern cetaceans==

===Baleen whales===
{{Main|Baleen whale#Evolutionary history}}
[[Image:VMNH megalodon.jpg|thumb|Artistic impression of two ''[[Eobalaenoptera]]'' pursued by the giant shark ''[[Megalodon|Otodus megalodon]]'']]
All modern baleen whales or mysticetes are filter-feeders which have [[baleen]] in place of teeth, though the exact means by which baleen is used differs among species (gulp-feeding within balaenopterids, skim-feeding within balaenids, and bottom plowing within eschrichtiids). The first members of both groups appeared during the middle [[Miocene]]. Filter feeding is very beneficial as it allows baleen whales to efficiently gain huge energy resources, which makes the large body size in modern varieties possible.<ref>{{cite journal|last1=Demere|first1=T.A.|last2=McGowen|first2=M.R.|last3=Berta|first3=A.|last4=Gatesy|first4=J.|title=Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales|journal=Systematic Biology|date=2008|volume=57|issue=1|pages=15–37|doi=10.1080/10635150701884632|pmid=18266181|doi-access=free}}</ref> The development of filter feeding may have been a result of worldwide environmental change and physical changes in the oceans. A large-scale change in ocean current and temperature could have contributed to the radiation of modern mysticetes.<ref>{{cite journal|last=Deméré|first=Thomas|author2=Michael R. McGowen |author3=Annalisa Berta |author4=John Gatesy |title=Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales|journal=Systematic Biology|date=September 2007|volume=57|issue=1|pages=15–37|doi=10.1080/10635150701884632|pmid=18266181|doi-access=free}}</ref> The earlier varieties of baleen whales, or "archaeomysticetes", such as ''[[Janjucetus]]'' and ''[[Mammalodon]]'' had very little baleen and relied mainly on their teeth.<ref>{{cite journal|last=M. G. Fitzgerald|first=Erich|year=2012|title=Archaeocete-like jaws in a baleen whale|journal=Biology Letters|volume=8|issue=1|pages=94–96|doi=10.1098/rsbl.2011.0690|pmid=21849306|pmc=3259978}}</ref>

There is also evidence of a genetic component of the evolution of toothless whales. Multiple mutations have been identified in genes related to the production of enamel in modern baleen whales.<ref name=":2">{{Cite journal|last1=Deméré|first1=Thomas A.|last2=McGowen|first2=Michael R.|last3=Berta|first3=Annalisa|last4=Gatesy|first4=John|date=1 February 2008|title=Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales|journal=Systematic Biology|language=en|volume=57|issue=1|pages=15–37|doi=10.1080/10635150701884632|issn=1063-5157|pmid=18266181|doi-access=free}}</ref> These are primarily insertion/deletion mutations that result in premature stop codons.<ref name=":2" /> It is hypothesized that these mutations occurred in cetaceans already possessing preliminary baleen structures, leading to the pseudogenization of a "genetic toolkit" for enamel production.<ref>{{Cite journal|last1=Meredith|first1=R. W.|last2=Gatesy|first2=J.|title=Pseudogenization of the tooth gene enamelysin (MMP20) in the common ancestor of extant baleen whales|journal=Proceedings of the Royal Society B|volume=278|issue=1708|pages=993–1002|doi=10.1098/rspb.2010.1280|pmid=20861053|pmc=3049022|year=2010}}</ref> Recent research has also indicated that the development of baleen and the loss of enamel-capped teeth both occurred once, and both occurred on the mysticete stem branch.

Generally it is speculated the four modern mysticete families have separate origins among the [[Cetotheriidae|cetotheres]]. Modern baleen whales, [[Balaenopteridae]] (rorquals and humpback whale, ''Megaptera novaengliae''), [[Balaenidae]] (right whales), [[Eschrichtiidae]] (gray whale, ''Eschrictius robustus''), and [[Neobalaenidae]] (pygmy right whale, ''Caperea marginata'') all have derived characteristics presently unknown in any cetothere and vice versa (such as a [[sagittal crest]]<ref>{{cite journal|first1=R.|last1=E. Fordyce|first2=Felix|last2=G. Marx|year=2012|title=The pygmy right whale ''Caperea marginata'': the last of the cetotheres|journal= Proceedings. Biological Sciences|volume=280|issue=1753|article-number=20122645|doi=10.1098/rspb.2012.2645|pmid=23256199|pmc=3574355}}</ref>).<ref>{{cite journal|last1=Bisconti|first1=Michelangelo|last2=Lambert|first2=Olivier|last3=Bosselaers|first3=Mark|year=2013|title=Taxonomic revision of ''Isocetus depauwi'' (Mammalia, Cetacea, Mysticeti) and the phylogenetic relationships of archaic 'cetothere' mysticetes|journal= Palaeontology|volume=56|issue=1|pages=95–127|doi=10.1111/j.1475-4983.2012.01168.x|bibcode=2013Palgy..56...95B |doi-access=free}}</ref> Mysticetes are also known for their gigantism, as baleen whales are among the largest organisms to ever have lived; they reach lengths greater than 20 m and weigh more than 100,000&nbsp;kg.<ref>{{Cite journal |last1=Bisconti |first1=Michelangelo |last2=Pellegrino |first2=Luca |last3=Carnevale |first3=Giorgio |date=2021-07-18 |title=Evolution of gigantism in right and bowhead whales (Cetacea: Mysticeti: Balaenidae) |journal=Biological Journal of the Linnean Society |volume=134 |issue=2 |pages=498–524 |doi=10.1093/biolinnean/blab086 |issn=0024-4066}}</ref> This gigantism is directly related to their feeding mechanism – mysticete size has been found to be dependent on the amount of baleen a mysticete can use to filter its prey.<ref>{{Cite journal |last1=Bianucci |first1=Giovanni |last2=Marx |first2=Felix G. |last3=Collareta |first3=Alberto |last4=Di Stefano |first4=Agata |last5=Landini |first5=Walter |last6=Morigi |first6=Caterina |last7=Varola |first7=Angelo |date=May 2019 |title=Rise of the titans: baleen whales became giants earlier than thought |journal=Biology Letters |volume=15 |issue=5 |article-number=20190175 |doi=10.1098/rsbl.2019.0175 |pmid=31039728 |pmc=6548731 |issn=1744-9561}}</ref> Additionally, size is a positively selected trait that gives mysticetes a boost in fitness.<ref>{{Cite journal |last1=Slater |first1=Graham J. |last2=Goldbogen |first2=Jeremy A. |last3=Pyenson |first3=Nicholas D. |date=2017-05-24 |title=Independent evolution of baleen whale gigantism linked to Plio-Pleistocene ocean dynamics |journal=Proceedings of the Royal Society B: Biological Sciences |volume=284 |issue=1855 |article-number=20170546 |doi=10.1098/rspb.2017.0546 |pmid=28539520 |pmc=5454272 |issn=0962-8452}}</ref> Mysticete populations will therefore slowly become even more gigantic as whales with larger amounts of baleen are selected.

===Toothed whales===

The adaptation of [[biosonar#Toothed whales|echolocation]] occurred when [[toothed whale]]s (Odontoceti) split apart from baleen whales, and distinguishes modern toothed whales from fully aquatic archaeocetes. This happened around 34 million years ago in a second cetacean [[evolutionary radiation|radiation]].<ref name="Steeman et al 2009">{{cite journal|author1=Mette E. Steeman|author2=Martin B. Hebsgaard|author3=R. Ewan Fordyce|author4=Simon Y. W. Ho|author5=Daniel L. Rabosky|author6=Rasmus Nielsen|author7=Carsten Rahbek|author8=Henrik Glenner|author9=Martin V. Sørensen|author10=Eske Willerslev|date=24 August 2009|title=Radiation of Extant Cetaceans Driven by Restructuring of the Oceans|journal=Systematic Biology|volume=58|issue=6|pages=573–585|doi=10.1093/sysbio/syp060|pmid=20525610|pmc=2777972}}</ref><ref>{{cite journal|author1=Alexander J. P. Houben1|author2=Peter K. Bijl|author3=Jörg Pross|author4=Steven M. Bohaty|author5=Sandra Passchier|author6=Catherine E. Stickley|author7=Ursula Röhl|author8=Saiko Sugisaki|author9=Lisa Tauxe|author10=Tina van de Flierdt|author11=Matthew Olney|author12=Francesca Sangiorgi|author13=Appy Sluijs|author14=Carlota Escutia|author15=Henk Brinkhuis|date=19 April 2013|title=Reorganization of Southern Ocean Plankton Ecosystem at the Onset of Antarctic Glaciation|journal=Science|volume=340|issue=6130|pages=341–344|doi=10.1126/science.1223646|pmid=23599491|bibcode=2013Sci...340..341H|hdl=1874/385683|s2cid=30549019|hdl-access=free}}</ref> Modern toothed whales do not rely on their sense of sight, but rather on their [[biosonar|sonar]] to hunt prey. [[Animal echolocation|Echolocation]] also allowed toothed whales to dive deeper in search of food, with light no longer necessary for navigation, which opened up new food sources.<ref name=fordyce/><ref>{{cite journal|first1=Sirpa|author2-link=Hans Thewissen|last1=Nummela1|first2=J. G. M.|last2=Thewissen|first3=Sunil|last3=Bajpai|first4=S.|last4=T. Hussain|first5=Kishor|last5=Kumar |year=2004|title=Eocene evolution of whale hearing|journal=Nature|volume=430|issue=7001|pages=776–778|doi=10.1038/nature02720|pmid=15306808|bibcode=2004Natur.430..776N|s2cid=4372872}}</ref> Toothed whales echolocate by creating a series of clicks emitted at various frequencies. Sound pulses are emitted, reflected off objects, and retrieved through the lower jaw. Skulls of ''[[Squalodon]]'' show evidence for the first hypothesized appearance of echolocation.<ref>{{cite journal|last1=Whitmore, Jr.|first1=F.C.|last2=Sanders|first2=A.E.|year=1976|title=Review of the Oligocene Cetacea|journal=Systematic Zoology|volume=25|issue=4|pages=304–320|doi=10.2307/2412507|jstor=2412507|url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1246&context=usgsstaffpub|url-access=subscription}}</ref> ''Squalodon'' lived from the early to middle [[Oligocene]] to the middle [[Miocene]], around 33–14 million years ago. ''Squalodon'' featured several commonalities with modern toothed whales: the cranium was well compressed (to make room for the melon, a part of the [[nose]]), the rostrum telescoped outward into a beak, a characteristic of the modern toothed whales that gave ''Squalodon'' an appearance similar to them. However, it is thought unlikely that squalodontids are direct ancestors of modern toothed whales.<ref>{{cite journal|last1=Cahuzac|first1=Bruno|last2=Buisson|first2=Sébastien|last3=Pommiès|first3=Michel|last4=Rocher|first4=Philippe|year=2006|title=Découverte de deux dents de Squalodon (Cetacea Odontoceti) dans le Burdigalien du SW de la France (Martillac, Léognan). Considérations sur les Squalodon d'Aquitaine, la paléoécologie de leurs gisements et l'espèce type du genre|language=fr|journal=Neues Jahrbuch für Geologie und Paläontologie|volume=238 |issue=3|pages=413–451|doi=10.1127/njgpa/238/2006/413}}</ref>

The first oceanic dolphins such as [[Kentriodontidae|kentriodonts]], evolved in the late Oligocene and diversified greatly during the mid-Miocene.<ref>{{cite book|url={{Google books|id=RbrbCwAAQBAJ|page=242|plainurl=yes}}|first1=Felix G.|last1=Marx|first2=Olivier|last2=Lambert|first3=Mark D.|last3=Uhen|year=2016|title=Cetacean Paleobiology|publisher= John Wiley's and Sons|pages=242–243|isbn=978-111-856127-0}}</ref> The first fossil cetaceans near shallow seas (where porpoises inhabit) were found around the North Pacific; species like ''[[Semirostrum]]'' were found along California (in what were then estuaries).<ref>{{Cite journal|author1=Rachel A. Racicot |author2=Thomas A. Deméré |author3=Brian L. Beatty |author4=Robert W. Boessenecker |year=2014 |title=Unique Feeding Morphology in a New Prognathous Extinct Porpoise from the Pliocene of California|journal=Current Biology |volume=24 |issue=7 |pages=774–779 |doi=10.1016/j.cub.2014.02.031 |pmid=24631245 |doi-access=free |bibcode=2014CBio...24..774R }}</ref> These animals spread to the European coasts and Southern Hemisphere only much later, during the [[Pliocene]].<ref name=EoM>{{cite book|editor= Macdonald, D.|author= Gaskin, David E.|year= 1984|title= The Encyclopedia of Mammals|publisher= Facts on File|location= New York|pages= [https://archive.org/details/encyclopediaofma00mals_0/page/196 196–199]|isbn= 978-0-87196-871-5|url= https://archive.org/details/encyclopediaofma00mals_0/page/196}}</ref> The earliest known ancestor of arctic whales is ''[[Denebola brachycephala]]'' from the late [[Miocene]] around 9–10 million years ago.<ref>{{cite book|author=Barnes, Lawrence G |title=Fossil odontocetes (Mammalia: Cetacea) from the Almejas Formation, Isla Cedros, Mexico |asin=B0006YRTU4 |publisher=University of California, Museum of Paleontology|page=46 }}</ref> A single fossil from [[Baja California Peninsula|Baja California]] indicates the family once inhabited warmer waters.<ref name=fordyce/><ref>{{cite journal|author=Barnes, L. G. |title=Outline of Eastern North Pacific Fossil Cetacean Assemblages |journal=Systematic Zoology |volume=25 |year=1977 |pages=321–343 |doi= 10.2307/2412508 |issue=4|jstor=2412508 }}</ref><ref>{{cite book|url= https://books.google.com/books?id=2rkHQpToi9sC&pg=PA214|author1=Perrin, William F. |author2=Würsig, Bernd G. |author3=[[Hans Thewissen|Thewissen]], J. G. M. |title=Encyclopaedia of marine mammals |publisher=Academic Press |edition=2 |year=2009 |page= 214|isbn=978-0-12-373553-9}}</ref>

[[File:Acrophyseter deinodon skull.jpg|thumb|left|''[[Acrophyseter]]'' skull]]
Ancient sperm whales differ from modern sperm whales in tooth count and the shape of the face and jaws. For example, ''[[Scaldicetus]]'' had a tapered [[rostrum (anatomy)|rostrum]]. Genera from the Oligocene and Miocene had teeth in their upper jaws. These anatomical differences suggest that these ancient species may not have necessarily been deep-sea squid hunters like the modern sperm whale, but that some genera mainly ate fish.<ref name=fordyce/><ref name="Bianucci, G. & Landini, W. 103–131">{{cite journal|title=Killer sperm whale: a new basal physeteroid (Mammalia, Cetacea) from the Late Miocene of Italy|author1=Bianucci, G.  |author2=Landini, W.|journal=Zoological Journal of the Linnean Society|volume=148|issue=1|pages=103–131|date=8 September 2006|doi=10.1111/j.1096-3642.2006.00228.x|doi-access=free}}</ref> Contrary to modern sperm whales, most ancient sperm whales were built to hunt whales. ''[[Livyatan]]'' had a short and wide rostrum measuring {{convert|10|ft|m}} across, which gave the whale the ability to inflict major damage on large struggling prey, such as other early whales. Species like these are collectively known as killer sperm whales or [[macroraptorial sperm whale]]s.<ref name="Bianucci, G. & Landini, W. 103–131"/><ref>{{cite newsgroup|url=https://www.theguardian.com/science/blog/2010/sep/20/sperm-whales-fossils|title=Sperm whales: a long and vicious history|author=Brian Switek|access-date=12 August 2015}}</ref>

[[Beaked whales]] consist of over 20 genera.<ref name=Bianucci>{{cite journal|url=http://www.scielo.org.za/scielo.php?pid=S0038-23532008000200013&script=sci_arttext|year=2008|title=Beaked whale mysteries revealed by seafloor fossils trawled off South Africa|volume=104|issue=3–4|pages=140–142|author1=Giovanni Bianucci|author2=Klaas Post|author3=Olivier Lambert|journal=University of Pisa, Natural History Museum of Rotterdam, Royal Institute of Natural Sciences of Belgium}}</ref><ref>{{Cite journal|last1=Bianucci|first1=Giovanni|last2=Miján|first2=Ismael|last3=Lambert|first3=Olivier|last4=Post|first4=Klaas|last5=Mateus|first5=Octávio|title=Bizarre fossil beaked whales (Odontoceti, Ziphiidae) fished from the Atlantic Ocean floor off the Iberian Peninsula|year=2013|journal=Geodiversitas|volume=35|issue=1|pages=105–153|doi=10.5252/g2013n1a6|s2cid=129787505|url=http://www.vliz.be/imisdocs/publications/294830.pdf}}<!--|access-date=January 2014--></ref> Earlier varieties were probably preyed upon by killer sperm whales and large sharks such as [[megalodon]]. In 2008, a large number of fossil ziphiids were discovered off the coast of South Africa, confirming the remaining ziphiid species might just be a remnant of a higher diversity that has since gone extinct. After studying numerous fossil skulls, researchers discovered the absence of functional [[maxillary teeth]] in all South African ziphiids, which is evidence that suction feeding had already developed in several beaked whale lineages during the Miocene. Extinct ziphiids also had robust skulls, suggesting that [[tusk]]s were used for male-male interactions.<ref name=Bianucci/>

==Skeletal evolution==
[[File:Ichthyosaur vs dolphin.svg|right|thumb|320px|[[Dolphin]]s ([[aquatic mammal]]s) and [[Ichthyosauria|ichthyosaur]]s (extinct [[marine reptile]]s) share a number of unique adaptations for fully aquatic lifestyle and are frequently used as extreme examples of [[convergent evolution]]]]

Modern cetaceans have internal, rudimentary hind limbs, such as reduced femurs, fibulas, and tibias, and a [[pelvic girdle]]. ''Indohyus'' has a thickened ectotympanic internal lip of the ear bone. This feature compares directly to that of modern cetaceans. Another similar feature was the composition of the teeth, which contained mostly [[calcium phosphate]] which is needed for eating and drinking by aquatic animals, though, unlike modern day toothed whales, they had a [[heterodont]] (more than one tooth morphology) dentition as opposed to a [[homodont]] (one tooth morphology present) dentition.<ref name="outreach">{{cite journal|first1=J. G. M.|last1=Thewissen|first2=Lisa N.|last2=Cooper|first3=John C.|last3=George|first4=Sunil|last4=Bajpai|year=2009|title=From Land to Water: the Origin of Whales, Dolphins, and Porpoises|journal=Evolution: Education and Outreach|volume=2|issue=2|pages=272–288|doi=10.1007/s12052-009-0135-2|doi-access=free|author1-link=Hans Thewissen}}</ref> Although they somewhat resembled a wolf, the fossils of pakicetids showed the [[eye socket]]s were much closer to the top of their head than that of other terrestrial mammals, but similar to the structure of the eyes in cetaceans. Their transition from land to water led to reshaping of the skull and food processing equipment because the eating habits were changing. The change in position of the eyes and limb bones is associated with the pakicetids becoming waders. The ambulocetids also began to develop long snouts, which is seen in current cetaceans. Their limbs (and hypothesized movement) were very similar to otters.<ref>{{cite journal|last1=Thewissen|first1=J. G. M.|last2=Williams|first2=E. M.|last3=Roe|first3=L. J.|last4=Hussain|first4=S. T.|title=Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls|year=2001|journal=Nature|volume=413|issue=6853|pages=277–281|url=http://www.faculty.virginia.edu/bio202/202-2002/Lectures%2020202/thesissen%20et%20al%202001.pdf|doi=10.1038/35095005|pmid=11565023|bibcode=2001Natur.413..277T|s2cid=4416684|author1-link=Hans Thewissen|access-date=16 January 2016|archive-date=4 March 2016|archive-url=https://web.archive.org/web/20160304030425/http://www.faculty.virginia.edu/bio202/202-2002/Lectures%2020202/thesissen%20et%20al%202001.pdf}}</ref>

[[File:Mystice pelvis (whale).png|225px|thumb|left|The skeleton of a [[bowhead whale]] with the hind limb and pelvic bone structure circled in red. This bone structure stays internal during the entire life of the species.]]
Limblessness in cetaceans does not represent a regression of fully formed limbs nor the absence of limb bud initiation, but rather arrest of limb bud development.<ref name=":0">{{cite journal|last=Bejder|first=L.|date=2002|title=Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss|journal=Evolution and Development|volume=4|issue=6|pages=445–58|doi=10.1046/j.1525-142X.2002.02033.x|pmid=12492145|s2cid=8448387}}<!--|access-date=10 October 2015--></ref> Limb buds develop normally in cetacean embryos.<ref name="science_news_3" /><ref name=":1" /> Limb buds progress to the condensation phase of early skeletogenesis, where nerves grow into the limb bud and the [[apical ectodermal ridge]] (AER), a structure that ensures proper limb development, appears functional.<ref name=":0" /><ref name=":1">{{cite journal|last1=Thewissen|first1=J. G. M.|last2=Cohn|first2=M. J.|last3=Stevens|first3=L. S.|last4=Bajpai|first4=S.|last5=Heyning|first5=J.|last6=Horton, Jr.|first6=W. E.|year=2006|title=Developmental basis for hind-limb loss in dolphins and origin of the cetacean bodyplan|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=103|pages=8414–8418|bibcode=2006PNAS..103.8414T|doi=10.1073/pnas.0602920103|pmc=1482506|pmid=16717186|number=22|author1-link=Hans Thewissen|doi-access=free}}<!--|access-date=26 October 2015--></ref> Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs ([[atavism]]).<ref>{{cite journal|first1=Lars|last1=Bejder|first2=Brian K.|last2=Hall|year=2002|title=Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss|journal=Evolution & Development|volume=4|issue=6|pages=445–458|doi=10.1046/j.1525-142X.2002.02033.x|pmid=12492145|s2cid=8448387}}</ref>

[[File:Pakicetus attocki Skelton.jpg|thumb|''Pakicetus attocki'' skeleton]]
{{anchor|Pelvic bones}}''Pakicetus'' had a pelvic bone most similar to that of terrestrial mammals. In later species, such as ''Basilosaurus'', the pelvic bone, no longer attached to the vertebrae and the [[ilium (bone)|ilium]], was reduced.<ref name=outreach/> Certain genes are believed to be responsible for the changes that occurred to the cetacean pelvic structure, such as [[BMP7]], [[PBX1]], [[PBX2]], [[PRRX1]], and [[PRRX2]].<ref name="ReferenceA">{{cite journal|last=Gol'din|first=Pavel|year=2014|title=Naming an Innominate: Pelvis and Hindlimbs of Miocene Whales Give an Insight into Evolution and Homology of Cetacean Pelvic Girdle| journal=Evolutionary Biology|volume=41|issue=3|pages=473–479|doi=10.1007/s11692-014-9281-8|bibcode=2014EvBio..41..473G |s2cid=15055515}}</ref> The pelvic girdle in modern cetaceans were once thought to be [[vestigial structure]]s that served no purpose at all. The pelvic girdle in male cetaceans is different in size compared to females, and the size is thought to be a result of sexual dimorphism.<ref>{{cite journal | last1 = Dines | first1 = James P. | display-authors = etal | year = 2014 | title = Sexual selection targets cetacean pelvic bones | journal = Evolution | volume = 68 | issue = 11| pages = 3296–3306 | doi = 10.1111/evo.12516 | pmid = 25186496 | pmc = 4213350 }}</ref> The pelvic bones of modern male cetaceans are more massive, longer, and larger than those of females. Due to the sexual dimorphism displayed, they were most likely involved in supporting [[Penis#Cetaceans|male genitalia that remain hidden behind abdominal walls]] until sexual reproduction occurs.<ref name="ReferenceA" /><ref name="ReferenceB">{{cite journal|first1=Onbe|last1=Kaori|first2=Shin|last2=Nishida|first3=Emi|last3=Sone|first4=Naohisa|last4=Kanda|first5=Mutsuo|last5=Goto|first6=Luis|last6=A. Pastene|first7=Shinsuke |last7=Tanabe|first8=Hiroko|last8=Koike|year=2007|title=Sequence Variation in the Tbx4 Gene in Marine Mammals|journal=Zoological Science|volume=24|issue=5|pages=449–464|pmid=17867844|doi=10.2108/zsj.24.449|s2cid=31047933}}</ref>
<ref>{{cite journal|last1=Tajima|first1=Yuko|last2=Yoshihiro|first2=Hayashi|last3=Tadasu|first3=Yamada|year=2004|title=Comparative Anatomical Study on the Relationships between the Vestigial Pelvic Bones and the Surrounding Structures of Finless Porpoises|journal=Journal of Veterinary Medical Science|volume=66|issue=7|pages=761–766|doi=10.1292/jvms.66.761|pmid=15297745|doi-access=free}}</ref>

Early archaeocetes such as ''[[Pakicetus]]'' had the nasal openings at the end of the [[snout]], but in later species such as ''[[Rodhocetus]]'', the openings had begun to drift toward the top of the skull. This is known as nasal drift.<ref>{{cite book|chapter-url=https://books.google.com/books?id=PyU2CK6CeoAC&pg=PA227|first=Arthur|last=T. Johnson|year=2011|title=Biology for Engineers|chapter=Principles of Biology|page=227|publisher=CRC Press |isbn=978-1-4200-7763-6}}</ref> The nostrils of modern cetaceans have become modified into [[blowhole (biology)|blowhole]]s that allow them to break to the surface, inhale, and submerge with convenience. The ears began to move inward as well, and, in the case of ''Basilosaurus'', the middle ears began to receive vibrations from the lower jaw. Today's modern toothed whales use their melon organ, a pad of fat, for [[Animal echolocation|echolocation]].<ref>{{cite journal|pmc=4356564|first1=Maya|last1=Yamato|first2=Nicholas D.|author2-link=Nicholas Pyenson|last2=Pyenson|year=2015|title=Early Development and Orientation of the Acoustic Funnel Provides Insight into the Evolution of Sound Reception Pathways in Cetaceans|journal=PLOS ONE|volume=10|issue=3|article-number=e0118582|doi=10.1371/journal.pone.0118582|pmid=25760328|bibcode=2015PLoSO..1018582Y|doi-access=free}}</ref>

== Radiation events ==
There are three major radiation events that mark diversification and speciation in the evolutionary history of Cetacea. The first occurred around the middle Eocene (40 Mya) when these early cetaceans abandoned riverine and shallow coastal habitats, setting the scene for Protocetidae – the first fully marine cetacean.<ref name="radiations" /> With the oceans and its nutrients at their disposal, rapidly diversifying protocetids were also responsible for the first major geographic expansion, dispersing throughout North Africa, Europe, and North America.<ref name=":4" /> The second of three major radiation events occurred near the start of the Oligocene (~34 Mya) when Neoceti diverged from Basilosauridae.<ref name=":3" /> This radiation event concurrently occurs with the breakup of Gondwana and the opening of the Southern Ocean, wildly changing ocean ecosystems, productivity, and temperature gradients.<ref name=":5">{{Citation |last=Fordyce |first=R. Ewan |title=Cetacean Evolution |date=2018-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780128043271000881 |encyclopedia=Encyclopedia of Marine Mammals (Third Edition) |pages=180–185 |editor-last=Würsig |editor-first=Bernd |publisher=Academic Press |language=en |isbn=978-0-12-804327-1 |access-date=2022-11-13 |editor2-last=Thewissen |editor2-first=J. G. M. |editor3-last=Kovacs |editor3-first=Kit M.}}</ref> The timing of this second radiation event is not coincidental, as the following diversification of cetaceans was likely due to new ecological opportunities the change in oceans gave them.<ref>{{Cite journal |last1=Fordyce |first1=R. Ewan |last2=Marx |first2=Felix G. |date=2018-05-21 |title=Gigantism Precedes Filter Feeding in Baleen Whale Evolution |journal=Current Biology |language=en |volume=28 |issue=10 |pages=1670–1676.e2 |doi=10.1016/j.cub.2018.04.027 |pmid=29754903 |s2cid=21680283 |issn=0960-9822|doi-access=free |bibcode=2018CBio...28E1670F }}</ref> The final major radiation event, occurring throughout the middle Miocene and into the Pliocene (12 Mya to 2 Mya), was not due to a specific event but is associated with widespread generic expansion of odontocetes and mysticetes.<ref name=":4" /><ref name=":5" /> Some modern genera of cetaceans began to emerge, including ''[[Balaenoptera]]'', a genus of rorquals that includes the blue whale.<ref name=":4" /><ref name=":5" /> Delphinidae, ocean dolphins, also arose during this radiation event in the late Miocene.<ref name=":5" />

== Ongoing evolution ==

=== Culture ===
[[File:The-Ecological-Conditions-That-Favor-Tool-Use-and-Innovation-in-Wild-Bottlenose-Dolphins-(Tursiops-pone.0022243.s002.ogv|thumb| Researchers pushed a pole with a conical sponge attached along the substrate to simulate sponging behavior by dolphins. They videotaped this experiment to learn what prey was available on the seafloor and why a sponge would be beneficial to foraging rather than [[Animal echolocation|echolocation]].<ref name="Patterson 2011">{{cite journal|last1=Patterson|first1=E.M.|last2=Mann|first2=J.|title=The ecological conditions that favor tool use and innovation in wild bottlenose dolphins (Tursiops sp.)|journal=PLOS ONE|date=2011|volume=6|article-number=e22243|doi=10.1371/journal.pone.0022243|pmid=21799801|pmc=3140497|bibcode=2011PLoSO...622243P|doi-access=free}}</ref>]]

Culture is group-specific behavior transferred by social learning. Tool use to aid with foraging is one example. Whether or not a dolphin uses a tool affects its eating behavior, which causes differences in diet. Also, using a tool allows a new niche and new prey to open up for that particular dolphin. Due to these differences, fitness levels change within the dolphins of a population, which further causes evolution to occur in the long run.<ref name="Cantor, M. 2013">{{cite journal|last1=Cantor|first1=M.|last2=Whitehead|first2=H.|year=2013|title=The interplay between social networks and culture: theoretically and among whales and dolphins|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=368|issue=1618|article-number=20120340|doi=10.1098/rstb.2012.0340|pmid=23569288|pmc=3638443}}</ref> Culture and social networks have played a large role in the evolution of modern cetaceans, as concluded in studies showing dolphins preferring mates with the same socially learned behaviors, and humpback whales using songs between breeding areas. For dolphins particularly, the largest non-genetic effects on their evolution are due to culture and social structure.<ref>{{cite journal | last1 = Cantor | first1 = M. | last2 = Whitehead | first2 = H. | year = 2013 | title = The interplay between social networks and culture: theoretically and among whales and dolphins | journal = Philosophical Transactions of the Royal Society B: Biological Sciences | volume = 368 | issue = 1618| article-number = 20120340 | doi = 10.1098/rstb.2012.0340 | pmid=23569288 | pmc=3638443}}</ref>

Based on a 2014 study, the population of [[Indo-Pacific bottlenose dolphin]]s (''Tursiops'' sp.) around Shark Bay of Western Australia can be divided into spongers and nonspongers. Spongers put [[sea sponge]]s on their snouts as protection against abrasions from sharp objects, stingray barbs, or toxic organisms. The sponges also help the dolphins target fish without swim bladders, since echolocation cannot detect these fish easily against a complex background. Spongers also specifically forage in deep channels, but nonspongers are found foraging in both deep and shallow channels.<ref name="Krutzen, M. 2014">{{cite journal|last1=Krutzen|first1=M.|last2=Kreicker |first2=S.|last3=D. Macleod|first3=C.|last4=Learmonth|first4=J.|last5=M. Kopps|first5=A.|last6=Walsham|first6=P.|last7=J. Allen|first7=S.|year=2014|title=Cultural transmission of tool use by Indo-Pacific bottlenose dolphins (''Tursiops'' sp.) provides access to a novel foraging niche|journal= Proceedings of the Royal Society B: Biological Sciences|volume=281|issue=1784|article-number=20140374|doi=10.1098/rspb.2014.0374|pmid=24759862|pmc=4043097}}</ref> This foraging behavior is mainly passed on from mother to child. Therefore, since this is a group behavior being passed down by social learning, this tool use is considered a cultural trait.<ref name="Cantor, M. 2013"/>

Researchers in a 2014 study in Shark Bay found the fatty acid analyses between the West and East Gulf populations to differ, which is due to the two areas having different food sources. However, when comparing data from within the West Gulf, the spongers vs. the nonspongers in the deep channels had very different fatty acid results even though they are in the same habitat. Nonspongers from deep and shallow channels had similar data. This suggests that sponging was the cause of the different data and not the deep vs. shallow channels. Sponging opened up a new niche for the dolphins and allowed them access to new prey, which caused long-term dietary changes. By producing different food sources within a population, there is less intrapopulation competition for resources, showing [[character displacement]]. As a result, the carrying capacity increases since the entire population does not depend on one food source. The fitness levels within the population also change, thus allowing this culture to evolve.<ref name="Krutzen, M. 2014"/>

==== Social structure ====

Social structure forms groups with individuals that interact with one another, and this allows for cultural traits to emerge, exchange, and evolve. This relationship is especially seen in the bottlenose dolphin populations in southwestern Australia, which have been known to beg for food from fishermen. This begging behavior was spread through the population due to individual (dolphins spending time around boats) and social (dolphins spending time with other dolphins who express begging behavior) learning.<ref name="Cantor, M. 2013"/>

Culture can, however, impact social structure by causing behavior matching and assortive mating. Individuals within a certain culture are more likely to mate with individuals using the same behaviors rather than a random individual, thus influencing social groups and structure. For example, the spongers of Shark Bay preferentially stick with other spongers.<ref name="Cantor, M. 2013"/> Also, some bottlenose dolphins in [[Moreton Bay]], Australia followed [[prawn]] [[fishing trawler|trawlers]] to feed on their debris, while other dolphins in the same population did not. The dolphins preferentially associated with individuals with same behavior even though they all lived in the same habitat. Later on, prawn trawlers were no longer present, and the dolphins integrated into one social network after a couple of years.<ref name="Cantor, M. 2013"/>

Social networks can still affect and cause evolution on their own by impending fitness differences on individuals.<ref name=Frere/> According to a 2012 study, male calves had a lower survival rate if they had stronger bonds with juvenile males. However, when other age and sex classes were tested, their survival rate did not significantly change.<ref name="Stanton, M. A. 2012">{{cite journal|last1=Stanton|first1=M. A.|last2=Mann|first2=J.|year=2012|title=Early Social Networks Predict Survival in Wild Bottlenose Dolphins|journal=PLOS ONE|volume=7|issue=10|article-number=e47508|doi=10.1371/journal.pone.0047508|pmid=23077627|bibcode=2012PLoSO...747508S|pmc=3471847|doi-access=free}}</ref> This suggests that juvenile males impose a social stress on their younger counterparts. In fact, it has been documented that juvenile males commonly perform acts of aggression, dominance, and intimidation against the male calves.<ref name="Stanton, M. A. 2012"/> According to a 2010 study, certain populations of Shark Bay dolphins had varying levels of fitness and calf success. This is either due to social learning (whether or not the mother passed on her knowledge of reproductive ability to the calves), or due to the strong association between mother dolphins in the population; by sticking in a group, an individual mother does not need to be as vigilant all the time for predators.<ref name=Frere>{{cite journal|last1=Frere|first1=C. H.|last2=Krutzen|first2=M.|last3=Mann|first3=J.|last4=Connor|first4=R. C.|last5=Bejder|first5=L.|last6=Sherwin|first6=W. B.|year=2010|title=Social and genetic interactions drive fitness variation in a free-living dolphin population|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=107|issue=46|pages=19,949–19,954|doi=10.1073/pnas.1007997107|bibcode=2010PNAS..10719949F|pmid=21041638|pmc=2993384|doi-access=free}}</ref>

Genetic studies conducted on [[Clymene dolphin]]s (''Stenella clymene'') focused on their natural histories, and the results show that the origin of the species was actually an outcome of [[hybrid speciation]].<ref>{{Cite journal | last1 = Amaral | first1 = A. R. | last2 = Lovewell | first2 = G. | last3 = Coelho | first3 = M. M. | last4 = Amato | first4 = G. | last5 = Rosenbaum | first5 = H. C. | editor1-last = Johnson | editor1-first = Norman | title = Hybrid Speciation in a Marine Mammal: The Clymene Dolphin (''Stenella clymene'') | doi = 10.1371/journal.pone.0083645 | journal = PLOS ONE| volume = 9 | issue = 1 | article-number = e83645 | year = 2014 | pmid =  24421898| pmc = 3885441| bibcode = 2014PLoSO...983645A | doi-access = free }}</ref><ref>{{cite web|url=http://news.nationalgeographic.com/news/2014/01/140111-hybrid-dolphin-species-ocean-animal-science/|archive-url=https://web.archive.org/web/20140112030121/http://news.nationalgeographic.com/news/2014/01/140111-hybrid-dolphin-species-ocean-animal-science/|archive-date=12 January 2014|last=Choi|first=Charles|year=2014|title=DNA Discovery Reveals Surprising Dolphin Origins|access-date=2 January 2016}}</ref> Hybridization between [[spinner dolphin]]s (''Stenella longirostris'') and [[striped dolphin]]s (''Stenella coeruleoalba'') in the North Atlantic was caused by constant habitat sharing of the two species. Relationships between these three species had been speculated according to notable resemblances between anatomies of the Clymene and the spinner dolphins, resulting in the former being regarded as subspecies of the latter until 1981,<ref>{{cite journal|jstor=1380405|first1=W. F.|last1=Perrin|first2=E. D.|last2=Mitchell|first3=J. G.|last3=Mead|first4=D. K.|last4=Caldwell|first5=P. J. H.|last5=van Bree|year=1981|title=''Stenella clymene'', a Rediscovered Tropical Dolphin of the Atlantic|journal=Journal of Mammalogy|volume=62|number=3|pages=583–598|doi=10.2307/1380405}}
</ref> and the possibility of the Clymene dolphin as a hybrid between the spinner and the striped dolphins have come to question based on anatomical and behavioral similarities between these two species.<ref>{{cite web|url= http://news.nationalgeographic.com/news/2014/01/140111-hybrid-dolphin-species-ocean-animal-science/|archive-url= https://web.archive.org/web/20140112030121/http://news.nationalgeographic.com/news/2014/01/140111-hybrid-dolphin-species-ocean-animal-science/|archive-date= 12 January 2014|first=Charles|last=Q. Choi|year=2014|title=DNA Discovery Reveals Surprising Dolphin Origins|work=National Geographic|access-date=20 January 2016}}</ref>

=== Environmental factors ===

Genome sequences done in 2013 revealed that the [[Yangtze river dolphin]], or "baiji" (''Lipotes vexillifer''), lacks [[single nucleotide polymorphism]]s in its genome. After reconstructing the history of the baiji genome for this dolphin species, researchers found that the major decrease in genetic diversity occurred most likely due to a [[population bottleneck|bottleneck]] event during the last [[deglaciation]] event. During this time period, sea levels were rising while global temperatures were increasing. Other historical climate events can be correlated and matched with the genome history of the Yangtze river dolphin as well. This shows how global and local [[climate change]] can drastically affect a genome, leading to changes in fitness, survival, and evolution of a species.<ref>{{cite journal|last1=Zhou|first1=X.|last2=Sun|first2=F.|last3=Xu|first3=S.|last4=Fan|first4=G.|last5=Zhu|first5=K.|last6=Liu|first6=X.|last7=Yang |first7=G.|year=2013 |title=Baiji genomes reveal low genetic variability and new insights into secondary aquatic adaptations|journal=Nature Communications|volume=4|page=2708|doi=10.1038/ncomms3708|pmid=24169659|pmc=3826649|bibcode=2013NatCo...4.2708Z}}</ref>

The European population of [[common dolphin]]s (''Delphinus delphis'') in the Mediterranean have differentiated into two types: eastern and western. According to a 2012 study, this seems to be due to a recent bottleneck as well, which drastically decreased the size of the eastern Mediterranean population. Also, the lack of population structure between the western and eastern regions seems contradictory of the distinct population structures between other regions of dolphins.<ref>{{cite journal|last1=Moura|first1=A. E.|last2=Natoli|first2=A.|last3=Rogan|first3=E.|last4=Hoelzel|first4=A. R.|year=2012|title=Atypical panmixia in a European dolphin species (''Delphinus delphis''): implications for the evolution of diversity across oceanic boundaries|journal=Journal of Evolutionary Biology|volume=26|issue=1|pages=63–75|doi=10.1111/jeb.12032|pmid=23205921|s2cid=7205948|doi-access=free}}</ref> Even though the dolphins in the Mediterranean area had no physical barrier between their regions, they still differentiated into two types due to ecology and biology. Therefore, the differences between the eastern and western dolphins most likely stems from highly specialized niche choice rather than just physical barriers. Through this, environment plays a large role in the differentiation and evolution of this dolphin species.<ref>{{cite iucn |author=Bearzi, G. |author2=Fortuna, C. |author3=Reeves, R. |date=2012 |title=''Tursiops truncatus Mediterranean subpopulation'' |volume=2012 |article-number=e.T16369383A16369386 |doi=10.2305/IUCN.UK.2012-1.RLTS.T16369383A16369386.en |access-date=19 November 2021}}</ref>

The divergence and speciation within bottlenose dolphins has been largely due to climate and environmental changes over history. According to research, the divisions within the genus correlate with periods of rapid climate change. For example, the changing temperatures could cause the coast landscape to change, niches to empty up, and opportunities for separation to appear.<ref>{{cite journal|last1=Moura|first1=A. E.|last2=Nielsen|first2=S. C. A.|last3=Vilstrup|first3=J. T.|last4=Moreno-Mayar|first4=J. V.|last5=Gilbert|first5=M. T. P.|last6=Gray|first6=H. W. I.|last7=Hoelzel|first7=A. R.|year=2013|title=Recent Diversification of a Marine Genus (''Tursiops'' spp.) Tracks Habitat Preference and Environmental Change|journal=Systematic Biology|volume=62|issue=6|pages=865–877|doi=10.1093/sysbio/syt051|pmid=23929779|doi-access=free}}</ref> In the Northeast Atlantic, specifically, genetic evidence suggests that the bottlenose dolphins have differentiated into coastal and pelagic types. Divergence seems most likely due to a founding event where a large group separated. Following this event, the separate groups adapted accordingly and formed their own niche specializations and social structures. These differences caused the two groups to diverge and to remain separated.<ref>{{cite journal|last1=Louis|first1=M.|last2=Viricel|first2=A.|last3=Lucas|first3=T.|last4=Peltier|first4=H.|last5=Alfonsi|first5=E.|last6=Berrow|first6=S.|last7=Simon-Bouhet|first7=B.|year=2014|title=Habitat-driven population structure of bottlenose dolphins, ''Tursiops truncatus'', in the North-East Atlantic|journal=Molecular Ecology|volume=23|issue=4|pages=857–874|doi=10.1111/mec.12653|pmid=24383934|bibcode=2014MolEc..23..857L |hdl=10261/92313|s2cid=22266293|hdl-access=free}}</ref>

Two endemic, distinctive types of [[short-finned pilot whale]], ''Tappanaga'' (or ''Shiogondou'') the larger, northern type and ''Magondou'' the smaller, southern type, can be found along the Japanese archipelago where distributions of these two types mostly do not overlap by the [[Front (oceanography)|oceanic front]] border around the [[Chōshi, Chiba|easternmost point of Honshu]]. It is thought that the local extinction of long-finned pilot whales in the [[North Pacific]] in the 12th century could have triggered the appearance of ''Tappanaga'', causing short-finned pilot whales to colonize the colder ranges of the long-finned variant.<ref>{{cite iucn|last1=L. Taylor|first1=B.|last2=Baird|first2=R.|last3=Barlow|first3=J.|last4=M. Dawson|first4=S.|last5=Ford|first5=J.|last6=G. Mead|first6=J.|last7=Notarbartolo di Sciara|first7=G.|last8=Wade|first8=P. |last9=L. Pitman|first9=R.|year=2011|title=''Globicephala macrorhynchus''|doi=10.2305/IUCN.UK.2011-2.RLTS.T9249A12972356.en}}</ref><ref>{{cite journal|author=Amano M.|title=みちのくの海のイルカたち(特集 みちのくの海と水族館の海棲哺乳類)  |url=http://isanakai.web.fc2.com/journal/ |journal=Isana 56|pages=60–65 |year=2012 |access-date=9 March 2017 }}</ref> Whales with similar characteristics to the ''Tappanaga'' can be found along [[Vancouver Island]] and northern US coasts as well.<ref>[[:ja:日高敏隆|Hidaka T.]] [[:ja:粕谷俊雄|Kasuya T.]] [[:ja:伊沢紘生|Izawa K.]] [[:ja:川道武男|Kawamichi T.]] 1996. The encyclopaedia of animals in Japan (2) – Mammals 2.
{{ISBN|9784582545524}} (9784582545517) (4582545521). [[:ja:平凡社|Heibonsha]]</ref>

==See also==
{{Portal|Mammals|Cetaceans|Evolutionary biology|Paleontology}}
* [[Aquatic adaptation]]
* [[Evolution of mammals]]
* [[Evolution of sirenians]]
* [[List of extinct cetaceans]]
* [[Transitional form]]

==References==
{{Reflist}}

==External links==
For a review of whale evolution, see {{Cite journal| last1 = Uhen | first1 = M. D.| title = The Origin(s) of Whales| journal = Annual Review of Earth and Planetary Sciences| volume = 38| issue = 1| pages = 189–219| year = 2010| doi = 10.1146/annurev-earth-040809-152453|bibcode = 2010AREPS..38..189U }}
* [https://web.archive.org/web/20130506135816/http://ocean.si.edu/ocean-over-time/did-whale-evolution-go-backwards Timeline of Whale Evolution ]- Smithsonian Ocean Portal
* [https://web.archive.org/web/20110822063925/http://palaeo.gly.bris.ac.uk/Palaeofiles/whales/archaeoceti.htm Cetacean Paleobiology]  – University of Bristol
* [https://news.bbc.co.uk/2/hi/science/nature/1974869.stm BBC: Whale's evolution]
* [https://h2g2.com/edited_entry/A1283186 BBC: Whale Evolution – The Fossil Evidence]
* [http://www.stephenjaygould.org/ctrl/gould_leviathan.html Hooking Leviathan by Its Past] by [[Stephen Jay Gould]]
* [http://www-personal.umich.edu/~gingeric/PDGwhales/Whales.htm Research on the Origin and Early Evolution of Whales (Cetacea)], Gingerich, P.D., University of Michigan
* [http://deepblue.lib.umich.edu/handle/2027.42/48501 Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan)]. Gingerich, P.D., 1981, Museum of Paleontology, The University of Michigan
* [http://www.nature.com/nature/journal/v413/n6853/abs/413277a0.html Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls], Nature 413, 277–281 (20 September 2001), J. G. M. [[Hans Thewissen|Thewissen]], E. M. Williams, L. J. Roe and S. T. Hussain
* [http://collections.tepapa.govt.nz/exhibitions/whales/Segment.aspx?irn=161 Evolution of Whales segment from the Whales Tohorā Exhibition Minisite of the Museum of New Zealand Te Papa Tongarewa]

{{Evolution|state=expanded}}
{{fins, limbs and wings}}

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[[Category:Cetaceans]]
[[Category:Evolution of mammals|Cetaceans]]
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