Editing Mammal (section)

===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>