Editing Cetacea (section)

== Evolution ==

{{main|Evolution of cetaceans}}

=== Fossil history ===

==== Origins ====

The direct ancestors of today's cetaceans are probably found within the [[Dorudontidae]] whose most famous member, ''[[Dorudon]]'', lived at the same time as ''[[Basilosaurus]]''. Both groups had already developed some of the typical anatomical features of today's whales, such as the fixed bulla, which replaces the mammalian [[eardrum]], as well as sound-conducting elements for submerged directional hearing. Their wrists were stiffened and probably contributed to the typical build of flippers. The hind legs existed, however, but were significantly reduced in size and with a vestigial pelvis connection.<ref name="Thewissen" />

==== Transition from land to sea ====

[[File:Adult female and fetal Maiacetus.jpg|thumb|Fossil of a ''Maiacetus'' (red, beige skull) with fetus (blue, red teeth) shortly before the end of gestation <ref name="Gingerich PD">{{cite journal|author1=Gingerich PD|author2=ul-Haq M|author3=von Koenigswald W|author4=WJ Sanders|author5=Smith BH|year=2009|title=New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and sexual dimorphism|journal=PLOS ONE|doi= 10.1371/journal.pone.0004366|bibcode = 2009PLoSO...4.4366G|volume=4|issue=2|pages=e4366|pmid=19194487|pmc=2629576|doi-access=free}}</ref>]]

The fossil record traces the gradual transition from terrestrial to aquatic life. The regression of the hind limbs allowed greater flexibility of the spine. This made it possible for whales to move around with the vertical tail hitting the water. The front legs transformed into flippers, costing them their mobility on land.<ref>{{cite web |url=https://evolution.berkeley.edu/what-are-evograms/the-evolution-of-whales/ |title=The Evolution of Whales |publisher=Understanding Evolution, University of California Museum of Paleontology |accessdate=21 January 2025}}</ref>

One of the oldest members of ancient cetaceans ([[Archaeoceti]]) is ''[[Pakicetus]]'' from the Middle Eocene of [[Pakistan]]. This is an animal the size of a wolf, whose skeleton is known only partially. It had functioning legs and lived near the shore. This suggests the animal could still move on land. The long snout had [[Carnivore|carnivorous]] dentition.<ref name="Thewissen"/>

The transition from land to sea dates to about 49 million years ago, with the ''[[Ambulocetus]]'' ("running whale"), also discovered in Pakistan. It was up to {{convert|3|m|ft|abbr=on}} long. The limbs of this archaeocete were leg-like, but it was already fully aquatic, indicating that a switch to a lifestyle independent from land happened extraordinarily quickly.<ref>{{Cite journal |last1=Ando |first1=Konami |last2=Fujiwara |first2=Shin-ichi |date=2016-07-10 |title=Farewell to life on land - thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals |journal=Journal of Anatomy |volume=229 |issue=6 |pages=768–777 |doi=10.1111/joa.12518 |issn=0021-8782 |pmc=5108153 |pmid=27396988}}</ref> The snout was elongated with overhead nostrils and eyes. The tail was strong and supported movement through water. ''Ambulocetus'' probably lived in mangroves in [[brackish water]] and fed in the [[riparian zone]] as a predator of fish and other vertebrates.<ref>{{cite journal |last=Thewissen |first=Hans |author-link=Hans Thewissen |year=1994 |title=Phylogenetic aspects of Cetacean origins: A morphological perspective |journal=Journal of Mammalian Evolution |pages=157–184 |doi=10.1007/bf01473527 |volume=2 |issue=3 |s2cid=27675176}}</ref>

Dating from about 45 million years ago are species such as ''[[Indocetus]]'', ''[[Kutchicetus]]'', ''[[Rodhocetus]]'' and ''[[Andrewsiphius]]'', all of which were adapted to life in water. The hind limbs of these species were regressed and their body shapes resemble modern whales. [[Protocetidae]] family member ''Rodhocetus'' is considered the first to be fully aquatic. The body was streamlined and delicate with extended hand and foot bones. The merged pelvic [[lumbar spine]] was present, making it possible to support the floating movement of the tail. It was likely a good swimmer, but could probably move only clumsily on land, much like a modern [[Pinniped|seal]].<ref name="Thewissen" />

==== Marine animals ====

Since the late Eocene, about 40 million years ago, cetaceans populated the subtropical oceans and no longer emerged on land. An example is the 18 metre long ''[[Basilosaurus]]'', sometimes called ''Zeuglodon''. The transition from land to water was completed in about 10 million years. The [[Wadi Al-Hitan]] ("Whale Valley") in Egypt contains numerous skeletons of ''Basilosaurus'', as well as other marine vertebrates.<ref>{{cite web |title=First intact fossil of prehistoric whale discovered in Wadi Al-Hitan |url=https://www.iucn.org/content/first-intact-fossil-prehistoric-whale-discovered-wadi-al-hitan |publisher=[[International Union for Conservation of Nature]] |date=9 June 2015}}</ref>

{{anchor|Classification}}
=== External phylogeny ===

[[Molecular biology]], [[immunology]], and fossils show that cetaceans are phylogenetically closely related with the [[even-toed ungulate]]s (Artiodactyla). Whales' direct lineage began in the early [[Eocene]], around 55.8 million years ago, with early artiodactyls.<ref name="Thewissen" /> Most molecular biological evidence suggests that [[Hippopotamidae|hippos]] are the closest living relatives. Common anatomical features include similarities in the [[Morphology (biology)|morphology]] of the posterior [[molars]], and the bony ring on the temporal bone (bulla) and the involucre, a skull feature that was previously associated only with cetaceans.<ref name="Thewissen">{{cite journal |last1=Thewissen|first1=J. G. M. |last2=Cooper |first2=Lisa Noelle|last3=Clementz |first3=Mark T. |last4=Bajpai |first4=Sunil |last5=Tiwari |first5=B. N. |title=Whales originated from aquatic artiodactyls in the Eocene epoch of India |year=2007 |journal=Nature |volume=450 |issue=7173 |pages=1190–4 |url=http://repository.ias.ac.in/4642/1/316.pdf |archive-url=https://web.archive.org/web/20131224104529/http://repository.ias.ac.in/4642/1/316.pdf |archive-date=2013-12-24 |url-status=live |doi=10.1038/nature06343 |pmid=18097400 |bibcode=2007Natur.450.1190T |s2cid=4416444 |author1-link=Hans Thewissen }}</ref> Since the fossil record suggests that the morphologically distinct hippo lineage dates back only about 15 million years, Cetacea and hippos apparently diverged from a common ancestor that was morphologically distinct from either.<ref name="DNA">{{cite journal|title = More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen|author = Gatesy, J.|journal = [[Molecular Biology and Evolution]]|date=1 May 1997| volume = 14|pages = 537–543|pmid = 9159931|issue = 5|doi = 10.1093/oxfordjournals.molbev.a025790|doi-access = free}}</ref><ref name="Cetartiodactyla">{{cite journal |author1=Boisserie, Jean-Renaud |author2=Lihoreau, Fabrice |author3=Brunet, Michel |year= 2005|title= The position of Hippopotamidae within Cetartiodactyla|journal= [[Proceedings of the National Academy of Sciences]] |volume= 102 |issue= 5|pages= 1537–1541|doi= 10.1073/pnas.0409518102|pmid= 15677331 |pmc=547867|bibcode= 2005PNAS..102.1537B|doi-access=free }}</ref><ref name="ScienceNews">{{cite web|title = Scientists find missing link between the dolphin, whale and its closest relative, the hippo|date = 2005-01-25|access-date = 2011-01-08|url = http://www.innovations-report.com/html/reports/life_sciences/report-39309.html|work = Science News Daily}}</ref> The most striking common feature is the [[talus bone|talus]], a bone in the upper ankle. Early cetaceans, [[archaeocetes]], show double castors, which occur only in even-toed ungulates. Corresponding findings are from [[Tethys Sea]] deposits in northern India and Pakistan. The Tethys Sea was a shallow sea between the Asian continent and northward-bound Indian plate.{{citation needed|date=October 2023}}

[[File:Ichthyosaur vs dolphin.svg|thumb|Cetaceans [[convergent evolution|evolved convergently]] as [[Streamlining theory|streamlined]] swimmers with fish and [[Ichthyosaur|aquatic reptiles]]. ]]

Molecular and morphological evidence suggests that [[artiodactyl]]s as traditionally defined are [[paraphyletic]] with respect to cetaceans. Cetaceans are deeply nested within the artiodactyls; the two groups together form a [[clade]], a natural group with a common ancestor, for which the name [[Cetartiodactyla]] is sometimes used. Modern nomenclature divides Artiodactyla (or Cetartiodactyla) into four subordinate taxa: camelids (Tylopoda), pigs and peccaries (Suina), ruminants (Ruminantia), and hippos plus whales (Whippomorpha). The Cetacea's presumed location within [[Artiodactyla]] can be represented in the following [[cladogram]]:<!--why have we got a heap of refs for a single cladogram?--><ref>
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 |last19= Spaulding   |first19= M.         |last20= Velazco |first20= P.M.
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}} (see e.g. Fig S10)
</ref>

{{Clade
   |style=font-size:100%;line-height:100%;width:820px;
   |label1=[[Artiodactyla]]
   |1={{Clade
     |1=[[Tylopoda]] (camels)[[File:Cladogram of Cetacea within Artiodactyla (Camelus bactrianus).png|50 px]]
     |label2=[[Artiofabula]]
     |2={{Clade
           |1=[[Suina]] (pigs)[[File:Recherches pour servir à l'histoire naturelle des mammifères (Pl. 80) (white background).jpg|50 px]]
           |label2=[[Cetruminantia]]
           |2={{Clade
               |label1=[[Ruminantia]] (ruminants)
               |1={{Clade
                  |1=[[Chevrotain|Tragulidae]] (mouse deer)[[File:Tragulus napu - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg|50 px]]
                  |2=[[Pecora]] (horn bearers)[[File:Walia ibex illustration white background.png|50 px]]
                  }}
               |label2=[[Cetancodonta]]/[[Whippomorpha]]
               |sublabel2=54 mya
               |2={{Clade
                  |1=[[Hippopotamidae]] (hippopotamuses)[[File:Voyage en Abyssinie Plate 2 (white background).jpg|50 px]]
                  |2='''Cetacea''' (whales)[[File:Bowhead-Whale1 (16273933365).jpg|50 px]]
                  }}
              }}
           }}
        }}
     }}

=== Internal phylogeny ===

{{Further|List of cetaceans|List of extinct cetaceans}}

Within Cetacea, the two [[parvorder]]s are [[baleen whales]] ([[Mysticeti]]) which owe their name to their baleen, and [[toothed whales]] ([[Odontoceti]]), which have teeth shaped like cones, spades, pegs, or tusks, and can perceive their environment through [[Animal echolocation|biosonar]].{{citation needed|date=October 2024}}

{{Cladogram
|title=<small>Relationship of extinct and extant cetaceans<ref>
{{cite journal
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 |author3=Chang, Joseph      |author4=Buell, Carl 
 |author5=Berta, Annalisa    |author6=Meredith, Robert W. 
 |author7=Springer, Mark S.  |author8=McGowen, Michael R. 
 |year=2012
 |title=A phylogenetic blueprint for a modern whale
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 |doi=10.1016/j.ympev.2012.10.012  |pmid=23103570
 |url=https://www.montclair.edu/profilepages/media/5008/user/Gatesy_et_al._2012_A_phylogenetic_blueprint_for_a_modern_whale.pdf |archive-url=https://web.archive.org/web/20130227091508/https://www.montclair.edu/profilepages/media/5008/user/Gatesy_et_al._2012_A_phylogenetic_blueprint_for_a_modern_whale.pdf |archive-date=2013-02-27 |url-status=live
 |access-date=4 September 2015
}}
</ref></small>: 
|caption={{center|† Extinct taxa}}
|align=left
|cladogram=
 {{Clade
 |style=font-size:85%;line-height:100%;width:580px;
 |label1=
 |1={{Clade
    |1=[[Raoellidae]] †
    |label2='''Cetacea'''
    |2={{Clade
       |1=[[Pakicetidae]] †
       |2={{Clade
          |1=[[Ambulocetidae]] † [[File:Ambulocetus BW.jpg|50 px]]
          |2={{Clade
             |1=[[Remingtonocetidae]] † [[File:Remingtonocetus cropped.png|50 px]]
             |2={{Clade
                |1=''[[Rodhocetus]]'' †
                |2={{Clade
                   |1=''[[Dorudon]]'' †
                   |2={{Clade
                      |1=''[[Basilosaurus]]'' † [[File:Basilosaurus cropped.png|50 px]]
                      |2={{Clade
                         |label1=[[Mysticeti]]
                         |sublabel1=([[baleen whales]], 25 mya)
                         |1={{Clade
                            |1=''[[Janjucetus]]'' †
                            |2={{Clade
                               |label1=[[Balaenidae]]
                               |1=[[Balaenidae|Right whales]] [[File:Balaena mysticetus NOAA.jpg|50 px]]
                               |2={{Clade
                                  |label1=[[Neobalaenidae]]
                                  |1=[[Pygmy right whale]] <span style="{{MirrorH}}">[[File:Caperea marginata 3.jpg|50 px]]</span>
                                  |2={{Clade
                                     |label1=[[Balaenopteridae]]
                                     |1=[[Rorqual]]s [[File:Megaptera novaeangliae NOAA.jpg|50 px]]
                                     |label2=[[Eschrichtiidae]]
                                     |2=[[Gray whale]]s [[File:Eschrichtius robustus NOAA.jpg|50 px]]
                                     }}
                                  }}
                               }}
                            }}
                         |label2=[[Odontoceti]]
                         |sublabel2=([[toothed whales]])
                         |2={{Clade
                            |1={{Clade
                               |label1=[[Kogiidae]]
                               |1=[[Dwarf sperm whale]]s [[File:Kogia sima (transparent background).png|50 px]]
                               |label2=[[Physeteridae]]
                               |2=[[Sperm whale]]s [[File:Physeter macrocephalus NOAA.jpg|50 px]]
                               }}
                            |2={{Clade
                               |label1=[[Platanistidae]]
                               |1=[[South Asian river dolphin]]s [[File:Ganges.png|50 px]]
                               |2={{Clade
                                  |label1=[[Ziphiidae]]
                                  |1=[[Beaked whale]]s [[File:Ziphius cavirostris NOAA.jpg|50 px]]
                                  |2={{Clade
                                     |label1=[[Inioidea]]
                                     |sublabel1=(South American<br/>river dolphins)
                                     |1={{Clade
                                        |label1=[[Lipotidae]]
                                        |1=[[Yangtze river dolphin]] [[File:Lipotes vexillifer.png|50 px]]
                                        |2={{Clade
                                           |1=[[Iniidae]] [[File:Inia geoffrensis.jpg|50 px]]
                                           |2=[[Pontoporiidae]] [[File:Pontoporia blainvillei.jpg|50 px]]
                                           }}
                                        }}
                                     |label2=[[Delphinoidea]]
                                     |2={{Clade
                                        |label1=[[Delphinidae]]
                                        |sublabel1=("true" dolphins)
                                        |1=[[Oceanic dolphins]] [[File:Orcinus orca NOAA 2.jpg|50 px]]
                                        |2={{Clade
                                           |label1=[[Monodontidae]]
                                           |1=[[beluga whale|Belugas]] & [[Narwhal]]s [[File:Delphinapterus leucas NOAA.jpg|50 px]]
                                           |label2=[[Phocoenidae]]
                                           |2=[[Porpoise]]s [[File:Vaquita swimming through the void. (Phocoena Sinus).png|50 px]]
                                           }}
                                        }}
                                     }}
                                  }}
                               }}
                            }}
                         }}
                      }}
                   }}
                }}
             }}
          }}
       }}
    }}
 }}
}}
{{clear}}

The terms ''whale'' and ''dolphin'' are informal:

*[[Mysticeti]]:
:*[[Whale]]s, with four families: [[Balaenidae]] (right and bowhead whales), [[Cetotheriidae]] (pygmy right whales), [[Balaenopteridae]] (rorquals), [[Eschrichtiidae]] (grey whales)
*[[Odontoceti]]:
:*[[Whale]]s: with four families: [[Monodontidae]] (belugas and narwhals), [[Physeteridae]] (sperm whales), [[Kogiidae]] (dwarf and pygmy sperm whales), and [[Ziphiidae]] (beaked whales)
:*[[Dolphin]]s, with five families: [[Delphinidae]] (oceanic dolphins), [[Platanistidae]] ([[South Asian river dolphin]]s), [[Lipotidae]] (old world river dolphins) [[Iniidae]] (new world river dolphins), and [[Pontoporiidae]] ([[La Plata dolphin]]s)
:*[[Porpoise]]s, with one family: [[Phocoenidae]]
The term 'great whales' covers those currently regulated by the [[International Whaling Commission]]:<ref name=iwcjuris>{{cite web |title=Small cetaceans |website=iwc.int |publisher=[[International Whaling Commission]] |url=https://iwc.int/smallcetacean |access-date=2018-04-08}}</ref> the Odontoceti families Physeteridae (sperm whales), Ziphiidae (beaked whales), and Kogiidae (pygmy and dwarf sperm whales); and Mysticeti families Balaenidae (right and bowhead whales), Cetotheriidae (pygmy right whales), Eschrichtiidae (grey whales), as well as part of the family Balaenopteridae (minke, Bryde's, sei, blue and fin; not Eden's and Omura's whales).<ref name=iwcgreat>{{Cite web |title=Lives of Whales |website=iwc.int |publisher=[[International Whaling Commission]] |url=https://iwc.int/lives |access-date=2018-04-08}}</ref>