Editing Marine mammal (section)

===Evolution===
[[File:Prorastomus BW.jpg|thumb|left|Illustration of †''[[Prorastomus]]'', an early sirenian (40 mya)|alt=A furry, streamlined mammal swimming through the water with toes visible on each foot, similar to those of an elephant. All limbs are thrust backwards or underneath the animal.]]

Marine mammals form a diverse group of 129 species that rely on the ocean for their existence.<ref name="kasc">{{Cite journal | doi = 10.1371/journal.pone.0019653| pmid = 21625431| title = Current and Future Patterns of Global Marine Mammal Biodiversity| journal = PLOS ONE| volume = 6| issue = 5| pages = e19653| year = 2011| last1 = Kaschner | first1 = K. | last2 = Tittensor | first2 = D. P. | last3 = Ready | first3 = J. | last4 = Gerrodette | first4 = T. | last5 = Worm | first5 = B. | bibcode = 2011PLoSO...619653K | pmc=3100303| doi-access = free}}</ref><ref name="pompa">{{Cite journal | last1 = Pompa | first1 = S. | last2 = Ehrlich | first2 = P. R. | last3 = Ceballos | first3 = G. | doi = 10.1073/pnas.1101525108 | title = Global distribution and conservation of marine mammals | journal = Proceedings of the National Academy of Sciences| volume = 108 | issue = 33 | pages = 13600–13605| date = 2011-08-16| pmid =  21808012| pmc = 3158205| bibcode = 2011PNAS..10813600P | doi-access = free }}</ref> They are an informal group unified only by their reliance on marine environments for feeding.<ref name="jeff">{{cite book|last1=Jefferson|first1=T. A.| last2=Webber|first2=M. A.|last3=Pitman|first3=R. L.|year=2009|title=Marine Mammals of the World A Comprehensive Guide to their Identification|location=London|publisher=Academic Press|isbn=978-0-12-383853-7|pages=7–16|oclc=326418543|edition=1st}}</ref> Despite the diversity in anatomy seen between groups, improved foraging efficiency has been the main driver in their [[evolution]].<ref>{{Cite journal | doi = 10.1002/ar.20545|pmid=17516441| title = Evolution of marine mammals: Back to the sea after 300 million years| journal = The Anatomical Record| volume = 290| issue = 6| pages = 514–22| year = 2007| last1 = Uhen | first1 = M. D. | doi-access = free}}{{open access}}</ref><ref name=savage/> The level of dependence on the marine environment varies considerably with species. For example, dolphins and whales are completely dependent on the marine environment for all stages of their life; [[Pinniped|seals]] feed in the ocean but breed on land; and polar bears must feed on land.<ref name=jeff/>

The cetaceans became aquatic around 50&nbsp;million years ago (mya).<ref>{{cite book|url=https://archive.org/stream/Marine_Biology_by_Peter_Castro#page/n211/mode/2up/search/pakicetus|first1=Peter|last1=Castro|first2=Michael E. |last2=Huber|year=2007|title=Marine Biology|edition=7th|publisher=McGraw-Hill|page=192|isbn=978-0-07-302819-4}}</ref> Based on molecular and morphological research, the cetaceans genetically and morphologically fall firmly within the [[Artiodactyla]] (even-toed ungulates).<ref>{{cite journal |title=Phylogenetic Relationships of Extinct Cetartiodactyls: Results of Simultaneous Analyses of Molecular, Morphological, and Stratigraphic Data |journal=Journal of Mammalian Evolution |year=2005 |last1=Geisler |first1=Jonathan H. |last2=Uden |first2=Mark D. |volume=12 |issue=1–2 |pages=145–160 |doi=10.1007/s10914-005-4963-8|s2cid=34683201 }}</ref><ref>{{cite journal |title=Molecular evidence for the inclusion of cetaceans within the order Artiodactyla |journal=Molecular Biology and Evolution |year=1994 |last1=Graur |first1=D. |last2=Higgins |first2=G. |volume=11 |issue=3 |pages=357–364 |url=http://nsmn1.uh.edu/dgraur/ArticlesPDFs/MBE_Artiodactyla.pdf |pmid=8015431|doi=10.1093/oxfordjournals.molbev.a040118 |doi-access=free }}{{open access}}</ref> The term "Cetartiodactyla" reflects the idea that whales evolved within the ungulates. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. Under this definition, the closest living land relative of the whales and dolphins is thought to be the [[hippopotamus]]es.<ref>{{cite journal |title=The phylogeny of Cetartiodactyla: the importance of dense taxon sampling, missing data, and the remarkable promise of cytochrome b to provide reliable species-level phylogenies |journal= Molecular Phylogenetics and Evolution |year=2008 |last1=Agnarsson |first1=I. |last2=May-Collado |first2=LJ. |volume=48 |issue=3 |pages=964–985  |pmid=18590827 |doi=10.1016/j.ympev.2008.05.046}}{{open access}}</ref><ref>{{cite journal |title=A complete phylogeny of the whales, dolphins and even-toed hoofed mammals – Cetartiodactyla |journal=Biological Reviews of the Cambridge Philosophical Society|year=2005 |last1=Price |first1=SA. |last2=Bininda-Emonds |first2=OR. |last3=Gittleman |first3=JL. |volume=80 |issue=3 |pages=445–473  |pmid=16094808 |doi=10.1017/s1464793105006743|s2cid=45056197}}{{open access}}</ref><ref>{{cite journal |title=Phylogenetic relationships of artiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S RNA mitochondrial sequences |journal=Molecular Biology and Evolution |year=1997 |last1=Montgelard |first1=C. |last2=Catzeflis |first2=FM. |last3=Douzery |first3=E. |volume=14 |issue=5 |pages=550–559 |pmid=9159933 |doi=10.1093/oxfordjournals.molbev.a025792|doi-access=free }}{{open access}}</ref><ref>{{cite journal |title=Relationships of Cetacea -Artiodactyla- Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution |journal=PLOS ONE |volume=4 |issue=9 |pages=e7062 |year=2009 |last1= Spaulding |first1=M. |last2=O'Leary |first2=MA. |last3=Gatesy |first3=J. |doi=10.1371/journal.pone.0007062 |pmid=19774069 |bibcode=2009PLoSO...4.7062S |pmc=2740860|doi-access=free }}{{open access}}</ref>

Sirenians, the sea cows, became aquatic around 40&nbsp;million years ago. The first appearance of sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian, †''[[Prorastomus]]'', was found in Jamaica,<ref name=savage>{{cite journal|first1=R. J. G.|last1=Savage|first2=Daryl P.|last2=Domning|first3=J. G. M.|last3=Thewissen|year=1994|title=Fossil Sirenia of the West Atlantic and Caribbean Region. V. the Most Primitive Known Sirenian, ''Prorastomus sirenoides'' Owen, 1855|jstor=4523580|journal=Journal of Vertebrate Paleontology|volume=14|number=3| pages=427–449|doi=10.1080/02724634.1994.10011569|bibcode=1994JVPal..14..427S }}</ref> unlike other marine mammals which originated from the [[Old World]] (such as cetaceans<ref>{{cite journal|first1=J. G. M.|last1=Thewissen|first2=Sunil|last2=Bajpai|year=2001|title=Whale Origins as a Poster Child for Macroevolution|journal=BioScience|volume=51| issue=12|pages=1037–1049|doi=10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2|doi-access=free}}{{open access}}</ref>). The first known quadrupedal sirenian was †''[[Pezosiren]]'' from the early middle Eocene.<ref>{{cite journal|last=Domning DP|journal=Nature|title=The Earliest Known Fully Quadrupedal Sirenian|volume=413|issue=6856 |pages=625–627|date=2001|doi=10.1038/35098072|pmid=11675784|bibcode=2001Natur.413..625D|s2cid=22005691}}</ref> The earliest known sea cows, of the families †[[Prorastomidae]] and †[[Protosirenidae]], were both confined to the Eocene, and were pig-sized, four-legged, amphibious creatures.<ref name=prins>{{cite book|chapter-url={{Google books| plainurl=yes|id=EZB2AgAAQBAJ|page=123}}|editor1-first=Herbert H. T.|editor1-last=Prins|editor2-first=Iain J.|editor2-last=Gordon|year=2014|title=Invasion Biology and Ecological Theory|publisher=Cambridge University Press|location=Cambridge|page=123|isbn=978-1-107-03581-2|chapter=The Biological Invasion of Sirenia into Australasia|oclc=850909221}}</ref> The first members of [[Dugongidae]] appeared by the middle Eocene.<ref>{{cite journal |last=Samonds |first=K. E. |author2=Zalmout, I. S. |author3=Irwin, M. T. |author4=Krause, D. W. |author5=Rogers, R. R. |author6= Raharivony, L. L.  |year=2009 |title=''Eotheroides lambondrano'', new Middle Eocene seacow (Mammalia, Sirenia) from the Mahajanga Basin, Northwestern Madagascar |journal=Journal of Vertebrate Paleontology |volume=29 |issue=4 |pages=1233–1243 |doi=10.1671/039.029.0417|bibcode=2009JVPal..29.1233S |s2cid=59466434 }}</ref> At this point, sea cows were fully aquatic.<ref name=prins/>

Pinnipeds [[Cladogenesis|split]] from other caniforms 50 mya during the [[Eocene]]. Their evolutionary link to terrestrial mammals was unknown until the 2007 discovery of †''[[Puijila darwini]]'' in [[early Miocene]] deposits in [[Nunavut]], Canada. Like a modern otter, †''Puijila'' had a long tail, short limbs and webbed feet instead of flippers.<ref name="Dawson2009">{{Cite journal |author1=Rybczynski, N. |author2=Dawson, M. R. |author3=Tedford, R. H. |doi=10.1038/nature07985 |title=A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia |journal=Nature |volume=458 |issue=7241 |pages=1021–24 |year=2009 |pmid=19396145|bibcode=2009Natur.458.1021R |s2cid=4371413 }}</ref> The lineages of [[Otariidae]] (eared seals) and [[Odobenidae]] (walrus) split almost 28 mya.<ref name="Arnason2006">{{cite journal|last1=Arnason|first1=U.|last2=Gullberg|first2=A.|last3=Janke|first3=A.|last4=Kullberg|first4=M.|last5=Lehman|first5=N.|last6=Petrov|first6=E. A.|last7=Väinölä|first7=R.|year=2006|title=Pinniped phylogeny and a new hypothesis for their origin and dispersal|journal=Molecular Phylogenetics and Evolution|volume=41|issue=2|pages=345–354|doi=10.1016/j.ympev.2006.05.022|pmid=16815048}}</ref> [[Earless seal|Phocids]] (earless seals) are known to have existed for at least 15 mya,<ref>* {{cite book |last1=Perrin |first1=W. F. |last2=Wursig |first2=B. |last3=Thewissen |first3=J. G. M. |year=2009 |url={{Google books|plainurl=yes |id=2rkHQpToi9sC|page=front}} |title=Encyclopedia of Marine Mammals |edition=2nd |publisher=Academic Press |isbn=978-0-0809-1993-5 |oclc=316226747 |location=San Diego |ref={{harvid|Perrin|2009}} |pages=861–866}}</ref> and molecular evidence supports a divergence of the [[Monk seal|Monachinae]] (monk seals) and Phocinae lineages 22 mya.<ref name="Arnason2006"/>

Fossil evidence indicates the sea otter (''Enhydra'') lineage became isolated in the North Pacific approximately two mya, giving rise to the now-extinct †''[[Enhydra macrodonta]]'' and the modern sea otter, ''Enhydra lutris''. The sea otter evolved initially in northern [[Hokkaidō]] and Russia, and then spread east to the [[Aleutian Islands]], mainland [[Alaska]], and down the North American coast. In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50, 40, and 20 mya, respectively, the sea otter is a relative newcomer to marine life. In some respects though, the sea otter is more fully adapted to water than pinnipeds, which must haul out on land or ice to give birth.<ref name=fulcrum>{{cite book |title= Sea Otters|last= Love|first=John A. |year= 1992|publisher=Fulcrum Publishing|location=Golden, Colorado|pages=4–16|isbn=978-1-55591-123-2|oclc=25747993}}</ref>

Polar bears are thought to have diverged from a population of [[brown bear]]s, ''Ursus arctos'', that became isolated during a period of glaciation in the [[Pleistocene]]<ref name="DeMaster1981">{{cite journal |last1=DeMaster |first1=Douglas P. |last2=Stirling |first2=Ian |author2-link=Ian Stirling (biologist) |date=8 May 1981 |title=Ursus Maritimus|jstor=3503828 |journal=Mammalian Species |pages=1–7 |doi=10.2307/3503828|issue=145 |doi-access=free }}</ref> or from the eastern part of [[Siberia]], (from [[Kamchatka]] and the Kolym Peninsula).<ref name="Refere improving foraging efficiencynceC">{{cite journal|last1=Kurtén|first1=B|title=The evolution of the polar bear, ''Ursus maritimus'' Phipps|journal=Acta Zoologica Fennica|date=1964|volume=108|pages=1–30|url=https://helda.helsinki.fi/handle/10138/37762}}</ref> The oldest known polar bear fossil is a 130,000-to-110,000-year-old jaw bone, found on [[Prince Charles Foreland]] in 2004.<ref name="Lindqvist">{{cite journal |doi=10.1073/pnas.0914266107|pmid=20194737|pmc=2841953 |title=Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=11 |pages=5053–57 |year=2010 |last1=Lindqvist |first1=C. |last2=Schuster |first2=S. C. |last3=Sun |first3=Y. |last4=Talbot |first4=S. L. |last5=Qi |first5=J. |last6=Ratan |first6=A. |last7=Tomsho |first7=L. P. |last8=Kasson |first8=L. |last9=Zeyl |first9=E. |last10=Aars |first10=J. |last11=Miller |first11=W. |last12=Ingolfsson |first12=O. |last13=Bachmann |first13=L. |last14=Wiig |first14=O. |bibcode=2010PNAS..107.5053L|doi-access=free}}</ref> The [[mitochondrial DNA]] (mtDNA) of the polar bear diverged from the brown bear roughly 150,000 years ago.<ref name="Lindqvist"/> Further, some [[clade]]s of brown bear, as assessed by their mtDNA, are more closely related to polar bears than to other brown bears,<ref name="Waits">{{cite journal |doi=10.1111/j.1523-1739.1998.96351.x|jstor=2387511 |title=Mitochondrial DNA Phylogeography of the North American Brown Bear and Implications for Conservation |journal=Conservation Biology |volume=12 |issue=2 |pages=408–417 |year=2008 |last1=Waits |first1=L. P. |last2=Talbot |first2=S. L. |last3=Ward |first3=R. H. |last4=Shields |first4=G. F.|s2cid=86172292 }}</ref> meaning that the polar bear might not be considered a species under some [[species concepts]].<ref>{{cite journal |last=Marris |first=E. |year=2007 |journal=Nature |title=Linnaeus at 300: The species and the specious |volume=446 |issue=7133 |pages=250–253 |doi=10.1038/446250a |pmid=17361153 |bibcode=2007Natur.446..250M |s2cid=4420048 |doi-access=free }}</ref>

In general, terrestrial [[amniote]] invasions of the sea have become more frequent in the Cenozoic than they were in the Mesozoic. Factors contributing to this trend include the increasing productivity of near-shore marine environments, and the role of endothermy in facilitating this transition.<ref name="Vermeij2018">{{cite journal|last1= Vermeij|first1=G. J.|last2= Motani|first2= R.|title= Land to sea transitions in vertebrates: the dynamics of colonization|journal= Paleobiology|volume=44|issue=2|year= 2018|pages= 237–250|doi= 10.1017/pab.2017.37|bibcode=2018Pbio...44..237V |s2cid=91116726}}</ref>