Editing Mammal (section)

==Evolution==
{{Main|Evolution of mammals}}

===Origins===
[[Synapsida]], a clade that contains mammals and their extinct relatives, originated during the [[Pennsylvanian (geology)|Pennsylvanian subperiod]] (~323&nbsp;million to ~300&nbsp;million years ago), when they split from the reptile lineage. Crown group mammals evolved from earlier [[Mammaliaformes|mammaliaforms]] during the [[Early Jurassic]]. The cladogram takes Mammalia to be the crown group.<ref name=Liaoconodon2011>{{cite journal | vauthors = Meng J, Wang Y, Li C | title = Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont | journal = Nature | volume = 472 | issue = 7342 | pages = 181–185 | date = April 2011 | pmid = 21490668 | doi = 10.1038/nature09921 | bibcode = 2011Natur.472..181M | s2cid = 4428972 }}</ref>

{{clade
 |label1=[[Mammaliaformes]]
  |1={{clade
            |1=[[Morganucodontidae]] [[File:Morganucodon.jpg|50px]]
            |2={{clade
                 |1=[[Docodonta]] [[File:Docofossor NT flipped.jpg|50px]]
                 |2={{clade
                           |1=''[[Haldanodon]]''
                           |label2='''Mammalia'''
                           |2={{clade
                                |1=[[Australosphenida]] (incl. [[Monotremata]]) [[File:Steropodon BW.jpg|50px]]
                                |2={{clade
                                  |1=''[[Fruitafossor]]''
                                  |2={{clade
                                     |1={{clade
                                         |1=''[[Haramiyavia]]''
                                         |2=[[Multituberculata]] [[File:Sunnyodon.jpg|50px]]
                                   }}
                                     |2=''[[Tinodon]]''
                                     |3=[[Eutriconodonta]] (incl. [[Gobiconodonta]]) [[File:Repenomamus BW.jpg|50px]]
                                     |4=[[Trechnotheria]] (incl. [[Theria]]) [[File:Juramaia NT.jpg|50px]]
                               }}
                         }}
                    }}
               }}
          }}
     }}
}}

===Evolution from older amniotes===
[[File:Skull synapsida 1.png|thumb|The original synapsid skull structure contains one [[temporal fenestrae|temporal opening]] behind the [[eye socket|orbitals]], in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength.]]

The first fully terrestrial [[vertebrate]]s were [[amniote]]s. Like their amphibious early [[tetrapod]] predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing [[embryo]] to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while [[amphibian]]s generally need to lay their eggs in water.

The first amniotes apparently arose in the Pennsylvanian subperiod of the [[Carboniferous]]. They descended from earlier [[Reptiliomorpha|reptiliomorph]] amphibious tetrapods,<ref name="AhlbergMilner1994OriginOfTetrapods">{{cite journal| vauthors = Ahlberg PE, Milner AR |date=April 1994| title=The Origin and Early Diversification of Tetrapods | journal= Nature | volume=368 | pages=507–514| doi=10.1038/368507a0| issue=6471|bibcode = 1994Natur.368..507A |s2cid=4369342}}</ref> which lived on land that was already inhabited by [[insect]]s and other invertebrates as well as [[fern]]s, [[moss]]es and other plants. Within a few million years, two important amniote lineages became distinct: the [[synapsid]]s, which would later include the common ancestor of the mammals; and the [[sauropsid]]s, which now include [[turtle]]s, [[lizard]]s, [[snake]]s, [[crocodilian]]s and [[dinosaur]]s (including [[bird]]s).<ref>{{cite web | url = https://palaeos.com/Vertebrates/Units/190Reptilomorpha/190.400.html#Amniota | archive-url = https://web.archive.org/web/20101220194106/http://palaeos.com/Vertebrates/Units/190Reptilomorpha/190.400.html#Amniota | archive-date = 20 December 2010 | title = Amniota – Palaeos}}</ref> Synapsids have a single hole ([[temporal fenestra]]) low on each side of the skull. Primitive synapsids included the largest and fiercest animals of the early [[Permian]] such as ''[[Dimetrodon]]''.<ref>{{cite web | url=https://palaeos.com/Vertebrates/Units/390Synapsida/390.000.html | archive-url=https://web.archive.org/web/20101220193822/http://palaeos.com/Vertebrates/Units/390Synapsida/390.000.html | archive-date=20 December 2010 | title=Synapsida overview – Palaeos}}</ref> Nonmammalian synapsids were traditionally—and incorrectly—called "mammal-like reptiles" or [[pelycosaur]]s; we now know they were neither reptiles nor part of reptile lineage.<ref name=Kemp2006/><ref name=Bennett1986/>

[[Therapsid]]s, a group of synapsids, evolved in the [[Guadalupian|Middle Permian]], about 265&nbsp;million years ago, and became the dominant land vertebrates.<ref name=Kemp2006>{{cite journal | vauthors = Kemp TS | title = The origin and early radiation of the therapsid mammal-like reptiles: a palaeobiological hypothesis | journal = Journal of Evolutionary Biology | volume = 19 | issue = 4 | pages = 1231–1247 | date = July 2006 | pmid = 16780524 | doi = 10.1111/j.1420-9101.2005.01076.x | s2cid = 3184629 | url = https://users.ox.ac.uk/~tskemp/pdfs/jeb2006.pdf | access-date = 14 January 2012 | archive-date = 8 March 2021 | archive-url = https://web.archive.org/web/20210308061139/http://users.ox.ac.uk/~tskemp/pdfs/jeb2006.pdf | url-status = dead }}</ref> They differ from basal [[Eupelycosauria|eupelycosaurs]] in several features of the skull and jaws, including: larger skulls and [[incisor]]s which are equal in size in therapsids, but not for eupelycosaurs.<ref name=Kemp2006/> The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their early synapsid ancestors and ending with [[probainognathia]]n [[cynodont]]s, some of which could easily be mistaken for mammals. Those stages were characterised by:<ref name="Kermack1984">{{cite book | vauthors = Kermack DM, Kermack KA | title=The evolution of mammalian characters|location=Washington, DC | publisher=Croom Helm | year=1984 | isbn=978-0-7099-1534-8 |oclc=10710687}}</ref>
* The gradual development of a bony secondary [[hard palate|palate]].
* Abrupt acquisition of [[endothermy]] among [[Mammaliamorpha]], thus prior to the origin of mammals by 30–50 millions of years '''''<ref>{{cite journal |last1=Araújo |title=Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy |journal=Nature |volume=607 |date= 28 July 2022 |issue=7920 |pages=726–731 |doi=10.1038/s41586-022-04963-z |pmid=35859179 |bibcode=2022Natur.607..726A |s2cid=236245230 |display-authors=etal}}</ref>'''''.
* Progression towards an erect limb posture, which would increase the animals' stamina by avoiding [[Carrier's constraint]]. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
* The [[Mandible#Other vertebrates|dentary]] gradually became the main bone of the lower jaw which, by the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).

===First mammals===

The [[Permian–Triassic extinction event]] about 252&nbsp;million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids.<ref>{{cite journal| vauthors = Tanner LH, Lucas SG, Chapman MG |title=Assessing the record and causes of Late Triassic extinctions |journal=Earth-Science Reviews |volume=65 |issue=1–2 |pages=103–139 |year=2004 |doi=10.1016/S0012-8252(03)00082-5 |url=https://nmnaturalhistory.org/pdf_files/TJB.pdf |bibcode=2004ESRv...65..103T |url-status=dead |archive-url=https://web.archive.org/web/20071025225841/http://nmnaturalhistory.org/pdf_files/TJB.pdf |archive-date=25 October 2007 }}</ref> In the early Triassic, most medium to large land carnivore niches were taken over by [[archosaur]]s<ref>{{cite journal | vauthors = Brusatte SL, Benton MJ, Ruta M, Lloyd GT | title = Superiority, competition, and opportunism in the evolutionary radiation of dinosaurs | journal = Science | volume = 321 | issue = 5895 | pages = 1485–1488 | date = September 2008 | pmid = 18787166 | doi = 10.1126/science.1161833 | bibcode = 2008Sci...321.1485B | hdl = 20.500.11820/00556baf-6575-44d9-af39-bdd0b072ad2b | s2cid = 13393888 | url = https://www.pure.ed.ac.uk/ws/files/8232088/PDF_Brusatteetal2008SuperiorityCompetition.pdf | access-date = 12 October 2019 | archive-date = 19 July 2018 | archive-url = https://web.archive.org/web/20180719005836/https://www.pure.ed.ac.uk/ws/files/8232088/PDF_Brusatteetal2008SuperiorityCompetition.pdf | url-status = live }}</ref> which, over an extended period (35&nbsp;million years), came to include the [[Crocodylomorpha|crocodylomorphs]],<ref>{{cite book | vauthors = Gauthier JA |year=1986 |chapter=Saurischian monophyly and the origin of birds |title=The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Sciences | veditors = Padian K |volume=8 |publisher=California Academy of Sciences |location=San Francisco |pages=1–55}}</ref> the [[pterosaur]]s and the dinosaurs;<ref>{{cite journal| vauthors = Sereno PC |year=1991|title=Basal archosaurs: phylogenetic relationships and functional implications|journal=Memoirs of the Society of Vertebrate Paleontology|volume=2|pages=1–53|doi=10.2307/3889336|jstor=3889336}}</ref> however, large cynodonts like ''[[Trucidocynodon]]'' and [[Traversodontidae|traversodontids]] still occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.<ref>{{cite journal| vauthors = MacLeod N, Rawson PF, Forey PL, Banner FT, Boudagher-Fadel MK, Bown PR, Burnett JA, Chambers P, Culver S, Evans SE, Jeffery C | display-authors = 6 |title=The Cretaceous–Tertiary biotic transition|year=1997|journal=Journal of the Geological Society|volume=154|issue=2|pages=265–292|doi=10.1144/gsjgs.154.2.0265|bibcode=1997JGSoc.154..265M | s2cid = 129654916 }}</ref>

The first mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225&nbsp;million years ago), 40&nbsp;million years after the first therapsids. They expanded out of their nocturnal [[insectivore]] niche from the mid-Jurassic onwards;<ref>{{cite book|url={{Google books|plainurl=yes|id=APWwBAAAQBAJ|page=73}}| vauthors = Hunt DM, Hankins MW, Collin SP, Marshall NJ |title=Evolution of Visual and Non-visual Pigments|year= 2014|location=London|publisher=Springer|page=73|isbn=978-1-4614-4354-4|oclc=892735337}}</ref> the Jurassic ''[[Castorocauda]]'', for example, was a close relative of true mammals that had adaptations for swimming, digging and catching fish.<ref>{{cite web | url=https://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html| archive-url=https://web.archive.org/web/20060303071809/http://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html| url-status=dead| archive-date=3 March 2006| vauthors = Bakalar N |year=2006| title=Jurassic "Beaver" Found; Rewrites History of Mammals|access-date=28 May 2016|work=National Geographic News}}</ref> Most, if not all, are thought to have remained nocturnal (the [[nocturnal bottleneck]]), accounting for much of the typical mammalian traits.<ref>{{cite journal | vauthors = Hall MI, Kamilar JM, Kirk EC | title = Eye shape and the nocturnal bottleneck of mammals | journal = Proceedings of the Royal Society B: Biological Sciences| volume = 279 | issue = 1749 | pages = 4962–4968 | date = December 2012 | pmid = 23097513 | pmc = 3497252 | doi = 10.1098/rspb.2012.2258 }}</ref> The majority of the mammal species that existed in the [[Mesozoic|Mesozoic Era]] were multituberculates, eutriconodonts and [[spalacotheriid]]s.<ref name=Luo2007>{{cite journal | vauthors = Luo ZX | title = Transformation and diversification in early mammal evolution | journal = Nature | volume = 450 | issue = 7172 | pages = 1011–1019 | date = December 2007 | pmid = 18075580 | doi = 10.1038/nature06277 | bibcode = 2007Natur.450.1011L | s2cid = 4317817 }}</ref> The earliest-known fossil of the [[Metatheria]] ("changed beasts") is ''[[Sinodelphys]]'', found in 125-million-year-old [[Early Cretaceous]] [[shale]] in China's northeastern [[Liaoning Province]]. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.<ref>{{cite web | url=https://news.nationalgeographic.com/news/2003/12/1215_031215_oldestmarsupial.html | archive-url=https://web.archive.org/web/20031217024049/http://news.nationalgeographic.com/news/2003/12/1215_031215_oldestmarsupial.html | url-status=dead | archive-date=17 December 2003 | vauthors = Pickrell J |year=2003| title=Oldest Marsupial Fossil Found in China| publisher=National Geographic News|access-date=28 May 2016}}</ref>

[[File:Juramaia NT.jpg|thumb|Restoration of ''[[Juramaia|Juramaia sinensis]]'', the oldest-known [[eutheria]]n (160 mya)<ref name=Juramaia/>]]

The oldest-known fossil among the [[Eutheria]] ("true beasts") is the small shrewlike ''[[Juramaia|Juramaia sinensis]]'', or "Jurassic mother from China", dated to 160&nbsp;million years ago in the late Jurassic.<ref name=Juramaia>{{cite journal | vauthors = Luo ZX, Yuan CX, Meng QJ, Ji Q | title = A Jurassic eutherian mammal and divergence of marsupials and placentals | journal = Nature | volume = 476 | issue = 7361 | pages = 442–5 | date = August 2011 | pmid = 21866158 | doi = 10.1038/nature10291 | bibcode = 2011Natur.476..442L | s2cid = 205225806 }}</ref> A later eutherian relative, ''[[Eomaia]]'', dated to 125&nbsp;million years ago in the early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage.<ref>{{cite journal | vauthors = Ji Q, Luo ZX, Yuan CX, Wible JR, Zhang JP, Georgi JA | title = The earliest known eutherian mammal | journal = Nature | volume = 416 | issue = 6883 | pages = 816–822 | date = April 2002 | pmid = 11976675 | doi = 10.1038/416816a | bibcode = 2002Natur.416..816J | s2cid = 4330626 }}</ref> In particular, the [[epipubic bone]]s extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, other nontherian mammals and ''[[Ukhaatherium]]'', an early Cretaceous animal in the eutherian order [[Asioryctitheria]]. This also applies to the multituberculates.<ref name="Epipubic bones in eutherian mammals">{{cite journal | vauthors = Novacek MJ, Rougier GW, Wible JR, McKenna MC, Dashzeveg D, Horovitz I | title = Epipubic bones in eutherian mammals from the late Cretaceous of Mongolia | journal = Nature | volume = 389 | issue = 6650 | pages = 483–486 | date = October 1997 | pmid = 9333234 | doi = 10.1038/39020 | bibcode = 1997Natur.389..483N | s2cid = 205026882 }}</ref> They are apparently an ancestral feature, which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles during locomotion, reducing the amount of space being presented, which placentals require to contain their [[fetus]] during gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and therefore [[Pregnancy (mammals)|pregnancy]] was short, as in modern marsupials. This suggests that the placenta was a later development.<ref>{{cite book|chapter-url={{Google books|plainurl=yes|id=G1exWxU3QHIC|page=68}} | vauthors = Power ML, Schulkin J |year=2012|title=Evolution of the Human Placenta|publisher=Johns Hopkins University Press|location=Baltimore| isbn=978-1-4214-0643-5|page=68|chapter=Evolution of Live Birth in Mammals}}</ref>

One of the earliest-known monotremes was ''[[Teinolophos]]'', which lived about 120&nbsp;million years ago in Australia.<ref>{{cite journal | vauthors = Rowe T, Rich TH, Vickers-Rich P, Springer M, Woodburne MO | title = The oldest platypus and its bearing on divergence timing of the platypus and echidna clades | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 4 | pages = 1238–1242 | date = January 2008 | pmid = 18216270 | pmc = 2234122 | doi = 10.1073/pnas.0706385105 | bibcode = 2008PNAS..105.1238R | doi-access = free }}</ref> Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce ([[cloaca]])—as reptiles and birds also do—<ref>{{cite book|chapter-url={{Google books|plainurl=yes|id=FASJWgDhxIsC|page=55}}| vauthors = Grant T |year=1995|title=The Platypus: A Unique Mammal|chapter=Reproduction|publisher=University of New South Wales|location= Sydney|page=55|isbn=978-0-86840-143-0|oclc=33842474}}</ref> and they lay [[Egg (biology)|eggs]] which are leathery and uncalcified.<ref>{{cite journal | vauthors = Goldman AS | title = Evolution of immune functions of the mammary gland and protection of the infant | journal = Breastfeeding Medicine | volume = 7 | issue = 3 | pages = 132–142 | date = June 2012 | pmid = 22577734 | doi = 10.1089/bfm.2012.0025 }}</ref>

===Earliest appearances of features===
''[[Hadrocodium]]'', whose fossils date from approximately 195&nbsp;million years ago, in the early [[Jurassic]], provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.<ref name=jawbone2006>{{cite book|url={{Google books|plainurl=yes|id=lyGqD_GWQ7oC&|page=82}}| vauthors = Rose KD |year=2006|title=The Beginning of the Age of Mammals|location=Baltimore|publisher=Johns Hopkins University Press|pages=82–83|isbn=978-0-8018-8472-6|oclc=646769601}}</ref>

[[File:Thrinaxodon Lionhinus.jpg|left|thumb|Fossil of ''[[Thrinaxodon]]'' at the [[National Museum of Natural History]]]]
The earliest clear evidence of hair or fur is in fossils of ''[[Castorocauda]]'' and ''[[Megaconus]]'', from 164&nbsp;million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in the [[maxilla]]e and [[premaxilla]]e (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae ([[whiskers]]) and so were evidence of hair or fur;<ref name="Brink1955">{{cite journal | vauthors = Brink AS | title=A study on the skeleton of ''Diademodon'' | journal=Palaeontologia Africana | volume=3 | pages=3–39 |year=1955 }}</ref><ref name="Kemp1982">{{cite book | vauthors = Kemp TS | title=Mammal-like reptiles and the origin of mammals | publisher=Academic Press | year=1982 | location=London | page=363 | isbn=978-0-12-404120-2|oclc=8613180}}</ref> it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard ''[[Tupinambis]]'' has foramina that are almost identical to those found in the nonmammalian cynodont ''[[Thrinaxodon]]''.<ref name=Bennett1986>{{cite book| vauthors = Bennett AF, Ruben JA |year=1986|chapter=The metabolic and thermoregulatory status of therapsids|pages=207–218| veditors = Hotton III N, MacLean JJ, Roth J, Roth EC |title=The ecology and biology of mammal-like reptiles|publisher=Smithsonian Institution Press|location=Washington, DC|isbn=978-0-87474-524-5}}</ref><ref>{{cite journal | vauthors = Estes R | title=Cranial anatomy of the cynodont reptile ''Thrinaxodon liorhinus'' | journal=Bulletin of the Museum of Comparative Zoology | pages=165–180 |year=1961|issue=1253 }}</ref> Popular sources, nevertheless, continue to attribute whiskers to ''Thrinaxodon''.<ref>{{cite web |url=https://news.nationalgeographic.com/news/2009/02/photogalleries/darwin-birthday-evolution/#/thrinaxodon-missing-link_7787_600x450.jpg |archive-url=https://web.archive.org/web/20090214205210/http://news.nationalgeographic.com/news/2009/02/photogalleries/darwin-birthday-evolution/#/thrinaxodon-missing-link_7787_600x450.jpg |url-status=dead |archive-date=14 February 2009 |title=''Thrinaxodon:'' The Emerging Mammal |date=11 February 2009 |publisher=National Geographic Daily News |access-date=26 August 2012}}</ref> Studies on Permian [[coprolites]] suggest that non-mammalian [[synapsids]] of the epoch already had fur, setting the evolution of hairs possibly as far back as [[dicynodont]]s.<ref name=piotr>{{cite journal |title= Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia | vauthors = Bajdek P, Qvarnström M, Owocki K, Sulej T, Sennikov AG, Golubev VK, Niedźwiedzki G |year=2015 |journal=Lethaia |volume=49 |issue=4 |pages=455–477 |doi=10.1111/let.12156 }}</ref>

When [[endothermy]] first appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammalian [[therapsids]].<ref name=piotr/><ref>{{cite journal | vauthors = Botha-Brink J, Angielczyk KD |title=Do extraordinarily high growth rates in Permo–Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction? |year=2010 |doi= 10.1111/j.1096-3642.2009.00601.x |volume=160 |issue=2 |pages=341–365 |journal=Zoological Journal of the Linnean Society |doi-access=free}}</ref> Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,<ref name="Paul1988">{{cite book | vauthors = Paul GS | title=Predatory Dinosaurs of the World | publisher=Simon and Schuster | year=1988 | location=New York | page=[https://archive.org/details/predatorydinosau00paul/page/464 464] | isbn=978-0-671-61946-6 | oclc=18350868 | url=https://archive.org/details/predatorydinosau00paul/page/464 }}</ref> but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians.<ref>{{Cite journal|journal=Australian Journal of Zoology|title=Monotreme Cell-Cycles and the Evolution of Homeothermy| vauthors = Watson JM, Graves JA |volume=36|issue=5|pages=573–584|year=1988| doi = 10.1071/ZO9880573}}</ref> Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.<ref>{{cite journal| vauthors = McNab BK |year=1980|title=Energetics and the limits to the temperate distribution in armadillos|journal=Journal of Mammalogy|volume=61|issue=4|pages=606–627|doi=10.2307/1380307|jstor=1380307}}</ref>

The evolution of erect limbs in mammals is incomplete—living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian ''Eomaia'' and the metatherian ''Sinodelphys'', both dated to 125&nbsp;million years ago.<ref>{{cite journal | vauthors=Kielan-Jaworowska Z, Hurum JH | title=Limb posture in early mammals: Sprawling or parasagittal | journal=Acta Palaeontologica Polonica | volume=51 | issue=3 | pages=10237–10239 | year=2006 | url=https://app.pan.pl/archive/published/app51/app51-393.pdf | access-date=25 January 2024 | archive-date=25 January 2024 | archive-url=https://web.archive.org/web/20240125191351/https://app.pan.pl/archive/published/app51/app51-393.pdf | url-status=live }}</ref> [[Epipubic]] bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in [[Tritylodontidae]], suggesting that it is a [[synapomorphy]] between them and [[Mammaliaformes]]. They are omnipresent in non-placental Mammaliaformes, though ''[[Megazostrodon]]'' and ''[[Erythrotherium]]'' appear to have lacked them.<ref>{{cite book| vauthors = Lillegraven JA, Kielan-Jaworowska Z, Clemens WA |title=Mesozoic Mammals: The First Two-Thirds of Mammalian History |publisher=University of California Press |year=1979 |page=321 |isbn=978-0-520-03951-3 |oclc=5910695}}</ref>

It has been suggested that the original function of [[lactation]] ([[milk]] production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals.<ref>{{cite journal | vauthors = Oftedal OT | title = The mammary gland and its origin during synapsid evolution | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 225–252 | date = July 2002 | pmid = 12751889 | doi = 10.1023/A:1022896515287 | s2cid = 25806501 }}
</ref><ref>{{cite journal | vauthors = Oftedal OT | title = The origin of lactation as a water source for parchment-shelled eggs | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 253–266 | date = July 2002 | pmid = 12751890 | doi = 10.1023/A:1022848632125 | s2cid = 8319185 }}</ref> In human females, mammary glands become fully developed during puberty, regardless of pregnancy.<ref>{{cite web|url = https://www.texaschildrens.org/health/breast-development |title = Breast Development | work = Texas Children's Hospital |access-date =13 January 2021|archive-url=https://web.archive.org/web/20210113081725/https://www.texaschildrens.org/health/breast-development|archive-date=13 January 2021}}</ref>

===Rise of the mammals===
[[File:Hyaenodon horridus, Niobrara County, Wyoming, USA, Late Oligocene - Royal Ontario Museum - DSC00114.JPG|thumb|left|''[[Hyaenodon]] horridus'' at the [[Royal Ontario Museum]]. The genus ''Hyaenodon'' was among the most successful mammals of the late [[Eocene]]-early [[Miocene]] epochs spanning for most of the [[Paleogene]] and some of the [[Neogene]] periods, undergoing many endemic radiations in North America, Europe, and Asia.<ref>{{cite journal|last1=Pfaff|first1=Cathrin|last2=Nagel|first2=Doris|last3=Gunnell|first3=Gregg|last4=Weber|first4=Gerhard W.|last5=Kriwet|first5=Jürgen|last6=Morlo|first6=Michael|last7=Bastl|first7=Katharina|year=2017|title=Palaeobiology of Hyaenodon exiguus (Hyaenodonta, Mammalia) based on morphometric analysis of the bony labyrinth|journal=Journal of Anatomy|volume=230|issue=2|pages=282–289|doi=10.1111/joa.12545|pmid=27666133 |pmc=5244453 }}</ref>]]
Therians took over the medium- to large-sized ecological niches in the [[Cenozoic]], after the [[Cretaceous–Paleogene extinction event]] approximately 66&nbsp;million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups,<ref name="SahneyBentonFerry2010LinksDiversityVertebrates">{{cite journal | vauthors = Sahney S, Benton MJ, Ferry PA | title = Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land | journal = Biology Letters | volume = 6 | issue = 4 | pages = 544–547 | date = August 2010 | pmid = 20106856 | pmc = 2936204 | doi = 10.1098/rsbl.2009.1024 }}</ref> and underwent an exponential increase in body size ([[megafauna#In terrestrial mammals|megafauna]]).<ref name="F.A.Smith">{{cite journal | vauthors = Smith FA, Boyer AG, Brown JH, Costa DP, Dayan T, Ernest SK, Evans AR, Fortelius M, Gittleman JL, Hamilton MJ, Harding LE, Lintulaakso K, Lyons SK, McCain C, Okie JG, Saarinen JJ, Sibly RM, Stephens PR, Theodor J, Uhen MD | display-authors = 6 | title = The evolution of maximum body size of terrestrial mammals | journal = Science | volume = 330 | issue = 6008 | pages = 1216–1219 | date = November 2010 | pmid = 21109666 | doi = 10.1126/science.1194830 | citeseerx = 10.1.1.383.8581 | bibcode = 2010Sci...330.1216S | s2cid = 17272200 }}</ref> The increase in mammalian diversity was not, however, solely because of expansion into large-bodied niches.<ref>{{Cite journal |last1=Benevento |first1=Gemma Louise |last2=Benson |first2=Roger B. J. |last3=Close |first3=Roger A. |last4=Butler |first4=Richard J. |date=16 June 2023 |title=Early Cenozoic increases in mammal diversity cannot be explained solely by expansion into larger body sizes |journal=[[Palaeontology (journal)|Palaeontology]] |language=en |volume=66 |issue=3 |page=12653 |doi=10.1111/pala.12653 |issn=0031-0239 |doi-access=free |bibcode=2023Palgy..6612653B }}</ref> Mammals diversified very quickly, displaying an exponential rise in diversity.<ref name="SahneyBentonFerry2010LinksDiversityVertebrates"/> For example, the earliest-known bat dates from about 50&nbsp;million years ago, only 16&nbsp;million years after the extinction of the non-avian dinosaurs.<ref>{{cite journal | vauthors = Simmons NB, Seymour KL, Habersetzer J, Gunnell GF | title = Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation | journal = Nature | volume = 451 | issue = 7180 | pages = 818–821 | date = February 2008 | pmid = 18270539 | doi = 10.1038/nature06549 | url = https://www.nature.com/articles/nature06549.epdf?referrer_access_token=vvnCZKFsJpI9PyHTMCW0AtRgN0jAjWel9jnR3ZoTv0OIjnS1VOVvLEmlM3pVKicSGILcK5-gC0KUTwfjLtGsWX-Jl3sk3aQbBWjeluiuMfZh8gMqZJ4qV9dfir4OcYHZFaqbm8GbWK-9JqFbcMEjN3G_-d7t8hJWLm5RDaih5vZhy47BSgJOQDuz8aRMBtHCvZEWV7affpdxsSefk_6x80U5fE1N1SjLKUHVdainKEM%3D&tracking_referrer=arstechnica.com | bibcode = 2008Natur.451..818S | hdl = 2027.42/62816 | s2cid = 4356708 | hdl-access = free | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191352/https://www.nature.com/articles/nature06549.epdf?referrer_access_token=vvnCZKFsJpI9PyHTMCW0AtRgN0jAjWel9jnR3ZoTv0OIjnS1VOVvLEmlM3pVKicSGILcK5-gC0KUTwfjLtGsWX-Jl3sk3aQbBWjeluiuMfZh8gMqZJ4qV9dfir4OcYHZFaqbm8GbWK-9JqFbcMEjN3G_-d7t8hJWLm5RDaih5vZhy47BSgJOQDuz8aRMBtHCvZEWV7affpdxsSefk_6x80U5fE1N1SjLKUHVdainKEM%3D&tracking_referrer=arstechnica.com | url-status = live }}</ref>

Molecular phylogenetic studies initially suggested that most placental orders diverged about 100 to 85&nbsp;million years ago and that modern families appeared in the period from the late [[Eocene]] through the [[Miocene]].<ref name="Bininda2007">{{cite journal | vauthors = Bininda-Emonds OR, Cardillo M, Jones KE, MacPhee RD, Beck RM, Grenyer R, Price SA, Vos RA, Gittleman JL, Purvis A | display-authors = 6 | title = The delayed rise of present-day mammals | journal = Nature | volume = 446 | issue = 7135 | pages = 507–512 | date = March 2007 | pmid = 17392779 | doi = 10.1038/nature05634 | url = https://www.utheria.org/uploads/nature05634.pdf | bibcode = 2007Natur.446..507B | s2cid = 4314965 | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191351/https://www.utheria.org/uploads/nature05634.pdf | url-status = live }}</ref> However, no placental fossils have been found from before the end of the Cretaceous.<ref name="Wible2007"/> The earliest undisputed fossils of placentals come from the early [[Paleocene]], after the extinction of the non-avian dinosaurs.<ref name="Wible2007">{{cite journal | vauthors = Wible JR, Rougier GW, Novacek MJ, Asher RJ | title = Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary | journal = Nature | volume = 447 | issue = 7147 | pages = 1003–1006 | date = June 2007 | pmid = 17581585 | doi = 10.1038/nature05854 | bibcode = 2007Natur.447.1003W | s2cid = 4334424 }}</ref> (Scientists identified an early Paleocene animal named ''[[Protungulatum donnae]]'' as one of the first placental mammals,<ref name="SCI-20130208">{{cite journal | vauthors = O'Leary MA, Bloch JI, Flynn JJ, Gaudin TJ, Giallombardo A, Giannini NP, Goldberg SL, Kraatz BP, Luo ZX, Meng J, Ni X, Novacek MJ, Perini FA, Randall ZS, Rougier GW, Sargis EJ, Silcox MT, Simmons NB, Spaulding M, Velazco PM, Weksler M, Wible JR, Cirranello AL | display-authors = 6 | title = The placental mammal ancestor and the post-K-Pg radiation of placentals | journal = Science | volume = 339 | issue = 6120 | pages = 662–667 | date = February 2013 | pmid = 23393258 | doi = 10.1126/science.1229237 | url = https://www.science.org/doi/abs/10.1126/science.1229237 | bibcode = 2013Sci...339..662O | hdl = 11336/7302 | s2cid = 206544776 | hdl-access = free | access-date = 30 June 2022 | archive-date = 10 November 2021 | archive-url = https://web.archive.org/web/20211110174432/https://www.science.org/doi/abs/10.1126/science.1229237 | url-status = live }}</ref> but it has since been reclassified as a non-placental eutherian.)<ref>{{cite journal | vauthors = Halliday TJ, Upchurch P, Goswami A | title = Resolving the relationships of Paleocene placental mammals | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 92 | issue = 1 | pages = 521–550 | date = February 2017 | pmid = 28075073 | pmc = 6849585 | doi = 10.1111/brv.12242 }}</ref> Recalibrations of genetic and morphological diversity rates have suggested a [[Maastrichtian|Late Cretaceous]] origin for placentals, and a Paleocene origin for most modern clades.<ref>{{cite journal | vauthors = Halliday TJ, Upchurch P, Goswami A | title = Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous-Palaeogene mass extinction | journal = Proceedings. Biological Sciences| volume = 283 | issue = 1833 | pages = 20153026 | date = June 2016 | pmid = 27358361 | pmc = 4936024 | doi = 10.1098/rspb.2015.3026 }}</ref>

The earliest-known ancestor of primates is ''[[Archicebus achilles]]''<ref name="NTR-20130606">{{cite journal | vauthors = Ni X, Gebo DL, Dagosto M, Meng J, Tafforeau P, Flynn JJ, Beard KC | title = The oldest known primate skeleton and early haplorhine evolution | journal = Nature | volume = 498 | issue = 7452 | pages = 60–64 | date = June 2013 | pmid = 23739424 | doi = 10.1038/nature12200 | bibcode = 2013Natur.498...60N | s2cid = 4321956 }}</ref> from around 55&nbsp;million years ago.<ref name="NTR-20130606"/> This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.<ref name="NTR-20130606"/>

{{anchor|Anatomy and morphology}}