Editing Placentalia

{{Short description|Infraclass of mammals in the clade Eutheria}}
{{Redirect|Placental|the organ interfacing between a placental mammalian mother and a fetus|Placenta}}
{{Automatic taxobox
| name = Placentals
| fossil_range = [[Maastrichtian]]-[[present]] {{fossilrange|66.3|earliest=90|0|refs=<ref name=Archibald2011>{{cite journal |first1=J. David |last1=Archibald |first2=Yue |last2=Zhang |first3=Tony |last3=Harper |first4=Richard L. |last4=Cifelli |date=May 6, 2011 |title=''Protungulatum'', confirmed Cretaceous occurrence of an otherwise Paleocene eutherian (placental?) mammal |journal=Journal of Mammalian Evolution |doi=10.1007/s10914-011-9162-1 |url=http://www.bio.sdsu.edu/faculty/archibald.html/ArchibaldEtAl.11JMEonline.pdf |access-date=April 28, 2013 |volume=18 |issue=3 |pages=153–161 |s2cid=16724836 |archive-url=https://web.archive.org/web/20160303221519/http://www.bio.sdsu.edu/faculty/archibald.html/ArchibaldEtAl.11JMEonline.pdf |archive-date=March 3, 2016 |url-status=dead}}</ref>}} Possible earlier [[Cretaceous]] records
| image = {{center|<imagemap>
File:Placental Mammals 2021 Montage.jpg|300px
rect 0 0 333 232 [[Common vampire bat]]
rect 0 232 333 470 [[Eastern gray squirrel]]
rect 0 696 333 470 [[Plains zebra]]
rect 0 928 333 700 [[Aardvark]]
rect 0 1160 333 930 [[Humpback whale]]
rect 0 1392 333 1160 [[Black and rufous elephant shrew]]

rect 666 0 333 232 [[Human]]
rect 666 232 333 470 [[Ground pangolin]]
rect 666 696 333 470 [[Sunda flying lemur]]
rect 666 928 333 700 [[West Indian manatee]]
rect 666 1160 333 930 [[European hedgehog]]
rect 666 1392 333 1160 [[Nine-banded armadillo]]

rect 999 0 666 232 [[Southern elephant seal]]
rect 999 232 666 470 [[Asian elephant]]
rect 999 696 666 470 [[Reindeer]]
rect 999 928 666 700 [[Giant anteater]]
rect 999 1160 666 930 [[Giant panda]]
rect 999 1392 666 1160 [[American pika]]
</imagemap>}}
| image_caption = Placentals from different orders.
| taxon = Placentalia
| authority = [[Richard Owen|Owen]], 1837
| subdivision_ranks = Subgroups
| subdivision = 
* Magnordo: [[Atlantogenata]]
* Magnordo: [[Boreoeutheria]]
* Order: †[[Palaeoryctidae|Palaeoryctida]]?<ref name="Bertrand&Shelley2022"/>
* Order: †[[Taeniodonta]]?<ref name="Bertrand&Shelley2022"/><ref name="Shelley2022"/><ref group=lower-alpha>Genera ''[[Alveugena]]'', ''[[Ambilestes]]'' and ''[[Procerberus]]'' are the immediate outgroups to order Taeniodonta, with genus ''Alveugena'' classified as a sister taxon to this order.</ref>
* ''Clade'': †[[Procerberus|Procerberinae]]?
* Genus: †''[[Alveugena]]''?
* Genus: †''[[Ambilestes]]''?
| synonyms = {{collapsible list |bullets=true |title=list of synonyms:
|Euplacentalia {{small|(Koenigswald, 2016)}}<ref>Wighart von Koenigswald (2016.) [http://www.rhinoresourcecenter.com/pdf_files/153/1533757716.pdf "Specialized wear facets and late ontogeny in mammalian dentitions"], Historical Biology, 30(1–2), 7–29.</ref>
|Eutheria {{small|(Huxley, 1880)}}<ref>T. H. Huxley (1880.) [https://www.biodiversitylibrary.org/item/90456#page/805/mode/1up "On the application of the laws of evolution to the arrangement of the Vertebrata, and more particularly of the Mammalia."] Proceedings of the Zoological Society, London 43:649-662</ref>
|Monodelphia {{small|(Gill, 1872)}}<ref>Gill, T. (1872.) [https://www.biodiversitylibrary.org/page/1336667#page/11/mode/1up "Arrangement of the Families of Mammals."] Smithsonian Misc. Coll. 11 (230), [i]-vi, 1-98.</ref>
|Placentaria {{small|(Fleming, 1822)}}<ref>Fleming, J. (1822.) [https://www.biodiversitylibrary.org/page/28230157#page/7/mode/1up "The philosophy of zoology: Or a general view of the structure, functions and classification of animals."] Edinburgh Vol. 2: 1-618</ref>
|Placentata {{small|(Turnbull, 1971)}}<ref>Turnbull, W. D. (1971.) "The trinity therians: Their bearing on evolution in marsupials and other therians." In: ed. A. A. Dahlberg [https://www.journals.uchicago.edu/doi/epdf/10.1086/407442 "Dental morphology and evolution"], Chicago: University of Chicago Press, pp. 151–179.</ref>
}}
}}

'''Placental mammals''' ([[infraclass]] '''Placentalia''' {{IPAc-en|p|l|æ|s|ə|n|ˈ|t|eɪ|l|i|ə}}) are one of the three extant subdivisions of the class [[Mammalia]], the other two being [[Monotremata]] and [[Marsupial]]ia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the [[fetus (biology)|fetus]] is carried in the [[uterus]] of its mother to a relatively late stage of development. The name is something of a misnomer, considering that marsupials also nourish their fetuses via a [[placenta]],<ref name="Renfree2010">{{cite journal |author=Renfree, M. B. |title=Review: Marsupials: placental mammals with a difference |journal=Placenta |volume=31 Supplement |pages=S21–6 |date=March 2010 |doi=10.1016/j.placenta.2009.12.023 |pmid=20079531}}</ref> though for a relatively briefer period, giving birth to less-developed young, which are then nurtured for a period inside the mother's [[pouch (marsupial)|pouch]]. Placentalia represents the only living group within [[Eutheria]], which contains all mammals that are more closely related to placentals than they are to marsupials.

== Anatomical features ==
Placental mammals are anatomically distinguished from other mammals by:
* a sufficiently wide opening at the bottom of the [[pelvis]] to allow the birth of a large baby relative to the size of the mother.<ref name="Weil2002MammalianEvolution">{{cite journal |author=Weil, A. |title=Mammalian evolution: Upwards and onwards |journal=Nature |volume=416 |pages=798–799 |date=April 2002 |doi=10.1038/416798a |pmid=11976661 |issue=6883|bibcode=2002Natur.416..798W |s2cid=4332049 }}</ref>
* the absence of [[epipubic bone]]s extending forward from the pelvis, which are found in all other mammals.<ref name=Reilly2003>{{cite journal |author1=Reilly, S. M. |author2=White, T. D. |name-list-style=amp |date=January 2003 |title=Hypaxial Motor Patterns and the Function of Epipubic Bones in Primitive Mammals |journal=Science |volume=299 |issue=5605 |pages=400–402 |doi=10.1126/science.1074905 |pmid=12532019 |bibcode=2003Sci...299..400R |s2cid=41470665 }}</ref> (Their function in non-placental mammals is to stiffen the body during locomotion,<ref name=Reilly2003/> but in placentals they would inhibit the expansion of the abdomen during pregnancy.)<ref>{{cite journal |author=Novacek, M. J., Rougier, G. W, Wible, J. R., McKenna, M. C, Dashzeveg, D. and Horovitz, I. |title=Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia |journal=Nature |volume=389 |pages=483–486 |date=October 1997 |doi=10.1038/39020 |pmid=9333234 |issue=6650|bibcode=1997Natur.389..483N |s2cid=205026882 }}</ref>
* the rearmost bones of the foot fit into a socket formed by the ends of the [[tibia]] and [[fibula]], forming a complete [[mortise and tenon]] upper ankle joint.<ref name="JiLuoYuan2002EarliestEutherian"/>
* the presence of a [[malleolus]] at the bottom of the fibula.<ref name="JiLuoYuan2002EarliestEutherian"/>
* instead of a [[cloaca]]<ref group=lower-alpha>Exceptional adult placentals that retain a cloaca are [[golden moles]], [[tenrec]]s, [[beaver]]s, and some [[shrew]]s.</ref> like monotremes, marsupials and most other [[vertebrate]]s, the [[Genitourinary system|urogenital ducts]] exit through the [[vulva]] or [[penis]] and the [[rectum]] opens as the [[anus]].<ref name="Wake1992">{{cite book|author=Marvalee H. Wake|title=Hyman's Comparative Vertebrate Anatomy|url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA583|access-date=6 May 2013|date=15 September 1992|publisher=University of Chicago Press|isbn=978-0-226-87013-7|page=583}}</ref>
* the presence of a [[corpus callosum]] in between the [[cerebral hemisphere]]s.<ref name="Velut">{{cite journal |last1=Velut |first1=S |last2=Destrieux |first2=C |last3=Kakou |first3=M |title=[Morphologic anatomy of the corpus callosum]. |journal=Neuro-Chirurgie |date=May 1998 |volume=44 |issue=1 Suppl |pages=17–30 |pmid=9757322}}</ref>

== Subdivisions ==
Analysis of molecular data led to rapid changes in assessments of the phylogeny of placental orders at the close of the 20th century. A novel phylogeny and classification of placental orders appeared with Waddell, Hasegawa and Okada in 1999.<ref name="Waddell1999">{{cite journal |last1=Waddell |first1=P. J. |last2=Okada |first2= N. |last3=Hasegawa |first3=M. |year=1999 |title=Towards Resolving the Interordinal Relationships of Placental Mammals|journal=Systematic Biology |volume=48 |issue=1 |pages=1–5|doi=10.1093/sysbio/48.1.1 }}</ref> "Jumping genes"-type [[retroposon]] presence/absence patterns have provided corroboration of phylogenetic relationships inferred from molecular sequences.<ref name=Kriegs2006>{{cite journal |last=Kriegs |first=Jan Ole |author2=Churakov, Gennady |author3=Kiefmann, Martin |author4=Jordan, Ursula |author5=Brosius, Jürgen |author6=Schmitz, Jürgen |title=Retroposed Elements as Archives for the Evolutionary History of Placental Mammals |journal=PLOS Biology |year=2006 |volume=4 |issue=4 |pages=e91 |doi=10.1371/journal.pbio.0040091 |pmid=16515367 |pmc=1395351 |doi-access=free }}</ref> It is now widely accepted that there are three major subdivisions or lineages of placental mammals: [[Boreoeutheria]], [[Xenarthra]], and [[Afrotheria]]. All of these diverged from common ancestors.

2022 studies of Bertrand, O. C. and Sarah L. Shelley have identified [[Palaeoryctidae|palaeoryctids]] and [[Taeniodonta|taeniodonts]] as basal placental mammal clades.<ref name="Bertrand&Shelley2022">{{Cite journal |last1=Bertrand |first1=O. C. |last2=Shelley |first2=S. L. |last3=Williamson |first3=T. E. |last4=Wible |first4=J. R. |last5=Chester |first5=S. G. B. |last6=Flynn |first6=J. J. |last7=Holbrook |first7=L. T. |last8=Lyson |first8=T. R. |last9=Meng |first9=J. |last10=Miller |first10=I. M. |last11=Püschel |first11=H. P. |last12=Smith |first12=T. |last13=Spaulding |first13=M. |last14=Tseng |first14=Z. J. |last15=Brusatte |first15=S. L. |year=2022 |title=Brawn before brains in placental mammals after the end-Cretaceous extinction |journal=Science |volume=376 |issue=6588 |pages=80–85 |doi=10.1126/science.abl5584 |pmid=35357913 |bibcode=2022Sci...376...80B |url=https://www.research.ed.ac.uk/en/publications/d7fb8c6e-886e-4c1d-9977-0cd6406fda20|hdl=20.500.11820/d7fb8c6e-886e-4c1d-9977-0cd6406fda20 |hdl-access=free }}</ref><ref name="Shelley2022">Sarah L. Shelley (2022.) "The phylogeny of Paleocene mammals and the evolution of Placentalia", in ''[https://vertpaleo.org/wp-content/uploads/2022/09/2022-SVP-Program-Abstract-Brochure_Preliminary.pdf "The Society of Vertebrate Paleontology 82nd annual meeting"]''</ref>

The 19 living orders of Placentalia in the three groups are:<ref>{{cite journal |vauthors=Archibald JD, Averianov AO, Ekdale EG |title=Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals |journal=Nature |volume=414 |issue=6859 |pages=62–5 |date=November 2001 |pmid=11689942 |doi=10.1038/35102048|bibcode=2001Natur.414...62A |doi-access=free }}</ref>

* Magnorder [[Atlantogenata]]
** Superorder [[Xenarthra]]
*** Order [[Cingulata]] ([[armadillo]]s)
*** Order [[Pilosa]] ([[sloth]]s and [[anteater]]s)
** Superorder [[Afrotheria]]
*** Grandorder [[Afroinsectiphilia]] 
**** Order [[Tubulidentata]] ([[aardvark]]s)
**** Mirorder [[Afroinsectivora]]
***** Order [[Afrosoricida]] ([[tenrec]]s, [[Potamogalidae|otter shrews]], and [[golden mole]]s)
***** Order [[Macroscelidea]] ([[elephant shrew]]s)
*** Grandorder [[Paenungulata]] 
**** Order [[Hyracoidea]] ([[hyrax]]es)
**** Mirorder [[Tethytheria]]
***** Order [[Proboscidea]] ([[elephant]]s)
***** Order [[Sirenia]] ([[dugong]]s and [[manatee]]s)
* Magnorder [[Boreoeutheria]]
** Superorder [[Euarchontoglires]]
*** Grandorder [[Gliriformes]]
**** Mirorder [[Glires]] 
***** Order [[Lagomorpha]] ([[rabbit]]s, [[hare]]s, and [[pika]]s) 
***** Order [[Rodent]]ia ([[rodent]]s: [[mouse|mice]], [[rat]]s, [[hamster]]s, [[Cavia|guinea pig]]s, [[chinchilla]]s, [[capybara]]s, [[porcupine]]s, [[vole]]s, [[squirrel]]s, [[beaver]]s, etc.)
*** Grandorder [[Euarchonta]] 
**** Order [[Scandentia]] ([[treeshrew]]s) 
**** Mirorder [[Primatomorpha]] 
***** Order [[Dermoptera]] ([[colugo]]s) 
***** Order [[Primates]] (primates: [[monkey]]s, [[ape]]s (including [[human]]s), [[lemur]]s, [[lorisid]]s, [[galago]]s, and [[tarsier]]s) 
** Superorder [[Laurasiatheria]]
*** Order [[Eulipotyphla]] ([[hedgehog]]s, [[gymnure]]s, [[shrew]]s, [[mole (animal)|moles]], and [[solenodon]]s)
*** Order [[Chiroptera]] ([[bat]]s)
*** Grandorder [[Ferungulata]]
**** Mirorder [[Euungulata]]
***** Order [[Artiodactyla]] ([[Artiodactyl|even-toed ungulates]]: [[Bos|cattle]], [[antelope]]s, [[Ovis|sheep]], [[deer]], [[Camelidae|camelids]], [[Sus (genus)|pigs]], [[giraffe]]s, [[cetacean]]s, [[Hippopotamidae|hippopotamuses]], [[Capra (genus)|goats]], [[Bubalina|buffalo]], etc.) 
***** Order [[Perissodactyla]] ([[Perissodactyla|odd-toed ungulates]]: [[Wild horse|horses]], [[Asinus|asses]], [[zebra]]s, [[rhinoceroses]], and [[tapir]]s)
**** Mirorder [[Ferae]]
***** Order [[Pholidota]] ([[pangolin]]s) 
***** Order [[Carnivora]] ([[carnivoran]]s: [[Canidae|dogs]], [[Felidae|cats]], [[bear]]s, [[pinniped]]s, [[mongoose]]s, [[raccoon]]s, [[skunk]]s, [[hyena]]s, [[weasel]]s, etc.)

The exact relationships among these three lineages is currently a subject of debate, and four different hypotheses have been proposed with respect to which group is [[Basal (phylogenetics)|basal]] or diverged first from other placentals. These hypotheses are [[Atlantogenata]] (basal Boreoeutheria), [[Epitheria]] (basal Xenarthra), [[Exafroplacentalia]] (basal Afrotheria) and a hypothesis supporting a near simultaneous divergence.<ref name=Nishiharaetal2009>{{cite journal |last=Nishihara |first=H. |author2=Maruyama, S. |author3=Okada, N. |title=Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals |journal=Proceedings of the National Academy of Sciences |year=2009 |volume=106 |issue=13 |pages=5235–5240 |doi=10.1073/pnas.0809297106 |pmid=19286970 |pmc=2655268|bibcode=2009PNAS..106.5235N |doi-access=free }}</ref> Estimates for the divergence times among these three placental groups mostly range from 105 to 120 million years ago (MYA), depending on the type of DNA, whether it is translated, and the phylogenetic method (e.g. [[nuclear DNA|nuclear]] or [[mitochondrial DNA|mitochondrial]]),<ref name="Waddell2001">{{cite journal |last1=Waddell |first1= P. J. |last2=Kishino |first2=H. |last3=Ota |first3=R. |year=2001 |title= A phylogenetic foundation for comparative mammalian genomics |journal=Genome Informatics Series |volume=12 |pages=141–154|pmid= 11791233 }}</ref><ref name=Springer2003>{{Cite journal |last1=Springer |first1=Mark S. |last2=Murphy |first2=William J. |last3=Eizirik |first3=Eduardo |last4=O'Brien |first4=Stephen J. |title=Placental mammal diversification and the Cretaceous–Tertiary boundary |doi=10.1073/pnas.0334222100 |journal=[[Proceedings of the National Academy of Sciences of the United States of America|Proceedings of the National Academy of Sciences]] |volume=100 |issue=3 |pages=1056–1061 |year=2003 |pmid=12552136 |pmc=298725|bibcode=2003PNAS..100.1056S |doi-access=free }}</ref> and varying interpretations of [[paleogeographic]] data.<ref name=Nishiharaetal2009/> In addition, a strict molecular clock does not hold, so it is necessary to assume models of how evolutionary rates change along lineages. These assumptions alone can make substantial differences to the relative ages of different mammal groups estimated with genomic data.<ref name=Kitazoe2006>{{Cite journal |last1=Kitazoe |first1=Y. |last2=Kishino |first2=H. |last3=Waddell |first3=P. J. |last4=Nakajima |first4=T.|last5=Okabayashi |first5=T.|last6=Watabe |first6=T.|last7=Okuhara |first7=Y. |title=Robust time estimation reconciles views of the antiquity of placental mammals|journal=PLOS ONE |volume=2 |issue=e384 |pages=1–11 |year=2007 |doi=10.1371/journal.pone.0000384 |doi-access=free |pmid=17440620 |pmc=1849890 |bibcode=2007PLoSO...2..384K }}</ref>

{{clade| style=font-size:100%;line-height:100%
|label1='''Placentalia'''
|1={{Clade
   |label1=[[Atlantogenata]]
   |1={{Clade
      |1=[[Xenarthra]]
      |2=[[Afrotheria]]
      }}
   |label2=[[Boreoeutheria]]
   |2={{Clade
      |label1=[[Euarchontoglires]]
      |1={{Clade
         |1=[[Glires]]
         |2=[[Euarchonta]]
         }}
      |label2=[[Laurasiatheria]]
      |2={{Clade
         |1=[[Eulipotyphla]]
         |label2=[[Scrotifera]]
         |2={{Clade
            |1=[[Chiroptera]]
            |label2=[[Ferungulata]]
            |2={{Clade
               |label1=[[Ferae]]
               |1={{Clade
                  |1=[[Pholidota]]
                  |2=[[Carnivora]]
                  }}
               |label2=[[Euungulata]]
               |2={{Clade
                  |1=[[Perissodactyla]]
                  |2=[[Artiodactyla]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

[[Cladogram]] and classification based on Amrine-Madsen, H. ''et al''. (2003)<ref>{{cite journal |last1=Amrine-Madsen |first1=H. |last2=Koepfli |first2= K. P. |last3=Wayne |first3=R. K. |last4=Springer |first4=M. S. |year=2003 |title=A new phylogenetic marker, apoliprotein B, provides compelling evidence for eutherian relationships |journal=Molecular Phylogenetics and Evolution |volume=28 |issue=2 |pages=225–240 |doi=10.1016/s1055-7903(03)00118-0 |pmid=12878460|bibcode=2003MolPE..28..225A }}</ref> and Asher, R. J. ''et al''. (2009)<ref>{{cite journal |last1=Asher |first1=R. J. |last2=Bennett |first2=N. |last3=Lehmann |first3=T. |year=2009 |title=The new framework for understanding placental mammal evolution |journal=BioEssays |volume=31 |issue=8 |pages=853–864 |doi=10.1002/bies.200900053 |pmid=19582725|doi-access=free }}</ref> Compare with Waddell, Hasegawa and Okada (1999)<ref name="Waddell1999"/> and Waddell et al. (2001).<ref name="Waddell2001"/>

== Genomics ==
{{As of|2020}}, the [[genome]] has been sequenced for at least one species in each extant placental order and in 83% of families (105 of 127 extant placental families).<ref>Zoonomia Consortium (2020) [https://www.nature.com/articles/s41586-020-2876-6 A comparative genomics multitool for scientific discovery and conservation.] ''Nature'' 587, 240–245</ref>

See [[list of sequenced animal genomes]].

== Evolutionary history ==
True placental mammals (the [[crown group]] including all modern placentals) arose from stem-group members of the clade [[Eutheria]], which had existed since at least the [[Middle Jurassic]] period, about 170 mya. These early eutherians were small, nocturnal insect eaters, with adaptations for life in trees.<ref name="JiLuoYuan2002EarliestEutherian">{{cite journal |author=Ji, Q., Luo, Z-X., Yuan, C-X., Wible, J. R., Zhang, J-P. and Georgi, J. A. |title=The earliest known eutherian mammal |journal=Nature |volume=416 |pages=816–822 |date=April 2002 |doi=10.1038/416816a |pmid=11976675 |issue=6883|bibcode=2002Natur.416..816J |s2cid=4330626 }}</ref>

True placentals may have originated in the [[Late Cretaceous]] around 90 mya, but the earliest undisputed fossils are dated to the [[Cretaceous–Paleogene boundary]] (K-Pg boundary). The genus ''[[Protungulatum]]'' is sometimes placed as a stem-ungulate,<ref name="SCI-20130208">{{cite journal |last1=O'Leary |first1=Maureen A. |last2=Bloch |first2=Jonathan I. |last3=Flynn |first3=John J. |last4=Gaudin |first4=Timothy J. |last5=Giallombardo |first5=Andres |last6=Giannini |first6=Norberto P. |last7=Goldberg |first7=Suzann L. |last8=Kraatz |first8=Brian P. |last9=Luo |first9=Zhe-Xi |last10=Meng |first10=Jin |last11=Ni |first11=Michael J. |last12=Novacek |first12=Fernando A. |last13=Perini |first13=Zachary S. |last14=Randall |first14=Guillermo |last15=Rougier |first15=Eric J. |last16=Sargis |first16=Mary T. |last17=Silcox |first17=Nancy b. |last18=Simmons |first18=Micelle |last19=Spaulding |first19=Paul M. |last20=Velazco |first20=Marcelo |last21=Weksler |first21=John r. |last22=Wible |first22=Andrea L. |title=The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals |date=8 February 2013 |journal=[[Science (journal)|Science]] |volume=339 |pages=662–667 |doi=10.1126/science.1229237 |pmid=23393258 |issue=6120 |last23=Cirranello |first23=A. L. |bibcode=2013Sci...339..662O |hdl=11336/7302 |s2cid=206544776 |hdl-access=free }}</ref> with probably the earliest known species ''P. coombsi'' from the strata within the [[Hell Creek Formation]] specifically dated to at least 300,000 years before the K-Pg boundary.<ref name=Archibald2011/> The genus ''[[Purgatorius]]'', sometimes considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary.<ref>{{cite journal |last1=Fox |first1=R. C. |last2=Scott |first2=C. S. |year=2011 |title=A new, early Puercan (earliest Paleocene) species of Purgatorius (Plesiadapiformes, Primates) from Saskatchewan, Canada |journal=Journal of Paleontology |volume=85 |issue=3 |pages=537–548 |doi=10.1666/10-059.1|bibcode=2011JPal...85..537F |s2cid=131519722 }}</ref> One study has recovered both genera to be closely related and as stem-[[eutherian]]s outside modern placental mammals,<ref>{{cite journal |doi=10.1111/brv.12242 |pmid=28075073 |pmc=6849585 |title=Resolving the relationships of Paleocene placental mammals |year=2015 |journal=Biological Reviews |volume=92 |issue=1 |pages=521–550 |last1=Halliday |first1=Thomas J. D. }}</ref> but others have recovered ''Protungulatum'' as a [[Pan-Euungulata|pan-euungulate]] based on phylogenetic analysis and inner ear anatomy different from non-placentals.<ref>{{Cite journal| doi = 10.5252/g2015n4a1| issn = 1280-9659| volume = 37| issue = 4| pages = 397| last1 = de Muizon| first1 = Christian| last2 = Billet| first2 = Guillaume| last3 = Argot| first3 = Christine| last4 = Ladevèze| first4 = Sandrine| last5 = Goussard| first5 = Florent| title = Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene of Bolivia: anatomy, phylogeny and palaeobiology| journal = Geodiversitas| year = 2015 | s2cid = 131556174| url = https://bioone.org/journals/geodiversitas/volume-37/issue-4/g2015n4a1/Alcidedorbignya-inopinata-a-basal-pantodont-Placentalia-Mammalia-from-the-early/10.5252/g2015n4a1.full}}</ref><ref>{{Cite journal |last1=Orliac |first1=M. J. |last2=o'Leary |first2=M. A. |title=The inner ear of ''Protungulatum'' (Pan-Euungulata, Mammalia) |url=https://link.springer.com/article/10.1007/s10914-016-9327-z |year=2016 |journal=Journal of Mammalian Evolution |volume=23 |issue=4 |pages=337–352 |doi=10.1007/s10914-016-9327-z|s2cid=133815599 }}</ref> The rapid appearance of placentals after the mass extinction at the end of the [[Cretaceous]] suggests that the group had already originated and undergone an initial diversification in the Late Cretaceous, as suggested by [[molecular clock]]s.<ref name="Reis, M. 2012">{{cite journal |last1=dos Reis |first1=M. |last2=Inoue |first2=J. |last3=Hasegawa |first3=M. |last4=Asher |first4=R. J. |last5=Donoghue |first5=P. C. J. |last6=Yang |first6=Z. |year=2012 |title=Phylogenomic datasets provide both precision and accuracy in estimating the timescale of placental mammal phylogeny |journal=Proceedings of the Royal Society B |volume=279 |issue=1742 |pages=3491–3500 |doi=10.1098/rspb.2012.0683 |pmid=22628470 |pmc=3396900}}</ref> The lineages leading to Xenarthra and Afrotheria probably originated around 90 mya, and Boreoeutheria underwent an initial diversification around 70-80 mya,<ref name="Reis, M. 2012"/> producing the lineages that eventually would lead to modern primates, rodents, [[insectivores]], [[artiodactyls]], and [[carnivorans]].

However, modern members of the placental orders originated in the [[Paleogene]] around 66 to 23 mya, following the Cretaceous–Paleogene extinction event. The evolution of crown orders such modern primates, rodents, and carnivores appears to be part of an adaptive radiation<ref>{{cite journal |last1=Alroy |first1=J |year=1999 |title=The fossil record of North American Mammals: evidence for a Palaeocene evolutionary radiation |journal=Systematic Biology |volume=48 |issue=1 |pages=107–118 |doi=10.1080/106351599260472 |pmid=12078635 |doi-access=free}}</ref> that took place as mammals quickly evolved to take advantage of ecological [[Ecological niche|niches]] that were left open when most dinosaurs and other animals disappeared following the [[Chicxulub impactor|Chicxulub asteroid impact]]. As they occupied new niches, mammals rapidly increased in body size, and began to take over the large herbivore and large carnivore niches that had been left open by the decimation of the dinosaurs (and perhaps more relevantly competing [[synapsid]]s<ref name="Brocklehurst et al 2021">{{cite journal |last1=Brocklehurst |first1=Neil |last2=Panciroli |first2=Elsa |last3=Benevento |first3=Gemma Louise |last4=Benson |first4=Roger B.J. |title=Mammaliaform extinctions as a driver of the morphological radiation of Cenozoic mammals |journal=Current Biology |date=July 2021 |volume=31 |issue=13 |pages=2955–2963.e4 |doi=10.1016/j.cub.2021.04.044 |pmid=34004143 |s2cid=234782605 |url=https://ora.ox.ac.uk/objects/uuid:bda82407-db76-4c15-b061-ceb9ae271dd5 |doi-access=free |bibcode=2021CBio...31E2955B }}</ref>). Mammals also exploited niches that the non-avian dinosaurs had never touched: for example, [[bats]] evolved flight and echolocation, allowing them to be highly effective nocturnal, aerial insectivores; and whales first occupied freshwater lakes and rivers and then moved into the oceans. Primates, meanwhile, acquired specialized grasping hands and feet which allowed them to grasp branches, and large eyes with keener vision which allowed them to forage in the dark.

The evolution of land placentals followed different pathways on different continents since they cannot easily cross large bodies of water. An exception is smaller placentals such as rodents and primates, who left [[Laurasia]] and colonized Africa and then South America via [[Rafting event|rafting]].

In Africa, the [[Afrotheria]] underwent a major adaptive radiation, which led to elephants, [[elephant shrew]]s, [[tenrec]]s, [[golden mole]]s, [[aardvark]]s, and [[manatee]]s. In South America a similar event occurred, with radiation of the Xenarthra, which led to modern [[sloth]]s, [[anteater]]s, and [[armadillo]]s, as well as the extinct [[ground sloths]] and [[glyptodonts]]. Expansion in Laurasia was dominated by Boreoeutheria, which includes primates and rodents, [[insectivore]]s, carnivores, [[Perissodactyla|perissodactyls]] and [[artiodactyl]]s. These groups expanded beyond a single continent when land bridges formed linking Africa to Eurasia and South America to North America.

A study on eutherian diversity suggests that placental diversity was constrained during the [[Paleocene]], while [[Multituberculata|multituberculate]] mammals diversified; afterwards, multituberculates decline and placentals explode in diversity.<ref name="Brocklehurst et al 2021"/>

== Notes ==
{{notelist}}

== References ==
{{Reflist|30em}}
{{Reflist|group=note}}

== External links ==
{{Wikibooks|Dichotomous Key|Placentalia}}
{{Eutheria|E.|state=autocollapse}}
{{Mammals}}
{{Taxonbar|from=Q25833}}
{{Authority control}}

[[Category:Placentalia| ]]
[[Category:Mammal taxonomy]]
[[Category:Extant Paleocene first appearances]]
[[Category:Taxa named by Richard Owen]]