Editing Mammal (section)

===Distinguishing features===
Living mammal species can be identified by the presence of [[sweat gland]]s, including [[Mammary gland|those that are specialised to produce milk]] to nourish their young.<ref>{{cite book | vauthors = Romer SA, Parsons TS |year=1977 |title=The Vertebrate Body |location=Philadelphia |publisher=Holt-Saunders International |pages=129–145 |isbn=978-0-03-910284-5 |oclc=60007175}}</ref> In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.<ref>{{cite book |url= {{Google books|plainurl=yes|id=kS-h84pMJw4C|page=593}} | vauthors = Purves WK, Sadava DE, Orians GH, Helle HC |year=2001 |title= Life: The Science of Biology|location=New York|edition=6th|publisher=Sinauer Associates, Inc. |page=593 |isbn=978-0-7167-3873-2 |oclc=874883911 }}</ref>

Many traits shared by all living mammals appeared among the earliest members of the group:
* '''[[Jaw|Jaw joint]]''' – The [[dentary]] (the lower jaw bone, which carries the teeth) and the [[squamosal]] (a small [[cranial]] bone) meet to form the joint. In most [[gnathostomes]], including early [[therapsids]], the joint consists of the [[articular]] (a small bone at the back of the lower jaw) and [[Quadrate bone|quadrate]] (a small bone at the back of the upper jaw).<ref name=jawbone2006/>
* '''[[Middle ear]]''' – In crown-group mammals, sound is carried from the [[eardrum]] by a chain of three bones, the [[malleus]], the [[incus]] and the [[stapes]]. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.<ref>{{cite journal | vauthors = Anthwal N, Joshi L, Tucker AS | title = Evolution of the mammalian middle ear and jaw: adaptations and novel structures | journal = Journal of Anatomy | volume = 222 | issue = 1 | pages = 147–160 | date = January 2013 | pmid = 22686855 | pmc = 3552421 | doi = 10.1111/j.1469-7580.2012.01526.x }}</ref>
* '''Tooth replacement''' – Teeth can be replaced once ([[diphyodonty]]) or (as in toothed whales and [[Muridae|murid]] rodents) not at all ([[Dentition|monophyodont]]y).<ref>{{cite journal | vauthors = van Nievelt AF, Smith KK |year=2005 |title=To replace or not to replace: the significance of reduced functional tooth replacement in marsupial and placental mammals |journal=Paleobiology |volume=31 |issue=2 |pages=324–346 |doi=10.1666/0094-8373(2005)031[0324:trontr]2.0.co;2|s2cid=37750062 }}</ref> Elephants, manatees, and kangaroos continually grow new teeth throughout their life ([[polyphyodont]]y).<ref>{{cite journal | vauthors = Libertini G, Ferrara N | title = Aging of perennial cells and organ parts according to the programmed aging paradigm | journal = Age | volume = 38 | issue = 2 | pages = 35 | date = April 2016 | pmid = 26957493 | pmc = 5005898 | doi = 10.1007/s11357-016-9895-0 }}</ref>
* '''Prismatic enamel''' – The [[tooth enamel|enamel]] coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the [[dentin]] to the tooth's surface.<ref>{{cite journal | vauthors = Mao F, Wang Y, Meng J | title = A Systematic Study on Tooth Enamel Microstructures of Lambdopsalis bulla (Multituberculate, Mammalia) – Implications for Multituberculate Biology and Phylogeny | journal = PLOS ONE | volume = 10 | issue = 5 | pages = e0128243 | year = 2015 | pmid = 26020958 | pmc = 4447277 | doi = 10.1371/journal.pone.0128243 | bibcode = 2015PLoSO..1028243M | doi-access = free }}</ref>
* '''[[Occipital condyle]]s''' – Two knobs at the base of the skull fit into the topmost [[Cervical vertebrae|neck vertebra]]; most other [[tetrapod]]s, in contrast, have only one such knob.<ref>{{cite journal |jstor=2453526 | vauthors = Osborn HF |year=1900 |title=Origin of the Mammalia, III. Occipital Condyles of Reptilian Tripartite Type|journal=The American Naturalist|volume=34|number=408| pages=943–947 |doi=10.1086/277821|doi-access=free | bibcode = 1900ANat...34..943O }}</ref>

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.<ref>{{cite journal| vauthors = Crompton AW, Jenkins Jr FA |year=1973|title=Mammals from Reptiles: A Review of Mammalian Origins|journal=Annual Review of Earth and Planetary Sciences|volume=1|pages=131–155|doi=10.1146/annurev.ea.01.050173.001023|bibcode=1973AREPS...1..131C}}</ref> Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.<ref name=schulkin/>