Editing Cetacea (section)

=== 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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</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]]
                  }}
              }}
           }}
        }}
     }}