View source for Evolution of mammals

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{{multiple image |caption_align=center |align=right |direction=vertical |image1=Skelton of Procynosuchus delaharpeae.jpg |width1=150px |caption1=Skeleton of ''[[Procynosuchus]]'', a basal non-mammalian [[cynodont]] of the [[Permian]]|image2=Plagiolophus 1.JPG |width2=150px |caption2=Skeleton of ''[[Plagiolophus (mammal)|Plagiolophus]]'', a fossil mammal genus of the Cenozoic}}
{{Evolutionary biology}}

The [[evolution]] of [[mammals]] has passed through many stages since the first appearance of their [[synapsid]] ancestors in the [[Pennsylvanian (geology)|Pennsylvanian]] sub-period of the late [[Carboniferous]] period. By the mid-[[Triassic]], there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the [[Jurassic]]; synapsids from this period include ''[[Dryolestes]]'', more closely related to extant [[placental]]s and [[marsupial]]s than to [[monotreme]]s, as well as ''[[Ambondro mahabo|Ambondro]]'', more closely related to monotremes.<ref>{{cite journal |last1=Rougier |first1=Guillermo W. |last2=Martinelli |first2=AgustíN G. |last3=Forasiepi |first3=AnalíA M. |last4=Novacek |first4=Michael J. |title=New Jurassic Mammals from Patagonia, Argentina: A Reappraisal of Australosphenidan Morphology and Interrelationships |journal=American Museum Novitates |date=2007 |issue=3566 |pages=1 |doi=10.1206/0003-0082(2007)507[1:NJMFPA]2.0.CO;2 |hdl=2246/5857 |s2cid=51885258 |url=https://www.biodiversitylibrary.org/item/281002 }}</ref> Later on, the [[eutheria]]n and [[metatheria]]n lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since ''[[Juramaia]]'', the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.

After the [[Cretaceous–Paleogene extinction event]] wiped out the non-avian dinosaurs ([[birds]] being the only surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the [[Paleogene]] and [[Neogene]], by the end of which all modern [[Order (biology)|orders]] had appeared.

The synapsid lineage became distinct from the [[sauropsid]] lineage in the late Carboniferous period, between 320 and 315 million years ago.<ref name=Amniota /> The only living synapsids are mammals,<ref>{{cite web |url=http://www.ucmp.berkeley.edu/synapsids/synapsida.html | first = Ben | last = Waggoner | name-list-style = vanc |title=Introduction to the Synapsida |date=February 2, 1997 |publisher=University of California Museum of Paleontology |access-date=April 28, 2012}}</ref>  while the sauropsids gave rise to today's [[reptile]]s; to the [[dinosaurs]], hence in turn to birds; and to all the extinct amniotes more closely related to them than to mammals.<ref name=Amniota>{{cite web|url=http://palaeos.com/Vertebrates/Units/190Reptilomorpha/190.400.html#Amniota |title=Amniota – Palaeos | vauthors = White AT |date=May 18, 2005 |access-date=January 23, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20101220205005/http://palaeos.com/Vertebrates/Units/190Reptilomorpha/190.400.html#Amniota |archive-date=December 20, 2010 }}</ref> Primitive synapsids were traditionally called "[[mammal-like reptiles]]" or "[[pelycosaur]]s", but both are now seen as outdated and disfavored [[paraphyletic]] terms, since they were not reptiles, nor part of reptile lineage. The modern term for these is "stem mammals", and sometimes "protomammals" or "paramammals".

Throughout the [[Permian]] period, the synapsids included the dominant [[carnivore]]s and several important [[herbivore]]s. In the subsequent Triassic period, however, a previously obscure group of sauropsids, the [[archosaur]]s, became the dominant vertebrates. The [[Mammaliaformes|mammaliaforms]] appeared during this period; their superior sense of smell, backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation. (Conversely, mammaliaforms' success in these niches may have prevented archosaurs from becoming smaller or nocturnal themselves.) The nocturnal lifestyle may have contributed greatly to the development of mammalian traits such as [[endothermy]] and [[hair]]. Later in the [[Mesozoic]], after [[Theropoda|theropod dinosaurs]] replaced [[rauisuchia]]ns as the dominant carnivores, mammals spread into other [[ecological niche]]s. For example, some became [[Castorocauda|aquatic]], some were [[Volaticotherium|gliders]], and some even [[Repenomamus|fed on juvenile dinosaurs]].<ref name="Hu et al 2005">{{cite journal |last1=Hu |first1=Yaoming |last2=Meng |first2=Jin |last3=Wang |first3=Yuanqing |last4=Li |first4=Chuankui |title=Large Mesozoic mammals fed on young dinosaurs |journal=Nature |date=13 January 2005 |volume=433 |issue=7022 |pages=149–152 |doi=10.1038/nature03102 |pmid=15650737 |bibcode=2005Natur.433..149H |s2cid=2306428 |url=http://doc.rero.ch/record/15291/files/PAL_E2590.pdf }}</ref>

Most of the evidence consists of [[fossils]]. For many years, fossils of Mesozoic mammals and their immediate ancestors were very rare and fragmentary; but, since the mid-1990s, there have been many important new finds, especially in China. The relatively new techniques of [[molecular phylogeny|molecular phylogenetics]] have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.{{citation needed|date=May 2020}}

Although [[mammary glands]] are a signature feature of modern mammals, little is known about the evolution of [[lactation]] as these soft tissues are not often preserved in the fossil record. Most research concerning the evolution of mammals centers on the shapes of the teeth, the hardest parts of the [[tetrapod]] body. Other important research characteristics include the evolution of the [[evolution of mammalian auditory ossicles|middle ear bones]], erect limb posture, a bony secondary [[palate]], [[fur]], hair, and [[warm-blooded|endothermy]].{{citation needed|date=May 2020}}

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==Definition of "mammal"==
[[File:Jaw joint - mammal n non-mammal.png|thumb|Figure 1:In mammals, the [[quadrate bone|quadrate]] and [[articular]] bones are small and part of the [[middle ear]]; the [[lower jaw]] consists only of [[dentary]] bone.]]
While living mammal species can be identified by the presence of milk-producing [[mammary gland]]s in the females, other features are required in classifying [[fossils]], because mammary glands and other soft-tissue features are not visible in fossils.

One such feature available for [[paleontology]], shared by all living mammals (including [[monotremes]]), but not present in any of the early [[Triassic]] [[therapsid]]s, is shown in Figure 1 (on the right), namely: mammals use [[Ossicles|two bones]] for hearing that all other [[Amniota|amniotes]] use for eating. The earliest amniotes had a jaw joint composed of the [[articular]] (a small bone at the back of the lower jaw) and the [[quadrate bone|quadrate]] (a small bone at the back of the upper jaw). All non-mammalian [[tetrapod]]s use this system including [[amphibian]]s, [[turtle]]s, [[lizard]]s, [[snake]]s, [[crocodilia]]ns, [[dinosaur]]s (including the [[birds]]), [[Ichthyosauria|ichthyosaur]]s, [[pterosaur]]s and therapsids. But mammals have a different jaw joint, composed only of the [[dentary]] (the lower jaw bone, which carries the teeth) and the [[squamosal]] (another small skull bone). In the Jurassic, their quadrate and articular bones evolved into the [[incus]] and [[malleus]] bones in the [[middle ear]].<ref name= "Mammalia_Overview">[http://www.palaeos.com/Vertebrates/Units/430Mammalia/430.000.html Mammalia: Overview – Palaeos] {{webarchive |url=https://web.archive.org/web/20080615211326/http://www.palaeos.com/Vertebrates/Units/430Mammalia/430.000.html |date=June 15, 2008 }}</ref><ref name = cowen>{{Cite book| vauthors = Cowen R | title=History of Life | publisher=Blackwell Science | year=2000 | location=Oxford |page=432| isbn=978-0-7266-0287-0}}</ref> Mammals also have a double [[occipital condyle]]; they have two knobs at the base of the skull that fit into the topmost neck vertebra, while other tetrapods have a single occipital condyle.<ref name="Mammalia_Overview" />

In a 1981 article, Kenneth A. Kermack and his co-authors argued for drawing the line between mammals and earlier synapsids at the point where the mammalian pattern of [[Molar (tooth)|molar]] [[Occlusion (dentistry)|occlusion]] was being acquired and the dentary-squamosal joint had appeared. The criterion chosen, they noted, is merely a matter of convenience; their choice was based on the fact that "the lower jaw is the most likely skeletal element of a Mesozoic mammal to be preserved."<ref name=Morganucodon>{{Cite journal|page=148 |author1=K. A. Kermack |author2=Frances Mussett |author3=H. W. RIgney | title = The skull of ''Morganucodon'' | journal = Zoological Journal of the Linnean Society | volume = 71 | issue = 1 |date=January 1981 | doi = 10.1111/j.1096-3642.1981.tb01127.x }}</ref> Today, most paleontologists consider that animals are mammals if they satisfy this criterion.<ref name=Kemp2005/>

==The ancestry of mammals==
{{barlabel | size=6
| at1=4 |label1='''[[Pelycosaurs]]''' | bar1=Chocolate
| cladogram=
{{clade
  |label1=[[Tetrapod]]s
  |1={{clade
    |1=[[Amphibian]]s [[File:Salamandra salamandra (white background).jpg|50px]]
    |label2=[[Amniote]]s
    |2={{clade
      |1=[[Sauropsid]]s (including dinosaurs) [[File:Deinosuchus illustration Andrey Atuchin.jpg|70px]]
      |label2=[[Synapsid]]s
      |2={{clade
        |label1=[[Caseidae|Caseids]]
        |1=''[[Cotylorhynchus]]'' <span style="{{MirrorH}}">[[File:CotylorhynchusDB2.jpg|60px]]</span>|barbegin1=Chocolate
        |label2=[[Eupelycosauria|Eupelycosaurs]]
        |2={{clade
          |label1=[[Edaphosauridae|Edaphosaurids]]
          |1 =''[[Edaphosaurus]]'' <span style="{{MirrorH}}">[[File:Ed novomexicanus1DB.jpg|60px]]</span>|bar1=Chocolate
          |label2=[[Sphenacodontia]]ns
          |2={{clade
            |1={{clade
              |label1=[[Sphenacodontidae|Sphenacodontids]]
              |1=''[[Dimetrodon]]'' <span style="{{MirrorH}}">[[File:Dimetrodon grandis.jpg|60px]]</span>|barend1=Chocolate
            }}
            |2= {{clade
              |label1= [[Therapsida|Therapsids]]
              |1=[[Mammal]]s [[File:Ruskea rotta.png|50px]]
              }}
            }}
          }}
        }}
      }}
    }}
  }}
}}

===Amniotes===
The first fully terrestrial [[vertebrate]]s were reptilian [[amniotes]] — their eggs had internal membranes that allowed the developing [[embryo]] to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water (a few amphibians, such as the [[common Suriname toad]], have [[evolution|evolved]] other ways of getting around this limitation). The first amniotes apparently arose in the middle [[Carboniferous]] from the ancestral [[reptiliomorpha|reptiliomorph]]s.<ref name=Carroll>[[Robert L. Carroll|Carroll R.L.]] (1991): The origin of reptiles. In: Schultze H.-P., Trueb L., (ed) ''Origins of the higher groups of tetrapods — controversy and consensus''. Ithaca: Cornell University Press, pp 331–353.</ref>

Within a few million years, two important amniote lineages became distinct: [[synapsid]]s, from which mammals are descended, and [[sauropsid]]s, from which [[lizard]]s, snakes, turtles/tortoises, crocodilians, dinosaurs, and birds are descended.<ref name=Amniota/> The earliest known fossils of synapsids and sauropsids (such as ''[[Archaeothyris]]'' and ''[[Hylonomus]]'', respectively) date from about 320 to 315 million years ago. The times of origin are difficult to know, because vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been [[Signor–Lipps effect|considerably earlier]] than the first fossil.<ref name=Synapsida_Varanopseidae>{{Cite web| url=http://palaeos.com/vertebrates/synapsida/varanopseidae.html |access-date=15 October 2013| title=Synapsida: Varanopseidae – Palaeos}}</ref>

=== Synapsids ===
[[File:Skull synapsida 1.png|thumb|The original synapsid skull structure has one [[temporal fenestrae|hole behind each eye]], in a fairly low position on the skull (lower right in this image).]]
[[Synapsid]] skulls are identified by the distinctive pattern of the [[temporal fenestrae|holes behind each eye]], which offered the following advantages:
*made the skull lighter without sacrificing strength.
*saved energy by using less bone.
*probably provided attachment points for jaw muscles. Having attachment points further away from the jaw made it possible for the muscles to be longer and therefore to exert a strong pull over a wide range of jaw movement without being stretched or contracted beyond their optimum range.

A number of creatures often – and incorrectly – believed to be [[dinosaur]]s, hence part of the reptile lineage and sauropsids, were in fact synapsids. This includes the well-known ''[[Dimetrodon]]''.<ref name="naish">{{Cite web|url=https://blogs.scientificamerican.com/tetrapod-zoology/the-stem-mammals-a-brief-primer|title = The Stem-Mammals—a Brief Primer}}</ref><ref name="angielczyk">{{Cite journal|doi = 10.1007/s12052-009-0117-4|title = Dimetrodon is Not a Dinosaur: Using Tree Thinking to Understand the Ancient Relatives of Mammals and their Evolution|year = 2009|last1 = Angielczyk|first1 = Kenneth D.|journal = Evolution: Education and Outreach|volume = 2|issue = 2|pages = 257–271|s2cid = 24110810|doi-access = free}}</ref>

====Terms used for discussing non-mammalian synapsids====
When referring to the ancestors and close relatives of mammals, paleontologists also use the following terms of convenience:

* '''[[Pelycosaur]]s''' — all synapsids, and all of their descendants, '''except for''' [[therapsid]]s – the eventual ancestor of mammals. The pelycosaurs included the largest land vertebrates of the [[Cisuralian|Early Permian]], such as the 6 metre (20 foot)-long ''[[Cotylorhynchus hancocki]]''. Among the other large pelycosaurs were ''[[Dimetrodon grandis]]'' and ''[[Edaphosaurus cruciger]]''.
* '''Stem mammals''' (sometimes called '''protomammals''' or '''paramammals''', and previously called mammal-like reptiles) — all synapsids, and all of their descendants, '''except for''' mammals themselves.<ref name="naish"/> Stem mammals therefore include all pelycosaurs, and also all non-mammalian therapsids. Traditionally these were known as "mammal-like reptiles", but this is incorrect;<ref name="angielczyk"/> terms such as "stem mammal" are preferred instead, because these synapsids were neither reptiles nor even part of reptile lineage.<ref name="naish" /><ref name="angielczyk" />

== Therapsids ==
[[File:Bonacynodon witout visible ears.png|thumb|Reconstruction of ''[[Bonacynodon schultzi]]'', a probainognathian [[cynodont]] closely related to the ancestors of mammals<ref>{{cite news |last1=Geggel |first1=L. |title=Meet the Ancient Reptile that Gave Rise to Mammals |date=2016 |url=https://www.scientificamerican.com/article/meet-the-ancient-reptile-that-gave-rise-to-mammals/ |work=Scientific American |language=en}}</ref>]]
[[Therapsid]]s descended from  [[Sphenacodontia|sphenacodonts]], a group of primitive synapsids, in the [[Guadalupian|middle Permian]], and took over from them as the dominant land vertebrates. They differ from earlier synapsids in several features of the skull and jaws, including larger [[temporal fenestrae]] and [[incisor]]s that are equal in size.<ref name=Therapsida>{{Cite web | url=http://www.palaeos.com/Vertebrates/Units/400Therapsida/100.html | title=Therapsida |work=Palaeos | url-status=dead | archive-url=https://web.archive.org/web/20070415120055/http://www.palaeos.com/Vertebrates/Units/400Therapsida/100.html | archive-date=2007-04-15 }}</ref>

The therapsid lineage then went through several stages, leading to the evolution of [[cynodont]]s in the late Permian, some of which had begun to resemble early mammals:<ref name="Kermack1984">{{Cite book| vauthors = Kermack DM, Kermack KA | title=The evolution of mammalian characters | publisher=Croom Helm | year=1984 | isbn=978-0709915348}}{{page needed|date=August 2021}}</ref>
*gradual development of a bony secondary [[hard palate|palate]]. Most books and articles interpret this as a prerequisite for the evolution of mammals' high [[metabolic rate]], because it enabled these animals to eat and breathe at the same time. But some scientists point out that some modern [[ectotherm]]s use a fleshy secondary palate to separate the mouth from the airway, and that a ''bony'' palate provides a surface on which the tongue can manipulate food, facilitating chewing rather than breathing.<ref name=Bennet_Ruben>{{cite book |last1=Bennett |first1=Albert F. |last2=Ruben |first2=John A. |chapter=The Metabolic and Thermoregulatory Status of Therapsids |pages=207–218 |chapter-url=https://compphys.bio.uci.edu/bennett/pubs/74.pdf |citeseerx=10.1.1.694.3599 |editor1-last=Hotton |editor1-first=Nicholas |editor2-last=MacLean |editor2-first=Paul D. |editor3-last=Roth |editor3-first=Jan J. |editor4-last=Roth |editor4-first=E. Carol |title=The Ecology and biology of mammal-like reptiles |date=1986 |publisher=Smithsonian Institution Press |oclc=775982686 |isbn=978-0-87474-524-5 }}</ref> The interpretation of the bony secondary palate as an aid to chewing also suggests the development of a faster metabolism, because chewing reduces the size of food particles delivered to the stomach and can therefore speed their digestion. In mammals, the palate is formed by two specific bones, but various Permian therapsids had other combinations of bones in the right places to function as a palate.
*the [[dentary]] gradually becomes the main bone of the lower jaw.

===Therapsid family tree===
A simplified phylogenetic tree showing only what is most relevant to the evolution of mammals<ref name=Therapsida/> is shown below:

{{clade| style=font-size:90%;line-height:100%
  |label1=[[Therapsids]]
  |1={{clade
    |1=[[Biarmosuchia]][[File:Biarmosuchus BW.jpg|50px]]
    |label2=Eutherapsida
    |2={{clade
      |1={{clade
        |1={{clade
          |1=[[Dinocephalia]][[File:Archaeosyodon praeventor.jpg|50px]]
          |label2=[[Neotherapsida]]
          |2={{clade
            |label1=[[Anomodont]]s
            |1={{clade
              |1=[[Dicynodont]]s [[File:Eodicynodon BW.jpg|50px]]
            }}
            |label2=[[Theriodontia]]
            |2={{clade
              |1=[[Gorgonopsia]][[File:Sauroctonus progressus09.jpg|50px]]
              |label2=Eutheriodontia
              |2={{clade
                |1=[[Therocephalia]] [[File:Scylacosaurus.jpg|50px]]
                |label2=[[Cynodontia]]
                |2={{clade
                  |1=(Mammals, eventually) [[File:Ruskea rotta.png|50px]]
                }}
              }}
            }}
          }}
        }}
      }}
    }}
  }}
}}

Only the dicynodonts, therocephalians, and cynodonts survived into the Triassic.

===Biarmosuchia===
The [[Biarmosuchia]] were the most primitive and pelycosaur-like of the therapsids.<ref name=Biarmosuchia>{{cite web|title=Therapsida: Biarmosuchia |work=Palaeos|url=http://palaeos.com/vertebrates/therapsida/biarmosuchia.html|access-date=16 October 2013}}</ref>

===Dinocephalians===
[[Dinocephalia]]ns ("terrible heads") included both carnivores and herbivores. They were large; ''[[Anteosaurus]]'' was up to {{cvt|20|ft}} long. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs. In many ways they were very primitive therapsids; for example, they had no secondary palate and their jaws were rather "reptilian".<ref name=Dinocephalia>{{Cite web|url=http://palaeos.com/vertebrates/therapsida/dinocephalia.html | title=Therapsida: Dinocephalia|website= Palaeos}}</ref>

===Anomodonts===
[[File:Lystrosaurus 1.JPG|thumb|''[[Lystrosaurus]]'', one of the few genera of dicynodonts that survived the [[Permian–Triassic extinction event]]]]
The [[Anomodontia|anomodonts]] ("anomalous teeth") were among the most successful of the herbivorous therapsids — one sub-group, the [[dicynodont]]s, survived to the end of the Triassic. But dicynodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw (lost in some derived [[kannemeyeriiformes]]) and it is generally agreed that they had beaks like those of birds or [[ceratopsian]]s.
<ref name=Anomodontia>{{cite web|title=Ammodontia |work=Palaeos|url=http://palaeos.com/vertebrates/therapsida/anomodontia.html|access-date=16 October 2013}}</ref>

===Theriodonts===
The [[theriodonts]] ("beast teeth") and their descendants had jaw joints in which the [[articular]] bone of the lower jaw tightly gripped the very small [[quadrate bone]] of the skull. This allowed a much wider gape and allowed one group, the carnivorous [[gorgonopsia]]ns ("gorgon faces"), to develop "[[saber-toothed predator|sabre teeth]]". However, the jaw hinge of the theriodont had a longer term significance — the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear.

The gorgonopsians still had some primitive features: no bony secondary palate (other bones in the right places perform the same functions); sprawling forelimbs; hindlimbs that could operate in both sprawling and erect postures. The [[therocephalia]]ns ("beast heads"), which appear to have arisen at about the same time as the gorgonopsians, had additional mammal-like features, e.g. their finger and toe bones had the same number of phalanges (segments) as in early mammals (and the same number that [[primates]] have, including humans).<ref name=Theriodontia>{{Cite web| url=http://palaeos.com/vertebrates/therapsida/theriodontia.html|access-date=2013-10-15| title=Theriodontia – Paleos }}</ref> Numerous [[Changhsingian]] [[coprolites]] that possibly belong to therocephalians and indeterminate basal [[archosaurs]] ([[Proterosuchidae|proterosuchids]]) contain elongated hollow structures that could be remains of [[hair]]. That means therapsids were covered in hair as early as 252 million years ago.<ref>{{cite book | year=2016 | first1=A. G. | last1=Sennikov | first2=V. K. | last2=Golubev | first3=G. | last3=Niedzwiedzki | first4=P. | last4=Bajdek | first5=K. | last5=Owocki | first6=T. | last6=Sulej | url=http://www.ginras.ru/materials/files/paleostrat2016.pdf#page=73|title=Древнейшая находка остатков волос в копролитах тетрапод из терминальной перми Владимирской области | trans-title=The earliest known remains of hair are tetrapod coprolites in Vladimir Oblast from the Changhsingian | language=ru | publication-place=Moscow | publisher=[[Paleontological Institute, Russian Academy of Sciences]] | pages=71–72 | archive-url=https://web.archive.org/web/20231103220552/http://www.ginras.ru/materials/files/paleostrat2016.pdf#page=73 | archive-date=2023-11-03 | url-status=live}}</ref>

====Cynodonts====
{{Further|Nocturnal bottleneck}}
[[File:Trirachodon.jpg|thumb|Artist's conception of the cynodont ''[[Trirachodon]]'' within a burrow]]
The [[Cynodontia|cynodonts]], a theriodont group that also arose in the late [[Permian]], include the ancestors of all mammals. Cynodonts' mammal-like features include further reduction in the number of bones in the lower jaw, a secondary bony palate, cheek teeth with a complex pattern in the crowns, and a brain which filled the endocranial cavity.<ref name=Cynodontia_Overview>{{Cite web| url=http://palaeos.com/vertebrates/cynodontia/overview.html | title=Cynodontia Overview |work=Palaeos}}</ref>

Multi-chambered burrows have been found, containing as many as 20 skeletons of the Early Triassic cynodont ''[[Trirachodon]]''; the animals are thought to have been drowned by a [[flash flood]]. The extensive shared burrows indicate that these animals were capable of complex social behaviors.<ref>{{cite journal |last1=Groenewald |first1=Gideon H. |last2=Welman |first2=Johann |last3=Maceachern |first3=James A. |title=Vertebrate Burrow Complexes from the Early Triassic Cynognathus Zone (Driekoppen Formation, Beaufort Group) of the Karoo Basin, South Africa |journal=PALAIOS |date=1 April 2001 |volume=16 |issue=2 |pages=148–160 |doi=10.1669/0883-1351(2001)016<0148:VBCFTE>2.0.CO;2 |bibcode=2001Palai..16..148G |s2cid=140537804 }}</ref>

Their primitive synapsid and therapsid ancestors were very large (between {{cvt|5|–|8|ft}}) but cynodonts gradually decreased in size (to {{cvt|1.5|–|5|ft}}) even before the [[Permian-Triassic extinction event]], probably due to competition with other therapsids. After the extinction event, the [[probainognathia]]n cynodont group rapidly decreased in size (to {{cvt|4|–|18|in}}) due to new competition with [[archosaurs]] and  transitioned to [[nocturnal bottleneck|nocturnality]], evolving nocturnal features, [[pulmonary alveoli]], [[bronchioles]] and a developed [[Thoracic diaphragm|diaphragm]] for a larger surface area for breathing, enucleated [[erythrocytes]], a [[large intestine]] which bears a true colon after the [[cecum]], [[endothermy]], a hairy, glandular and thermoregulatory skin (which releases [[sebum]] and [[sweat]]), and a 4-chambered [[heart]] to maintain their high metabolism, larger brains, and fully upright hindlimb (forelimbs remained semi sprawling, and became like that only later, in [[therians]]). Some skin glands may have evolved into [[mammary glands]] in females for fulfilling the metabolic demands of their offspring (which increased 10 times). Many skeletal changes occurred also: the [[dentary]] bone became stronger and held differentiated teeth, for example, and the pair of nasal openings in the skull became fused.

These evolutionary changes led to the first mammals (size around {{cvt|4|in}}). They appear to have evolved rapid growth and short lifespan, a life history trait also found in numerous modern small-bodied mammals.<ref>{{cite journal |last1=Funston |first1=Gregory F. |last2=dePolo |first2=Paige E. |last3=Sliwinski |first3=Jakub T. |last4=Dumont |first4=Matthew |last5=Shelley |first5=Sarah L. |last6=Pichevin |first6=Laetitia E. |last7=Cayzer |first7=Nicola J. |last8=Wible |first8=John R. |last9=Williamson |first9=Thomas E. |last10=Rae |first10=James W. B. |last11=Brusatte |first11=Stephen L. |title=The origin of placental mammal life histories |journal=Nature |date=6 October 2022 |volume=610 |issue=7930 |pages=107–111 |doi=10.1038/s41586-022-05150-w |pmid=36045293 |bibcode=2022Natur.610..107F |hdl=10023/27089 |s2cid=260606152 |hdl-access=free }}</ref> They also adapted to a [[fossorial|burrowing]] lifestyle, losing their large tail-based leg muscles which allowed dinosaurs to become bipedal, which may explain why bipedal mammals are so rare.<ref>{{Cite web |url=https://www.zmescience.com/science/news-science/dino-two-feet-mammals-four/ |title=New paper explains why predatory dinosaurs walked on two feet while mammals stayed on all fours |date=3 March 2017 |access-date=2022-02-02 |archive-date=2022-02-02 |archive-url=https://web.archive.org/web/20220202215306/https://www.zmescience.com/science/news-science/dino-two-feet-mammals-four/ |url-status=live }}</ref>

==Triassic takeover==
[[File:Purbeck lagoon.jpg|thumb|Artist's impression of the Purbeck lagoon at dusk: ''[[Durlstotherium]]'' (right and center foreground) and ''[[Durlstodon]]'' (left foreground) ventured out at night to hunt insects.]]
The catastrophic [[Permian–Triassic extinction event|mass extinction at the end of the Permian]], around 252 million years ago, killed off about 70% of [[Terrestrial ecoregion|terrestrial]] [[Chordate|vertebrate]] species and the majority of land plants.

As a result,<ref name=Olenekian>{{Cite web| url=http://www.palaeos.com/Mesozoic/Triassic/Olenekian.html| title=Olenekian Age of the Triassic – Palaeos| access-date=2007-04-18| archive-url=https://web.archive.org/web/20070416040544/http://www.palaeos.com/Mesozoic/Triassic/Olenekian.html| archive-date=2007-04-16| url-status=dead}}</ref> [[ecosystems]] and [[food chains]] collapsed, and the establishment of new stable ecosystems took about 30 million years. With the disappearance of the gorgonopsians, which were dominant predators in the late Permian,<ref name="Benton">{{Cite book | vauthors = Benton MJ |year=2004 |title=Vertebrate Palaeontology |edition=3rd |location=Oxford |publisher=Blackwell Science |isbn=978-0-632-05637-8 }}</ref> the cynodonts' principal competitors for dominance of the carnivorous niches were a previously obscure sauropsid group, the [[archosaur]]s, which includes the ancestors of crocodilians and dinosaurs.

The archosaurs quickly became the dominant carnivores,<ref name="Benton" /> a development often called the "Triassic takeover". Their success may have been due to the fact that the early Triassic was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage. All known archosaurs have glandless skins and eliminate [[Metabolic waste#Nitrogen wastes|nitrogenous waste]] in a [[uric acid]] paste containing little water, while the cynodonts probably excreted most such waste in a solution of [[urea]], as mammals do today; considerable water is required to keep urea dissolved.<ref>{{cite book| vauthors = Campbell JW |title=Comparative Animal Physiology|year=1979|publisher=W. B. Sauders|edition=3rd| veditors = Prosser CL |pages=279–316}}</ref>

However, this theory has been questioned, since it implies synapsids were necessarily less advantaged in water retention, that synapsid decline coincides with [[climate change]]s or archosaur diversity (neither of which has been tested) and the fact that desert-dwelling mammals are as well adapted in this department as archosaurs,<ref>[[Darren Naish]], [http://tetzoo.com/podcast/2015/1/22/episode-38-a-not-too-shabby-podcarts Episode 38: A Not Too Shabby Podcarts] {{Webarchive|url=https://web.archive.org/web/20160127144449/http://tetzoo.com/podcast/2015/1/22/episode-38-a-not-too-shabby-podcarts |date=2016-01-27 }}</ref> and some cynodonts like ''[[Trucidocynodon]]'' were large-sized predators.<ref>{{cite journal | vauthors = Oliveira TV, Soares MB, Schultz CL | year = 2010 | title = Trucidocynodon riograndensis gen. nov. et sp. nov. (Eucynodontia), a new cynodont from the Brazilian Upper Triassic (Santa Maria Formation) | journal = Zootaxa | volume = 2382 | pages = 1–71 | doi = 10.11646/zootaxa.2382.1.1 }}</ref>

The Triassic takeover was probably a vital factor in the evolution of the mammals. Two groups stemming from the early cynodonts were successful in niches that had minimal competition from the archosaurs: the [[Tritylodontidae|tritylodonts]], which were [[herbivore]]s, and the mammals, most of which were small nocturnal [[insectivore]]s (although some, like ''[[Sinoconodon]]'', were carnivores that fed on vertebrate prey, while still others were herbivores or [[omnivore]]s).{{sfn|Kielan-Jaworowska|Cifelli|Luo|2004|p=5}} As a result:
[[File:Aquilops americanus restoration.jpg|thumb|Cloverly formation environment with ''[[Gobiconodon]]'' pursuing juvenile ''[[Aquilops]]'' while adults attempt to protect them.]]
*The therapsid trend towards differentiated teeth with precise [[occlusion (dentistry)|occlusion]] accelerated, because of the need to hold captured [[arthropod]]s and crush their [[exoskeleton]]s.
*As the body length of the mammals' ancestors fell below {{cvt|4|in}}, advances in [[thermal insulation]] and [[temperature regulation]] would have become necessary for nocturnal life.<ref name="RubenJones2000FurAndFeathers">{{cite journal |last1=Ruben |first1=John A. |last2=Jones |first2=Terry D. |title=Selective Factors Associated with the Origin of Fur and Feathers |journal=American Zoologist |date=August 2000 |volume=40 |issue=4 |pages=585–596 |doi=10.1093/icb/40.4.585 |doi-access=free }}</ref>
*Acute senses of hearing and smell became vital.
**This accelerated the development of the mammalian middle ear (though the complete detachment of the middle ear bones from the jaw happened independently in [[monotremes]]).
**The increase in the size of the olfactory lobes of the brain increased brain weight as a percentage of total body weight.<ref>{{cite journal | vauthors = Rowe TB, Macrini TE, Luo ZX | title = Fossil evidence on origin of the mammalian brain | journal = Science | volume = 332 | issue = 6032 | pages = 955–7 | date = May 2011 | pmid = 21596988 | doi = 10.1126/science.1203117 | bibcode = 2011Sci...332..955R | s2cid = 940501 }}</ref> Brain tissue requires a disproportionate amount of energy.<ref>{{cite journal | vauthors = Raichle ME, Gusnard DA | title = Appraising the brain's energy budget | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 99 | issue = 16 | pages = 10237–9 | date = August 2002 | pmid = 12149485 | pmc = 124895 | doi = 10.1073/pnas.172399499 | bibcode = 2002PNAS...9910237R | doi-access = free }}</ref><ref>{{cite magazine| url= https://www.newscientist.com/blog/shortsharpscience/2006/09/brain-power.html | title=Brain power | date=13 September 2006 | magazine=New Scientist|archive-url =https://web.archive.org/web/20090415221810/https://www.newscientist.com/blog/shortsharpscience/2006/09/brain-power.html |archive-date = 15 April 2009|first = Roxanne |last =Khamsi}}</ref> The need for more food to support the enlarged brains increased the pressures for improvements in insulation, temperature regulation and feeding.
*Probably as a side-effect of the nocturnal life, mammals lost two of the four cone [[opsins]], photoreceptors in the [[retina]], present in the eyes of the earliest amniotes. Paradoxically, this might have improved their ability to discriminate colors in dim light.<ref>{{cite journal |last1=Vorobyev |first1=M. |year=2006 |title=Evolution of colour vision: The story of lost visual pigments |journal=Perception |volume=35 |url=http://www.perceptionweb.com/abstract.cgi?id=v060037 |access-date=2012-01-26 |archive-url=https://web.archive.org/web/20141006111136/http://www.perceptionweb.com/abstract.cgi?id=v060037 |archive-date=2014-10-06 |url-status=dead }}</ref>

This retreat to a nocturnal role is called a [[nocturnal bottleneck]], and is thought to explain many of the features of mammals.<ref>{{cite journal | vauthors = Gerkema MP, Davies WI, Foster RG, Menaker M, Hut RA | title = The nocturnal bottleneck and the evolution of activity patterns in mammals | journal = Proceedings. Biological Sciences | volume = 280 | issue = 1765 | pages = 20130508 | date = August 2013 | pmid = 23825205 | pmc = 3712437 | doi = 10.1098/rspb.2013.0508 }}</ref>

==From cynodonts to crown mammals==

===Fossil record===
Mesozoic synapsids that had evolved to the point of having a jaw joint composed of the dentary and squamosal bones are preserved in few good fossils, mainly because they were mostly smaller than rats:
*They were largely restricted to environments that are less likely to provide good [[fossils]]. [[Floodplains]] as the best terrestrial environments for fossilization provide few mammal fossils, because they are dominated by medium to large animals, and the mammals could not compete with [[archosaurs]] in the medium to large size range.
*Their delicate bones were vulnerable to being destroyed before they could be fossilized — by scavengers (including [[fungi]] and [[bacteria]]) and by being trodden on.
*Small fossils are harder to spot and more vulnerable to being destroyed by weathering and other natural stresses before they are discovered.
In the past {{roundup|{{age|format=raw|1977|3|11}}|-1}} years,<!-- Luo is citing Lillegraven et al. "Mesozoic Mammals: The First Two-thirds of Mammalian History"; its preface is dated March 11, 1977, the date used by the {{age}} template. --> however, the number of Mesozoic fossil mammals has increased decisively; only 116 genera were known in 1979, for example, but about 310 in 2007, with an increase in quality such that "at least 18 Mesozoic mammals are represented by nearly complete skeletons".<ref name=Luo2007>{{cite journal |last1=Luo |first1=Zhe-Xi |title=Transformation and diversification in early mammal evolution |journal=Nature |date=December 2007 |volume=450 |issue=7172 |pages=1011–1019 |doi=10.1038/nature06277 |pmid=18075580 |bibcode=2007Natur.450.1011L |s2cid=4317817 }}</ref>

===Mammals or mammaliaforms===
Some writers restrict the term "mammal" to the [[crown group]] mammals, the group consisting of the most recent common ancestor of the [[monotremes]], [[marsupials]], and [[placentals]], together with all the descendants of that ancestor. In an influential 1988 paper, Timothy Rowe advocated this restriction, arguing that "ancestry... provides the only means of properly defining taxa" and, in particular, that the divergence of the monotremes from the animals more closely related to marsupials and placentals "is of central interest to any study of Mammalia as a whole."<ref>{{cite journal |last1=Rowe |first1=Timothy |title=Definition, diagnosis, and origin of Mammalia |journal=Journal of Vertebrate Paleontology |date=23 September 1988 |volume=8 |issue=3 |pages=241–264 |doi=10.1080/02724634.1988.10011708 |bibcode=1988JVPal...8..241R }}</ref> To accommodate some related taxa falling outside the crown group, he defined the [[Mammaliaformes]] as comprising "the last common ancestor of Morganucodontidae and Mammalia [as he had defined the latter term] and all its descendants." Besides Morganucodontidae, the newly defined taxon includes Docodonta and Kuehneotheriidae. Though haramiyids have been referred to the mammals since the 1860s,<ref>{{cite web |url=https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=146454 |title=''Microlestes rhaeticus'' Dawkins 1864 (mammal) |work=Paleobiology Database |access-date=January 30, 2012}}</ref> Rowe excluded them from the Mammaliaformes as falling outside his definition, putting them in a larger clade, the [[Mammaliaformes|Mammaliamorpha]].

Some writers have adopted this terminology noting, to avoid misunderstanding, that they have done so. Most paleontologists, however, still think that animals with the dentary-squamosal jaw joint and the sort of molars characteristic of modern mammals should formally be members of Mammalia.<ref name=Kemp2005>{{cite book |title=The Origin and Evolution of Mammals |vauthors = Kemp TS | year=2005 |publisher=Oxford University Press |isbn=978-0-19-850760-4 |page=3}}</ref>

Where the ambiguity in the term "mammal" may be confusing, this article uses "mammaliaform" and "crown mammal".

===Family tree – cynodonts to crown group mammals===
(based on Cynodontia:Dendrogram – Palaeos<ref>{{cite web | url=http://palaeos.com/vertebrates/cynodontia/dendrogram.html | title=Palaeos Vertebrates Cynodontia: Dendrogram }}</ref>)

{{clade| style=font-size:90%;line-height:100%
  |label1=[[Cynodontia]]
  |1={{clade
    |1={{clade
      |1=''[[Dvinia]]''
      |2=[[Procynosuchidae]]
    }}
    |label2=[[Epicynodontia]]
    |2={{clade
      |1=''[[Thrinaxodon]]''
      |label2=[[Eucynodontia]]
      |2={{clade
        |1={{clade
          |1=''[[Cynognathus]]''
          |2={{clade
            |1=[[Traversodontidae]]
            |2=[[Tritylodontidae]]
          }}
        }}
        |label2=[[Probainognathia]]
        |2={{clade
          |1={{clade
            |1=[[Chiniquodontidae]]
            |2=[[Tritheledontidae]]
          }}
          |2={{clade
            |1=''[[Prozostrodon]]''
            |label2=[[Mammaliaformes]]
            |2={{clade
              |1=[[Morganucodon]]tidae
              |2={{clade
                |1=[[Docodonta]]
                |2={{clade
                  |1=''[[Hadrocodium]]''
                  |2={{clade
                    |1=[[Kuehneotheriidae]]
                    |2=[[crown group]] [[Mammals]]
                  }}
                }}
              }}
            }}
          }}
        }}
      }}
    }}
  }}
}}

[[File:Jaw joint - double.png|thumb|[[Morganucodon]]tidae and other transitional forms had both types of jaw joint: [[dentary]]-[[squamosal]] (front) and [[articular]]-[[quadrate bone|quadrate]] (rear).]]

===Morganucodontidae===
The [[Morganucodon]]tidae first appeared in the late Triassic, about 205 million years ago. They are an excellent example of transitional fossils, since they have both the dentary-squamosal and articular-quadrate jaw joints.<ref>{{Cite web | url=http://www.palaeos.com/Vertebrates/Units/Unit420/420.200.html | title=Morganucodontids & Docodonts – Palaeos | url-status=dead | archive-url=https://web.archive.org/web/20070416120501/http://www.palaeos.com/Vertebrates/Units/Unit420/420.200.html | archive-date=2007-04-16 }}</ref> They were also one of the first discovered and most thoroughly studied of the [[Mammaliaformes|mammaliaforms]] outside of the [[crown-group]] mammals, since an unusually large number of morganucodont fossils have been found.

===Docodonts===
[[File:Castorocauda BW.jpg|thumb|Reconstruction of ''[[Castorocauda]]''. Note the fur and the adaptations for swimming (broad, flat tail; webbed feet) and for digging (robust limbs and claws).]]
[[Docodonta|Docodonts]], among the most common Jurassic mammaliaforms, are noted for the sophistication of their molars. They are thought to have had general semi-aquatic tendencies, with the fish-eating ''[[Castorocauda]]'' ("beaver tail"), which lived in the mid-[[Jurassic]] about 164M years ago and was first discovered in 2004 and described in 2006, being the most well-understood example. ''Castorocauda'' was not a crown group mammal, but it is extremely important in the study of the evolution of mammals because the first find was an almost complete skeleton (a real luxury in paleontology) and it breaks the "small nocturnal insectivore" stereotype:<ref name="JiLuoYuanTabrumCastorocauda">{{cite journal | vauthors = Ji Q, Luo ZX, Yuan CX, Tabrum AR | title = A swimming mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals | journal = Science | volume = 311 | issue = 5764 | pages = 1123–7 | date = February 2006 | pmid = 16497926 | doi = 10.1126/science.1123026 | bibcode = 2006Sci...311.1123J | s2cid = 46067702 | url = http://doc.rero.ch/record/13437/files/PAL_E249.pdf }} See also the news item at {{Cite web| url=http://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html | title=Jurassic "Beaver" Found; Rewrites History of Mammals}}</ref>
*It was noticeably larger than most Mesozoic mammaliaform fossils — about {{cvt|17|in}} from its nose to the tip of its {{cvt|5|in}} tail, and may have weighed {{cvt|500|-|800|g}}.
*It provides the earliest absolutely certain evidence of hair and fur. Previously the earliest was ''[[Eomaia]]'', a crown group mammal from about 125M years ago.
*It had aquatic adaptations including flattened tail bones and remnants of soft tissue between the toes of the back feet, suggesting that they were webbed. Previously the earliest known semi-aquatic mammaliaforms were from the [[Eocene]], about 110M years later.
*''Castorocauda'''s powerful forelimbs look adapted for digging. This feature and the spurs on its ankles make it resemble the [[platypus]], which also swims and digs.
*Its teeth look adapted for eating fish: the first two molars had cusps in a straight row, which made them more suitable for gripping and slicing than for grinding; and these molars are curved backwards, to help in grasping slippery prey.

===''Hadrocodium''===
[[File:Hadrocodium-000.jpg| thumb | right | ''[[Hadrocodium]]'' skull. The jaw joint is fully mammalian ([[squamosal]]-[[dentary]] only) and farther forward than in earlier transitional forms.]]
The family tree above shows ''[[Hadrocodium]]'' as a close relative of crown-group mammals. This mammaliaform, dated about 195 million years ago in the very early Jurassic, exhibits some important features:<ref>{{cite journal |last1=Luo |first1=Zhe-Xi |last2=Crompton |first2=Alfred W. |last3=Sun |first3=Ai-Lin |title=A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics |journal=Science |date=25 May 2001 |volume=292 |issue=5521 |pages=1535–1540 |doi=10.1126/science.1058476 |pmid=11375489 |bibcode=2001Sci...292.1535L |s2cid=8738213 }}</ref>

*The jaw joint consists only of the squamosal and dentary bones, and the jaw contains no smaller bones to the rear of the dentary, unlike the therapsid design.
*In [[therapsids]] and early mammaliaforms, the [[eardrum]] may have stretched over a trough at the rear of the lower jaw. But ''Hadrocodium'' had no such trough, which suggests its ear was part of the [[cranium]], as it is in crown-group mammals — and hence that the former [[articular]] and [[quadrate bone|quadrate]] had migrated to the middle ear and become the [[malleus]] and [[incus]]. On the other hand, the dentary has a "bay" at the rear that mammals lack. This suggests that ''Hadrocodium's'' dentary bone retained the same shape that it would have had if the articular and quadrate had remained part of the jaw joint, and therefore that ''Hadrocodium'' or a very close ancestor may have been the first to have a fully mammalian middle ear.
*Therapsids and earlier mammaliaforms had their jaw joints very far back in the skull, partly because the ear was at the rear end of the jaw but also had to be close to the brain. This arrangement limited the size of the braincase, because it forced the jaw muscles to run round and over it. ''Hadrocodium's'' braincase and jaws were no longer bound to each other by the need to support the ear, and its jaw joint was further forward. In its descendants or those of animals with a similar arrangement, the brain case was free to expand without being constrained by the jaw and the jaw was free to change without being constrained by the need to keep the ear near the brain — in other words it now became possible for mammaliaforms both to develop large brains and to adapt their jaws and teeth in ways that were purely specialized for eating.

===Kuehneotheriidae===
The family [[Kuehneotheriidae]], known from the [[Late Triassic]] and [[Early Jurassic]], was originally classified as part of either '[[Symmetrodonta]]' or '[[Pantotheria]]' based on their tooth structure, with ''[[Kuehneotherium]]'' once being considered the oldest known representative of [[Theria]]. They have since been recovered as among the closest relatives of crown-group mammals. As only tooth fossils have been discovered, they nevertheless remain poorly known, and have rarely been included in [[Phylogeny|phylogenetic]] studies.<ref>{{cite journal |last=Debuysschere |first=Maxime |date=2016 |title=The Kuehneotheriidae (Mammaliaformes) from Saint-Nicolas-de-Port (Upper Triassic, France): a Systematic Review |journal=Journal of Mammalian Evolution |volume=24 |issue=2 |pages=127–146 |doi=10.1007/s10914-016-9335-z |url=https://link.springer.com/article/10.1007/s10914-016-9335-z|url-access=subscription }}</ref>

==Earliest crown mammals==
The [[crown group]] mammals, sometimes called 'true mammals', are the [[extant taxon|extant]] mammals and their close relatives back to their last common ancestor. Since this group has living members, [[DNA]] analysis can be applied in an attempt to explain the evolution of features that do not appear in fossils. This endeavor often involves [[molecular phylogenetics]], a technique that has become popular since the mid-1980s.

===Family tree of early crown mammals===
Cladogram after Z.-X Luo<ref name=Luo2007 /> († marks extinct groups) and Hackländer.<ref>{{Cite book |title=Mammals of Europe: past, present, and future |date=2020 |publisher=Springer |isbn=978-3-030-00280-0 |editor-last=Hackländer |editor-first=Klaus |series=Handbook of the mammals of Europe |location=Cham, Switzerland |editor-last2=Zachos |editor-first2=Frank E.}}</ref>

{{clade| style=font-size:90%;line-height:100%
  |label1=[[Crown group]] [[mammal]]s
  |1={{clade
    |label1=[[Australosphenida]]
    |1={{clade
      |1=[[Ausktribosphenidae]] †
      |2=[[Monotremes]]
    }}
    |2={{clade
       |1=[[Triconodonta|Eutriconodonta]] †
       |2={{clade
         |label1=[[Allotheria]]
         |1=[[Multituberculates]] †
         |label2=[[Trechnotheria]]
         |2={{clade
           |1=[[Spalacotheroidea]] †
           |label2=[[Cladotheria]]
           |2={{clade
             |1=[[Dryolestoidea]] †
             |label2=[[Theria]]
             |2={{clade
               |label1=[[Metatheria]]
               |1=[[Marsupials]]
               |label2=[[Eutheria]]
               |2=[[Placentals]]
            }}
          }}
        }}
      }}
    }}
  }}
}}

===Color vision===
Early amniotes had four [[opsin]]s in the cones of their retinas to use for distinguishing colours: one sensitive to red, one to green, and two corresponding to different shades of blue.<ref name=Jacobs2009>{{cite journal |last1=Jacobs |first1=Gerald H. |title=Evolution of colour vision in mammals |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=12 October 2009 |volume=364 |issue=1531 |pages=2957–2967 |doi=10.1098/rstb.2009.0039 |pmid=19720656 |pmc=2781854 }}</ref><ref>{{cite journal |last1=Bailes |first1=Helena J |last2=Davies |first2=Wayne L |last3=Trezise |first3=Ann EO |last4=Collin |first4=Shaun P |title=Visual pigments in a living fossil, the Australian lungfish Neoceratodus forsteri |journal=BMC Evolutionary Biology |date=2007 |volume=7 |issue=1 |pages=200 |doi=10.1186/1471-2148-7-200 |pmid=17961206 |pmc=2194722 |doi-access=free |bibcode=2007BMCEE...7..200B }}</ref> The green opsin was not inherited by any crown mammals, but all normal individuals did inherit the red one. Early crown mammals thus had three cone opsins, the red one and both of the blues.<ref name=Jacobs2009 /> All their extant descendants have lost one of the blue-sensitive opsins but not always the same one: monotremes retain one blue-sensitive opsin, while marsupials and placentals retain the other (except [[cetacea]]ns, which later lost the other blue opsin as well).<ref>{{cite journal |last1=Collin |first1=Shaun P. |title=Evolution and Ecology of Retinal Photoreception in Early Vertebrates |journal=Brain, Behavior and Evolution |date=2010 |volume=75 |issue=3 |pages=174–185 |doi=10.1159/000314904 |pmid=20733293 |s2cid=22974478 }}</ref> Some placentals and marsupials, including higher primates, subsequently evolved green-sensitive opsins; like early crown mammals, therefore, their vision is [[Trichromacy|trichromatic]].<ref>{{cite journal |last1=Dulai |first1=Kanwaljit S. |last2=Dornum |first2=Miranda von |last3=Mollon |first3=John D. |last4=Hunt |first4=David M. |title=The Evolution of Trichromatic Color Vision by Opsin Gene Duplication in New World and Old World Primates |journal=Genome Research |date=1 July 1999 |volume=9 |issue=7 |pages=629–638 |doi=10.1101/gr.9.7.629 |pmid=10413401 |s2cid=10637615 |url=http://genome.cshlp.org/cgi/pmidlookup?view=long&pmid=10413401 |doi-access=free }}</ref><ref>{{cite journal |last1=Arrese |first1=Catherine A. |last2=Hart |first2=Nathan S. |last3=Thomas |first3=Nicole |last4=Beazley |first4=Lyn D. |last5=Shand |first5=Julia |title=Trichromacy in Australian Marsupials |journal=Current Biology |date=April 2002 |volume=12 |issue=8 |pages=657–660 |doi=10.1016/s0960-9822(02)00772-8 |pmid=11967153 |s2cid=14604695 |doi-access=free |bibcode=2002CBio...12..657A }}</ref>

===Australosphenida and Ausktribosphenidae===
[[Ausktribosphenidae]] is a group name that has been given to some rather puzzling finds that:<ref name=Mammalia>{{Cite web | url=http://www.palaeos.com/Vertebrates/Units/430Mammalia/430.100.html | title=Mammalia – Palaeos | url-status=dead | archive-url=https://web.archive.org/web/20070412123957/http://www.palaeos.com/Vertebrates/Units/430Mammalia/430.100.html | archive-date=2007-04-12 }}</ref>
*appear to have [[tribosphenic molar]]s, a type of tooth that is otherwise known only in placentals and marsupials.<ref name="JacobsWinklerMurry1989MammalOriginsEvolutionary Grades">{{cite journal | vauthors = Jacobs LL, Winkler DA, Murry PA | title = Modern mammal origins: evolutionary grades in the Early Cretaceous of North America | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 86 | issue = 13 | pages = 4992–5 | date = July 1989 | pmid = 2740336 | pmc = 297542 | doi = 10.1073/pnas.86.13.4992 | bibcode = 1989PNAS...86.4992J | jstor = 34031 | doi-access = free }}</ref>
*come from mid-[[Cretaceous]] deposits in Australia — but Australia was connected only to Antarctica, and placentals originated in the Northern Hemisphere and were confined to it until [[continental drift]] formed land connections from North America to South America, from Asia to Africa and from Asia to India.
*are represented only by teeth and jaw fragments, which is not very helpful.
[[Australosphenida]] is a group that has been defined in order to include the Ausktribosphenidae and [[monotremes]]. ''Asfaltomylos'' (mid- to late Jurassic, from [[Patagonia]]) has been interpreted as a basal australosphenid (animal that has features shared with both Ausktribosphenidae and monotremes; lacks features that are peculiar to Ausktribosphenidae or monotremes; also lacks features that are absent in Ausktribosphenidae and monotremes) and as showing that australosphenids were widespread throughout [[Gondwanaland]] (the old Southern Hemisphere super-continent).<ref name="RauhutMartinEtAl2002JurassicMammal">{{cite journal | vauthors = Rauhut OW, Martin T, Ortiz-Jaureguizar E, Puerta P | title = A Jurassic mammal from South America | journal = Nature | volume = 416 | issue = 6877 | pages = 165–8 | date = March 2002 | pmid = 11894091 | doi = 10.1038/416165a | bibcode = 2002Natur.416..165R | hdl = 11336/99461 | s2cid = 4346804 | hdl-access = free }}</ref>

Recent analysis of ''[[Teinolophos]]'', which lived somewhere between 121 and 112.5 million years ago, suggests that it was a "crown group" (advanced and relatively specialised) monotreme. This was taken as evidence that the basal (most primitive) monotremes must have appeared considerably earlier, but this has been disputed (see the following section). The study also indicated that some alleged Australosphenids were also "crown group" monotremes (e.g. ''[[Steropodon]]'') and that other alleged Australosphenids (e.g. ''Ausktribosphenos'', ''Bishops'', ''Ambondro'', ''Asfaltomylos'') are more closely related to and possibly members of the Therian mammals (group that includes marsupials and placentals, see below).<ref name="RoweRichRichWoodburne2008" />

===Monotremes===
''[[Teinolophos]]'', from Australia, is the earliest known monotreme. A 2007 study (published 2008) suggests that it was not a basal (primitive, ancestral) monotreme but a full-fledged [[platypus]], and therefore that the platypus and [[echidna]] lineages diverged considerably earlier.<ref name="RoweRichRichWoodburne2008">{{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 = 1238–42 | date = January 2008 | pmid = 18216270 | pmc = 2234122 | doi = 10.1073/pnas.0706385105 | bibcode = 2008PNAS..105.1238R  | doi-access = free }}</ref> A more recent study (2009), however, has suggested that, while ''Teinolophos'' was a type of platypus, it was also a basal monotreme and predated the radiation of modern monotremes. The semi-aquatic lifestyle of platypuses prevented them from being outcompeted by the marsupials that migrated to Australia millions of years ago, since joeys need to remain attached to their mothers and would drown if their mothers ventured into water (though there are exceptions like the [[water opossum]] and the [[lutrine opossum]]; however, they both live in South America and thus do not come into contact with monotremes). Genetic evidence has determined that echidnas diverged from the platypus lineage as recently as 19-48M, when they made their transition from semi-aquatic to terrestrial lifestyle.<ref name="Phillips et al 2009">{{cite journal | vauthors = Phillips MJ, Bennett TH, Lee MS | title = Molecules, morphology, and ecology indicate a recent, amphibious ancestry for echidnas | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 106 | issue = 40 | pages = 17089–94 | date = October 2009 | pmid = 19805098 | pmc = 2761324 | doi = 10.1073/pnas.0904649106 | bibcode = 2009PNAS..10617089P | doi-access = free }}</ref>

Monotremes have some features that may be inherited from the [[cynodont]] ancestors:
*like lizards and birds, they use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole").
*they lay [[Egg (biology)|eggs]] that are leathery and uncalcified, like those of lizards, turtles and crocodilians.
Unlike other mammals, female monotremes do not have [[nipples]] and feed their young by "sweating" milk from patches on their bellies.

These features are not visible in fossils, and the main characteristics from paleontologists' point of view are:<ref name=Mammalia/>
*a slender [[dentary]] bone in which the [[coronoid process of the mandible|coronoid process]] is small or non-existent.
*the external opening of the ear lies at the posterior base of the jaw.
*the [[jugal]] bone is small or non-existent.
*a primitive [[pectoral girdle]] with strong [[ventral]] elements: [[coracoid]]s, [[clavicles]] and [[interclavicle]]. Note: [[theria]]n mammals have no interclavicle.<ref name="courses.washington.edu">{{Cite web| url=http://courses.washington.edu/chordate/453lectures/set2/453-appendskel-06.htm | title=Appendicular Skeleton}}</ref>
*sprawling or semi-sprawling forelimbs.

===Multituberculates===
[[File:Ptilodus skull BW.jpg|thumb|Skull of the multituberculate ''[[Ptilodus]]'']]

[[Multituberculates]] (named for the multiple [[Tubercle (anatomy)|tubercles]] on their "[[Molar (tooth)|molars]]") are often called the "rodents of the Mesozoic", but this is an example of [[convergent evolution]] rather than meaning that they are closely related to the [[Rodent]]ia. They existed for approximately 120 million years—the longest fossil history of any mammal lineage—but were eventually outcompeted by rodents, becoming extinct during the early [[Oligocene]].

Some authors have challenged the phylogeny represented by the cladogram above. They exclude the multituberculates from the mammalian crown group, holding that multituberculates are more distantly related to extant mammals than even the Morganucodontidae.<ref>{{cite journal |last1=Butler |first1=P. M. |title=Review of the early allotherian mammals |journal=Acta Palaeontologica Polonica | year=2000 |volume=45 |issue=4 |pages=317–342 |url=http://www.app.pan.pl/archive/published/app45/app45-317.pdf}}</ref><ref name=Mammaliaformes>{{cite web|url=http://palaeos.com/Vertebrates/Units/Unit420/420.100.html |title=Mammaliaformes—Palaeos |last1=White |first1=A. T. |date=May 21, 2005 |access-date=January 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20101220210140/http://palaeos.com/Vertebrates/Units/Unit420/420.100.html |archive-date=December 20, 2010 }}</ref> Multituberculates are like undisputed crown mammals in that their jaw joints consist of only the [[dentary]] and [[squamosal]] bones-whereas the [[quadrate bone|quadrate]] and [[articular]] bones are part of the middle ear; their teeth are differentiated, occlude, and have mammal-like [[cusp (dentistry)|cusp]]s; they have a [[zygomatic arch]]; and the structure of the [[pelvis]] suggests that they gave birth to tiny helpless young, like modern marsupials.{{sfn|Kielan-Jaworowska|Cifelli|Luo|2004|p=299}} On the other hand, they differ from modern mammals:

*Their "molars" have two parallel rows of tubercles, unlike the tribosphenic (three-peaked) molars of uncontested early crown mammals.
*The chewing action differs in that undisputed crown mammals chew with a side-to-side grinding action, which means that the molars usually occlude on only one side at a time, while multituberculates' jaws were incapable of side-to-side movement—they chewed, rather, by dragging the lower teeth backwards against the upper ones as the jaw closed.
*The anterior (forward) part of the zygomatic arch mostly consists of the [[maxilla]] (upper jawbone) rather than the [[jugal]], a small bone in a little slot in the maxillary process (extension).
*The [[squamosal]] does not form part of the [[braincase]].
*The [[rostrum (anatomy)|rostrum]] (snout) is unlike that of undisputed crown mammals; in fact it looks more like that of a [[pelycosaur]], such as ''[[Dimetrodon]]''. The multituberculate rostrum is box-like, with the large flat maxillae forming the sides, the [[Nasal bone|nasal]] the top, and the tall [[premaxilla]] at the front.

===Theria===
[[File:TheriaAnkle01.gif|thumb|upright=1.15|Therian form of [[crurotarsal]] ankle.<ref>http://www.palaeos.com/Vertebrates/Units/430Mammalia/430.500.html#Theria {{Webarchive|url=https://web.archive.org/web/20100911212001/http://www.palaeos.com/Vertebrates/Units/430Mammalia/430.500.html#Theria |date=2010-09-11 }}</ref>]]
[[Theria]] ("beasts") is the [[clade]] originating with the last common ancestor of the [[Eutheria]] (including [[Eutheria|placentals]]) and [[Metatheria]] (including marsupials). Common features include:<ref>{{Cite web| url=http://palaeos.com/vertebrates/mammalia/mammalia2.html#Theria | title=Mammalia: Spalacotheroidea & Cladotheria – Palaeos}}</ref>
*no [[interclavicle]].<ref name="courses.washington.edu"/>
*[[coracoid]] bones non-existent or fused with the [[scapula|shoulder blades]] to form [[coracoid process]]es.
*a type of [[crurotarsal]] ankle joint in which: the main joint is between the [[tibia]] and [[talus bone|astragalus]]; the [[calcaneum]] has no contact with the tibia but forms a heel to which muscles can attach. (The other well-known type of crurotarsal ankle is seen in crocodilians and works differently — most of the bending at the ankle is between the calcaneum and astragalus).
*tribosphenic molars.<ref name="JacobsWinklerMurry1989MammalOriginsEvolutionary Grades" />

===Metatheria===
The living [[Metatheria]] are all marsupials (animals with pouches). A few fossil [[genus|genera]], such as the Mongolian late Cretaceous ''Asiatherium'', may be marsupials or members of some other metatherian group(s).<ref>{{Cite web | url=http://www.palaeos.com/Vertebrates/Units/Unit440/440.100.html | title=Metatheria – Palaeos | access-date=2007-04-19 | archive-date=2007-06-08 | archive-url=https://web.archive.org/web/20070608042030/http://www.palaeos.com/Vertebrates/Units/Unit440/440.100.html | url-status=dead }}</ref><ref>{{cite journal |last1=Szalay |first1=Frederick S. |last2=Trofimov |first2=Boris A. |title=The Mongolian Late Cretaceous Asiatherium , and the early phylogeny and paleobiogeography of Metatheria |journal=Journal of Vertebrate Paleontology |date=19 September 1996 |volume=16 |issue=3 |pages=474–509 |doi=10.1080/02724634.1996.10011335 |bibcode=1996JVPal..16..474S }}</ref>

The oldest known metatherian is ''[[Sinodelphys]]'', found in 125M-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=http://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=December 17, 2003 | title=Oldest Marsupial Fossil Found in China | date=2003-12-15 | publisher=National Geographic News}}</ref>

[[Didelphimorphia]] (common opossums of the [[Western Hemisphere]]) first appeared in the late Cretaceous and still have living representatives, probably because they are mostly semi-[[arboreal]] unspecialized [[omnivores]].<ref>{{Cite web| url=http://palaeos.com/vertebrates/metatheria/metatheria.html#Didelphimorphia |access-date=2013-10-15| title=Didelphimorphia – Palaeos}}</ref>

Tracks from the Early Cretaceous of Angola show the existence of raccoon-size mammals 118 million years ago.<ref name = goncalves>{{cite journal |last1=Mateus |first1=Octávio |last2=Marzola |first2=Marco |last3=Schulp |first3=Anne S. |last4=Jacobs |first4=Louis L. |last5=Polcyn |first5=Michael J. |last6=Pervov |first6=Vladimir |last7=Gonçalves |first7=António Olímpio |last8=Morais |first8=Maria Luisa |title=Angolan ichnosite in a diamond mine shows the presence of a large terrestrial mammaliamorph, a crocodylomorph, and sauropod dinosaurs in the Early Cretaceous of Africa |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=April 2017 |volume=471 |pages=220–232 |doi=10.1016/j.palaeo.2016.12.049 |bibcode=2017PPP...471..220M }}</ref>

The best-known feature of marsupials is their method of reproduction:
*The mother develops a kind of [[yolk]] sack in her womb that delivers nutrients to the [[embryo]]. Embryos of [[bandicoots]], [[koalas]] and [[wombats]] additionally form placenta-like organs that connect them to the [[uterus|uterine]] wall, although the placenta-like organs are smaller than in placental mammals and it is not certain that they transfer nutrients from the mother to the embryo.<ref>{{Cite web| url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Peramelidae.html | title=Family Peramelidae (bandicoots and echymiperas)}}</ref>
*[[pregnancy (mammals)|Pregnancy]] is very short, typically four to five weeks. The embryo is born at a very early stage of development, and is usually less than {{cvt|2|in}} long at birth. It has been suggested that the short pregnancy is necessary to reduce the risk that the mother's [[immune system]] will attack the embryo.
*The newborn marsupial uses its forelimbs (with relatively strong hands) to climb to a [[nipple]], which is usually in a pouch on the mother's belly. The mother feeds the baby by contracting muscles over her [[mammary glands]], as the baby is too weak to suck. The newborn marsupial's need to use its forelimbs in climbing to the nipple was historically thought to have restricted metatherian evolution, as it was assumed that the forelimb could not become specialised intro structures like wings, hooves or flippers. However, several [[bandicoot]]s, most notably the [[pig-footed bandicoot]], have true hooves similar to those of placental ungulates, and several marsupial gliders have evolved.

[[File:Thylacine skull.jpg| thumb | right | Skull of [[thylacine]], showing [[marsupial]] pattern of molars]]
Although some marsupials look very like some placentals (the [[thylacine]], "marsupial tiger" or "marsupial wolf" is a good example), marsupial skeletons have some features that distinguish them from placentals:<ref>{{Cite web| url=http://lancelet.blogspot.com/2005/12/species-is-as-species-does-part-ii.html | title= Species is as species does... Part II| date= 2005-12-12}}</ref>{{self-published inline|date=August 2021}}
*Some, including the thylacine, have four molars; whereas no known placental has more than three.
*All have a pair of palatal fenestrae, window-like openings on the bottom of the skull (in addition to the smaller nostril openings).

Marsupials also have a pair of marsupial bones (sometimes called "[[epipubic bones]]"), which support the pouch in females. But these are not unique to marsupials, since they have been found in fossils of multituberculates, monotremes, and even eutherians — so they are probably a common ancestral feature that disappeared at some point after the ancestry of living placental mammals diverged from that of marsupials.<ref>{{Cite web| url=http://paleo.amnh.org/bjburger/fossilmammal/ma3.html | title=Marsupials |archive-url=https://web.archive.org/web/20030405212007/http://paleo.amnh.org/bjburger/fossilmammal/ma3.html |archive-date=5 April 2003}}</ref><ref>{{cite journal |last1=Novacek |first1=Michael J. |last2=Rougier |first2=Guillermo W. |last3=Wible |first3=John R. |last4=McKenna |first4=Malcolm C. |last5=Dashzeveg |first5=Demberelyin |last6=Horovitz |first6=Inés |title=Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia |journal=Nature |date=October 1997 |volume=389 |issue=6650 |pages=483–486 |doi=10.1038/39020 |pmid=9333234 |bibcode=1997Natur.389..483N |s2cid=205026882 }}</ref>
Some researchers think the epipubic bones' original function was to assist locomotion by supporting some of the muscles that pull the thigh forwards.<ref name="White1989Epipubic">{{cite journal |last1=White |first1=Thomas D. |title=An analysis of epipubic bone function in mammals using scaling theory |journal=Journal of Theoretical Biology |date=August 1989 |volume=139 |issue=3 |pages=343–357 |doi=10.1016/s0022-5193(89)80213-9 |pmid=2615378 |bibcode=1989JThBi.139..343W }}</ref>

===Eutheria===
{{Main|Eutheria}}
The time of appearance of the earliest eutherians has been a matter of controversy. On one hand, recently discovered fossils of ''[[Juramaia]]'' have been dated to 160 million years ago and classified as eutherian.<ref>{{cite journal |last1=Luo |first1=Zhe-Xi |last2=Yuan |first2=Chong-Xi |last3=Meng |first3=Qing-Jin |last4=Ji |first4=Qiang |title=A Jurassic eutherian mammal and divergence of marsupials and placentals |journal=Nature |date=August 2011 |volume=476 |issue=7361 |pages=442–445 |doi=10.1038/nature10291 |pmid=21866158 |bibcode=2011Natur.476..442L |s2cid=205225806 }}</ref> Fossils of ''[[Eomaia]]'' from 125 million years ago in the [[Early Cretaceous]] have also been classified as eutherian.<ref>{{Cite web| url=http://www.evolutionpages.com/Eomaia%20scansoria.htm | title=Eomaia scansoria: discovery of oldest known placental mammal}}</ref> A recent analysis of phenomic characters, however, classified ''Eomaia'' as pre-eutherian and reported that the earliest clearly eutherian specimens came from ''[[Maelestes]]'', dated to 91 million years ago.<ref name="The placental mammal ancestor and t">{{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 = 662–7 | date = February 2013 | pmid = 23393258 | doi = 10.1126/science.1229237 | bibcode = 2013Sci...339..662O | hdl = 11336/7302 | s2cid = 206544776 | hdl-access = free }}</ref> That study also reported that eutherians did not significantly diversify until after the catastrophic extinction at the [[Cretaceous–Paleogene boundary]], about 66 million years ago.

''Eomaia'' was found to have some features that are more like those of marsupials and earlier metatherians:

[[File:Eomaia scansoria.JPG|thumb|Fossil of ''[[Eomaia]]'' in the [[Hong Kong Science Museum]].]]

* [[Epipubic bones]] extending forwards from the pelvis, which are not found in any modern placental, but are found in all other mammals — early mammaliaforms, non-placental eutherians, marsupials, and [[monotremes]] — as well as in the [[cynodont]] [[therapsid]]s that are closest to mammals. Their function is to stiffen the body during locomotion.<ref>{{cite journal | vauthors = Reilly SM, White TD | title = Hypaxial motor patterns and the function of epipubic bones in primitive mammals | journal = Science | volume = 299 | issue = 5605 | pages = 400–2 | date = January 2003 | pmid = 12532019 | doi = 10.1126/science.1074905 | bibcode = 2003Sci...299..400R | s2cid = 41470665 }}</ref> This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.<ref>{{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 = 483–6 | date = October 1997 | pmid = 9333234 | doi = 10.1038/39020  | bibcode = 1997Natur.389..483N | s2cid = 205026882 }}</ref>
* A narrow pelvic outlet, which indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
* Five incisors in each side of the upper jaw. This number is typical of metatherians, and the [[Dentition|maximum number]] in modern placentals is three, except for [[homodont]]s, such as the [[armadillo]]. But ''Eomaia's'' molar to [[premolar]] ratio (it has more pre-molars than molars) is typical of eutherians, including placentals, and not normal in marsupials.

''Eomaia'' also has a [[Meckelian groove]], a primitive feature of the lower jaw that is not found in modern placental mammals.

These intermediate features are consistent with [[molecular phylogenetics]] estimates that the placentals diversified about 110M years ago, 15M years after the date of the ''Eomaia'' fossil.{{citation needed|date=December 2024}}

''Eomaia'' also has many features that strongly suggest it was a climber, including several features of the feet and toes; well-developed attachment points for muscles that are used a lot in climbing; and a tail that is twice as long as the rest of the spine.

Placentals' best-known feature is their method of reproduction:

* The embryo attaches itself to the [[uterus]] via a large [[placenta]] via which the mother supplies food and oxygen and removes waste products.
* Pregnancy is relatively long and the young are fairly well developed at birth. In some species (especially herbivores living on plains) the young can walk and even run within an hour of birth.

It has been suggested that the evolution of placental reproduction was made possible by [[retrovirus]]es that:<ref>{{Cite news| vauthors = Fox D | title=Why we don't lay eggs | newspaper=New Scientist | year=1999 | url=https://www.newscientist.com/article/mg16221904.800-why-we-dont-lay-eggs.html}}</ref><ref>{{cite journal |last1=Villarreal |first1=Luis P. |title=Viruses and the placenta: the essential virus first view |journal=APMIS |date=January 2016 |volume=124 |issue=1–2 |pages=20–30 |doi=10.1111/apm.12485 |pmid=26818259 |s2cid=12042851 |url=https://escholarship.org/content/qt42x2q1b3/qt42x2q1b3.pdf?t=o6yprx }}</ref>

* make the interface between the placenta and uterus into a [[syncytium]], i.e. a thin layer of cells with a shared external membrane. This allows the passage of oxygen, nutrients and waste products, but prevents the passage of blood and other cells that would cause the mother's [[immune system]] to attack the [[fetus]].
* reduce the aggressiveness of the mother's immune system, which is good for the foetus but makes the mother more vulnerable to infections.

From a paleontologist's point of view, eutherians are mainly distinguished by various features of their teeth,<ref>{{Cite web | url=http://www.palaeos.com/Vertebrates/Units/450Eutheria/450.100.html#Eutheria | title=Eutheria – Palaeos | access-date=2007-04-19 | archive-date=2010-09-11 | archive-url=https://archive.today/20100911000000/http://www.palaeos.com/Vertebrates/Units/450Eutheria/450.100.html#Eutheria | url-status=dead }}</ref> ankles and feet.<ref name="JiLuoYuan2002EarliestEutherian">{{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 = 816–22 | date = April 2002 | pmid = 11976675 | doi = 10.1038/416816a  | bibcode = 2002Natur.416..816J | s2cid = 4330626 }}</ref>
{{Clear}}

==Expansion of ecological niches in the Mesozoic==
[[File:Volaticotherium_Life_Reconstruction.png|thumb|Restoration of ''[[Volaticotherium]]'', a Middle Jurassic [[eutriconodont]] and the earliest known [[Gliding flight|gliding]] mammal.]][[File:Didelphodon Skull Clean.png|thumb|Skull cast of Late Cretaceous ''[[Didelphodon]]'', showing its robust teeth adapted to a [[durophagous]] diet.]]

Generally speaking, most species of mammaliaforms did occupy the niche of small, nocturnal insectivores, but recent finds, mainly in [[China]], show that some species and especially crown group mammals were larger and that there was a larger variety of lifestyles than previously thought. For example:

* The [[theria]]n ''[[Patagomaia]]'', found in the [[Late Cretaceous]] [[Chorrillo Formation]] (Argentina) is the largest known Mesozoic mammal, weighing an estimated {{convert|14|kg|lb}}.<ref name="Patagomaia">{{cite journal |last1=Chimento |first1=Nicolás R. |last2=Agnolín |first2=Federico L. |last3=García-Marsà |first3=Jordi |last4=Manabe |first4=Makoto |last5=Tsuihiji |first5=Takanobu |last6=Novas |first6=Fernando E. |title=A large therian mammal from the Late Cretaceous of South America |journal=Scientific Reports |date=3 February 2024 |volume=14 |issue=1 |page=2854 |doi=10.1038/s41598-024-53156-3 |pmc=10838296 |pmid=38310138 |bibcode=2024NatSR..14.2854C }}</ref>
* ''[[Adalatherium hui]]'' is a large sized, erect limbed herbivore from the [[Cretaceous]] of [[Madagascar]].<ref>{{cite journal |last1=Krause |first1=David W. |last2=Hoffmann |first2=Simone |last3=Hu |first3=Yaoming |last4=Wible |first4=John R. |last5=Rougier |first5=Guillermo W. |last6=Kirk |first6=E. Christopher |last7=Groenke |first7=Joseph R. |last8=Rogers |first8=Raymond R. |last9=Rossie |first9=James B. |last10=Schultz |first10=Julia A. |last11=Evans |first11=Alistair R. |last12=von Koenigswald |first12=Wighart |last13=Rahantarisoa |first13=Lydia J. |title=Skeleton of a Cretaceous mammal from Madagascar reflects long-term insularity |journal=Nature |date=29 April 2020 |volume=581 |issue=7809 |pages=421–427 |doi=10.1038/s41586-020-2234-8 |pmid=32461642 |bibcode=2020Natur.581..421K |s2cid=216650606 |url=https://research.monash.edu/en/publications/885b5cf0-4087-4b3a-a3ac-bfffd478f2ba }}</ref>
*''[[Castorocauda]]'', a member of Docodonta which lived in the middle Jurassic about 164 million years, was about {{cvt|42.5|cm}} long, weighed {{cvt|500|-|800|g}}, had a [[beaver]]-like tail that was adapted for swimming, limbs adapted for swimming and digging, and teeth adapted for eating fish.<ref name="JiLuoYuanTabrumCastorocauda" /> Another docodont, ''[[Haldanodon]]'', also had semi-aquatic habits, and indeed aquatic tendencies were probably common among docodonts based on their prevalence in wetland environments.<ref>{{cite book |editor1-last=Foster |editor1-first=John R. |editor2-last=Lucas |editor2-first=Spencer G. |title=Paleontology and Geology of the Upper Jurassic Morrison Formation: Bulletin 36 |date=2006 |publisher=New Mexico Museum of Natural History and Science |oclc=785821219 |url=https://nmdigital.unm.edu/digital/collection/bulletins/id/803/ }}{{page needed|date=August 2021}}</ref> The [[eutriconodonts]] ''[[Liaoconodon]]'' and ''[[Yanoconodon]]'' have more recently also have been suggested to be freshwater swimmers, lacking ''Castorocauda'''s powerful tail but possessing paddle-like limbs;<ref>{{cite journal |last1=Chen |first1=Meng |last2=Wilson |first2=Gregory P. |title=A multivariate approach to infer locomotor modes in Mesozoic mammals |journal=Paleobiology |date=24 February 2015 |volume=41 |issue=2 |pages=280–312 |doi=10.1017/pab.2014.14 |bibcode=2015Pbio...41..280C |s2cid=86087687 }}</ref> the eutriconodont ''[[Astroconodon]]'' has similarly been suggested as being semi-aquatic in the past, albeit to less convincing evidence.
* [[Multituberculates]] are allotherians that survived for over 125 million years (from mid-Jurassic, about 160M years ago, to late [[Eocene]], about 35M years ago) are often called the "rodents of the Mesozoic". As noted above, they may have given birth to tiny live neonates rather than laying eggs.
* ''[[Fruitafossor]]'', from the late Jurassic period about 150 million years ago, was about the size of a [[chipmunk]] and its teeth, forelimbs and back suggest that it broke open the nest of social insects to prey on them (probably [[termites]], as ants had not yet appeared).<ref>{{cite journal | vauthors = Luo ZX, Wible JR | title = A Late Jurassic digging mammal and early mammalian diversification | journal = Science | volume = 308 | issue = 5718 | pages = 103–7 | date = April 2005 | pmid = 15802602 | doi = 10.1126/science.1108875 | bibcode = 2005Sci...308..103L | s2cid = 7031381 }}</ref>
* Similarly, the [[Gobiconodontidae|gobiconodontid]] ''[[Spinolestes]]'' possessed adaptations for fossoriality and convergent traits with placental [[xenarthra]]ns like scutes and xenarthrous vertebrae, so it too might have had anteater like habits. It is also notable for the presence of quills akin to those of modern [[Spiny mouse|spiny mice]].
* ''[[Volaticotherium]]'', from the boundary the early Cretaceous about 125M years ago, is the earliest-known gliding mammal and had a gliding membrane that stretched out between its limbs, rather like that of a modern [[flying squirrel]]. This also suggests it was active mainly during the day.<ref name="mengetal06">{{cite journal | vauthors = Meng J, Hu Y, Wang Y, Wang X, Li C | title = A Mesozoic gliding mammal from northeastern China | journal = Nature | volume = 444 | issue = 7121 | pages = 889–93 | date = December 2006 | pmid = 17167478 | doi = 10.1038/nature05234 | bibcode = 2006Natur.444..889M | s2cid = 28414039 | url = http://doc.rero.ch/record/15510/files/PAL_E2912.pdf }}</ref> The closely related ''[[Argentoconodon]]'' also shows similar adaptations that may also suggest aerial locomotion.<ref name="Gaetano, L.C. 2011">{{cite journal | vauthors = Gaetano LC, Rougier GW | year = 2011 | title = New materials of Argentoconodon fariasorum (Mammaliaformes, Triconodontidae) from the Jurassic of Argentina and its bearing on triconodont phylogeny | journal = Journal of Vertebrate Paleontology | volume = 31 | issue = 4| pages = 829–843 | doi = 10.1080/02724634.2011.589877 | bibcode = 2011JVPal..31..829G | hdl = 11336/68497 | s2cid = 85069761 | hdl-access = free }}</ref>
* ''[[Repenomamus]]'', a [[Eutriconodonta|eutriconodont]] from the early Cretaceous 130 million years ago, was a stocky, [[badger]]-like predator that sometimes preyed on young dinosaurs. Two species have been recognized, one more than {{cvt|1|m}} long and weighing about {{cvt|12|-|14|kg}}, the other less than {{cvt|0.5|m}} long and weighing {{cvt|4|-|6|kg}}.<ref>{{cite journal |last1=Li |first1=Jinling |last2=Wang |first2=Yuan |last3=Wang |first3=Yuanqing |last4=Li |first4=Chuankui |title=A new family of primitive mammal from the Mesozoic of western Liaoning, China |journal=Chinese Science Bulletin |date=May 2001 |volume=46 |issue=9 |pages=782–785 |doi=10.1007/BF03187223 |bibcode=2001ChSBu..46..782L |s2cid=129025369 }}</ref><ref name="Hu et al 2005" />
* ''[[Schowalteria]]'' is a Late Cretaceous species almost as large if not larger than ''R. giganticus'' that shows speciations towards herbivory, comparable to those of modern ungulates.
* [[Zhelestidae]] is a lineage of Late Cretaceous herbivorous [[eutheria]]ns, to the point of being mistaken for stem-[[ungulate]]s.<ref>{{cite book |last1=Benton |first1=Michael J. |last2=Shishkin |first2=Mikhail A. |last3=Unwin |first3=David M. |last4=Kurochkin |first4=Evgenii N. |title=The Age of Dinosaurs in Russia and Mongolia |date=2003 |publisher=Cambridge University Press |isbn=978-0-521-54582-2 }}{{page needed|date=August 2021}}</ref>
* Similarly, [[mesungulatid]]s are also fairly large sized herbivorous mammals from the Late Cretaceous
* [[Deltatheroida]]ns were [[metatheria]]ns that were specialised towards carnivorous habits,<ref>{{cite journal |last1=Muizon |first1=Christian |last2=Lange-Badré |first2=Brigitte |title=Carnivorous dental adaptations in tribosphenic mammals and phylogenetic reconstruction |journal=Lethaia |date=29 March 2007 |volume=30 |issue=4 |pages=353–366 |doi=10.1111/j.1502-3931.1997.tb00481.x |bibcode=1997Letha..30..353D }}</ref>{{sfn|Kielan-Jaworowska|Cifelli|Luo|2004|loc="Metatherians", pp. 425–262}} and possible forms like ''[[Oxlestes]]'' and ''[[Khudulestes]]'' might have been among the largest Mesozoic mammals, though their status as deltatheroidans is questionable.
* ''[[Ichthyoconodon]]'', a eutriconodont from the [[Berriasian]] of Morocco, is currently known from molariforms found in marine deposits. These teeth are sharp-cusped and similar in shape to those of piscivorous mammals, and unlike the teeth of contemporary mammals they do not show degradation, so rather than being carried down by river deposits the animal died ''in situ'' or close. This has been taken to mean that it was a [[marine mammal]], likely one of the few examples known from the Mesozoic.<ref>{{cite journal |last1=Sigogneau-Russell |first1=Denise |title=Two possibly aquatic triconodont mammals from the Early Cretaceous of Morocco – Acta Palaeontologica Polonica |journal=Acta Palaeontologica Polonica |date=1995 |volume=40 |issue=2 |pages=149–162 |url=https://www.app.pan.pl/article/item/app40-149.html }}</ref>
* ''[[Didelphodon]]'' is a Late Cretaceous riverine species of [[Stagodontidae|stagodontid]] [[Metatheria|marsupialiform]] with a [[durophagous]] dentition, robust jaws similar to a modern [[Tasmanian devil]], and a postcranial skeleton very similar in size and shape to an [[otter]]. This animal has been lauded as the strongest bite of all Mesozoic mammals. It possibly specialized on eating [[freshwater crab]]s and [[mollusc]]s.
* Tracks of a raccoon-sized [[Mammaliaformes|mammaliaform]] representing the morphofamily Ameghinichnidae are described from the [[Early Cretaceous]] (late [[Aptian]]) [[Calonda Formation]] ([[Angola]]) by [[Octávio Mateus|Mateus]] ''et al.'' (2017), who name a new ichnotaxon ''[[Catocapes]] angolanus''.<ref name="goncalves" />
* A gobiconodontid was preserved attacking a substantially larger dinosaur.<ref>{{cite journal | doi=10.1038/s41598-023-37545-8 | title=An extraordinary fossil captures the struggle for existence during the Mesozoic | year=2023 | last1=Han | first1=Gang | last2=Mallon | first2=Jordan C. | last3=Lussier | first3=Aaron J. | last4=Wu | first4=Xiao-Chun | last5=Mitchell | first5=Robert | last6=Li | first6=Ling-Ji | journal=Scientific Reports | volume=13 | issue=1 | page=11221 | pmid=37464026 | pmc=10354204 | bibcode=2023NatSR..1311221H }}</ref>

A study on Mesozoic mammaliaforms suggests that they were a primary factor in constraining mammalian body size, rather than solely competition from dinosaurs.<ref>{{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 |doi-access=free |bibcode=2021CBio...31E2955B }}</ref> In general, it appears mammal faunas on southern continents had attained larger body sizes than those of northern continents.<ref>{{cite journal |last1=Chimento |first1=Nicolás R. |last2=Agnolín |first2=Federico L. |last3=García-Marsà |first3=Jordi |last4=Manabe |first4=Makoto |last5=Tsuihiji |first5=Takanobu |last6=Novas |first6=Fernando E. |title=A large therian mammal from the Late Cretaceous of South America |journal=Scientific Reports |date=3 February 2024 |volume=14 |issue=1 |pages=2854 |doi=10.1038/s41598-024-53156-3 |pmid=38310138 |pmc=10838296 |bibcode=2024NatSR..14.2854C }}</ref>

==Evolution of major groups of living mammals==
{{Further|Mammal classification}}

There are currently vigorous debates between traditional [[paleontologists]] and [[molecular phylogenetics|molecular phylogeneticists]] about how and when the modern groups of mammals diversified, especially the placentals. Generally, the traditional paleontologists date the appearance of a particular group by the earliest known fossil whose features make it likely to be a member of that group, while the molecular phylogeneticists suggest that each lineage diverged earlier (usually in the Cretaceous) and that the earliest members of each group were anatomically very similar to early members of other groups and differed only in their [[genes|genetics]]. These debates extend to the definition of and relationships between the major groups of placentals<!-- — the controversy about [[Afrotheria]] is a good example -->.
<!--
===Fossil-based family tree of placental mammals===
Here is a very simplified version of a typical family tree based on fossils.<ref>https://web.archive.org/web/20101220204717/http://palaeos.com/Vertebrates/Lists/Cladograms/360Mammalia.html {{bare URL inline|date=February 2024}}</ref> It tries to show the nearest thing there is at present to a consensus view, but some paleontologists have very different views, for example:<ref name=WibleRougierEtAl2007a>{{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 = 1003–6 | date = June 2007 | pmid = 17581585 | doi = 10.1038/nature05854  | bibcode = 2007Natur.447.1003W }}</ref>
* The most common view is that placentals originated in the Southern Hemisphere, but some paleontologists argue that they first appeared in [[Laurasia]] (old supercontinent containing modern Asia, N. America and Europe).
* Paleontologists differ as to when the first placentals appeared, with estimates ranging from 20M years before the end of the Cretaceous to just after the end of the Cretaceous. [[Molecular phylogeny|Molecular biologists]] argue for a much earlier origin, even suggesting appearance in the Middle Jurassic.<ref>{{cite journal | vauthors = Waddell PJ, Cao Y, Hasegawa M, Mindell DP | title = Assessing the Cretaceous superordinal divergence times within birds and placental mammals by using whole mitochondrial protein sequences and an extended statistical framework | journal = Systematic Biology | volume = 48 | issue = 1 | pages = 119–37 | date = March 1999 | pmid = 12078636 | doi = 10.1080/106351599260481 }}</ref>
* Molecular data suggest that either [[Xenarthra]], [[Afrotheria]], or [[Atlantogenata]] (Xenarthra + Afrotheria), was the earliest-diverging group from the rest of the placental mammals.<ref name="The placental mammal ancestor and t"/><ref name="Meredith 2011" /><ref>{{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 = 507–12 | date = March 2007 | pmid = 17392779 | doi = 10.1038/nature05634 | bibcode = 2007Natur.446..507B }}</ref>

For the sake of brevity and simplicity, the diagram omits some extinct groups in order to focus on the ancestry of well-known modern groups of placentals — '''†''' marks extinct groups. The diagram also shows the following:
*the age of the oldest known fossils in many groups, since one of the major debates between traditional paleontologists and molecular phylogeneticists is about when various groups first became distinct.
*well-known modern members of most groups.

{{clade| style=font-size:70%;line-height:100%
  |label1=[[Eutheria]]
  |1={{clade
    |1=[[Xenarthra]] (late cretaceous)<br />([[armadillo]]s, [[anteater]]s, [[sloth]]s)
    |2={{clade
      |1=[[Pholidota]] (late cretaceous)<br />(pangolins)
      |label2=[[Epitheria]]&nbsp;(latest&nbsp;Cretaceous)
      |2={{clade
        |1=(some extinct groups) '''†'''
        |2={{clade
          |1=[[Insectivora]] (latest [[Cretaceous]])<br />([[hedgehog]]s, [[shrew]]s, [[Mole (animal)|mole]]s, [[tenrec]]s)
          |2={{clade
            |1={{clade
              |label1=[[Anagalida]]
              |1={{clade
                |1=[[Zalambdalestidae]] '''†''' (late [[Cretaceous]])
                |2={{clade
                  |1=[[Macroscelidea]] (late [[Eocene]])<br />([[elephant shrew]]s)
                  |2={{clade
                    |1=[[Anagaloidea]] '''†'''
                    |label2=[[Glires]]&nbsp;(early&nbsp;[[Paleocene]])
                    |2={{clade
                      |1=[[Lagomorpha]] ([[Eocene]])<br />([[rabbit]]s, [[hare]]s, [[pika]]s)
                      |2=[[Rodent]]ia (late [[Paleocene]])<br />(mice & rats, squirrels, porcupines)
                    }}
                  }}
                }}
              }}
              |label2=[[Archonta]]
              |2={{clade
                |1={{clade
                  |1=[[Scandentia]] (mid-[[Eocene]])<br />([[tree shrews]])
                  |label2=[[Primatomorpha]]
                  |2={{clade
                    |1=[[Plesiadapiformes]] '''†'''
                    |2=[[Primate]]s (early [[Paleocene]])<br />([[tarsier]]s, [[lemur]]s, [[monkey]]s, [[ape]]s including [[human]]s)
                  }}
                }}
                |2={{clade
                  |1=[[Dermoptera]] (late [[Eocene]])<br />([[colugos]])
                  |2=[[Chiroptera]] (late [[Paleocene]])<br />([[bat]]s)
                }}
              }}
            }}
            |2={{clade
              |1=[[Carnivora]] (early [[Paleocene]])<br />(cats, dogs, bears, seals)
              |label2=[[Ungulatomorpha]]&nbsp;(late&nbsp;[[Cretaceous]])
              |2={{clade
                |label1=[[Eparctocyona]]&nbsp;(late&nbsp;[[Cretaceous]])
                |1={{clade
                  |1=(some extinct groups) '''†'''
                  |2={{clade
                    |1=[[Arctostylopida]] '''†''' (late [[Paleocene]])
                    |2={{clade
                      |1=[[Mesonychia]] '''†''' (mid-[[Paleocene]])<br />(predators / scavengers, but not closely related to modern [[carnivores]])
                      |label2=[[Cetartiodactyla]]
                      |2={{clade
                        |1=[[Cetacea]] (early Eocene)<br />(whales, dolphins, porpoises)
                        |2=[[Artiodactyla]] (early Eocene)<br />(even-toed ungulates: pigs, hippos, camels, giraffes, cattle, deer)
                      }}
                    }}
                  }}
                }}
                |label2=[[Altungulata]]
                |2={{clade
                  |1=[[Hilalia]] '''†'''
                  |2={{clade
                    |1={{clade
                      |1=[[Perissodactyla]] (late [[Paleocene]])<br />(odd-toed ungulates: horses, rhinos, tapirs)
                      |2=[[Tubulidentata]] (early [[Miocene]])<br />([[aardvarks]])
                    }}
                    |label2=[[Paenungulata]]&nbsp;("not&nbsp;quite&nbsp;ungulates")
                    |2={{clade
                      |1=[[Hyracoidea]] (early Eocene)<br />([[hyraxes]])
                      |2={{clade
                        |1=[[Sirenia]] (early Eocene)<br />([[manatees]], [[dugongs]])
                        |2=[[Proboscidea]] (early Eocene)<br />(elephants)
                      }}
                    }}
                  }}
                }}
              }}
            }}
          }}
        }}
      }}
    }}
  }}
}}

This family tree contains some surprises and puzzles. For example:
*The closest living relatives of cetaceans (whales, dolphins, porpoises) are artiodactyls, hoofed animals, which are almost all pure herbivores.
*Bats are fairly close relatives of primates.
*The closest living relatives of elephants are the aquatic sirenians, while their next relatives are hyraxes, which look more like well-fed guinea pigs.
*There is little correspondence between the structure of the family (what was descended from what) and the dates of the earliest fossils of each group. For example, the earliest fossils of perissodactyls (the living members of which are horses, rhinos and tapirs) date from the late Paleocene, but the earliest fossils of their "sister group", the Tubulidentata, date from the early Miocene, nearly 50M years later. Paleontologists are fairly confident about the family relationships, which are based on [[cladistic]] analyses, and believe that fossils of the ancestors of modern aardvarks have simply not been found yet.
-->

===Molecular phylogenetics-based family tree of placental mammals===
[[Molecular systematics|Molecular phylogenetics]] uses features of organisms' [[gene]]s to work out family trees in much the same way as paleontologists do with features of fossils — if two organisms' genes are more similar to each other than to those of a third organism, the two organisms are more closely related to each other than to the third.

Molecular phylogeneticists have proposed a family tree that is both broadly similar to but has notable differences from that of the paleontologists. Like paleontologists, molecular phylogeneticists have differing ideas about various details, but here is a typical family tree according to molecular phylogenetics:<ref>{{cite journal | vauthors = Murphy WJ, Eizirik E, O'Brien SJ, Madsen O, Scally M, Douady CJ, Teeling E, Ryder OA, Stanhope MJ, de Jong WW, Springer MS | display-authors = 6 | title = Resolution of the early placental mammal radiation using Bayesian phylogenetics | journal = Science | volume = 294 | issue = 5550 | pages = 2348–51 | date = December 2001 | pmid = 11743200 | doi = 10.1126/science.1067179 | bibcode = 2001Sci...294.2348M | s2cid = 34367609 }}</ref><ref>{{cite journal | vauthors = Kriegs JO, Churakov G, Kiefmann M, Jordan U, Brosius J, Schmitz J | title = Retroposed elements as archives for the evolutionary history of placental mammals | journal = PLOS Biology | volume = 4 | issue = 4 | pages = e91 | date = April 2006 | pmid = 16515367 | pmc = 1395351 | doi = 10.1371/journal.pbio.0040091 | doi-access = free |url=http://biology.plosjournals.org/perlserv/?request=get-pdf&file=10.1371_journal.pbio.0040091-S.pdf}}</ref> Note that the diagram shown here omits extinct groups, as one cannot extract DNA from most fossils. Some finer-level subdivisions are glossed-over.

{{clade
  |style=font-size:90%;line-height:100%
  |label1='''[[Eutheria]]'''
  |1={{clade
    |label1=[[Atlantogenata]]
    |sublabel1=("born&nbsp;round the<br/>Atlantic&nbsp;ocean")
|1={{Clade
        |label1=[[Xenarthra]] 
        |1={{Clade
          |1=[[Cingulata]] armadillos [[File:Nine-banded-Armadillo white background.jpg|50 px|''Dasypus novemcinctus'']]
          |2=[[Pilosa]] anteaters, sloths [[File:Natural history of the animal kingdom for the use of young people (Plate XV) (Myrmecophaga tridactyla).jpg|50 px|''Myrmecophaga tridactyla'']]
           }}
      |label2=[[Afrotheria]]
      |2={{clade
        |label1=[[Afrosoricida]]
        |1=golden moles, tenrecs, otter shrews 
        |2={{clade
          |1={{clade
            |label1=[[Macroscelidea]]
            |1=elephant shrews [[File:Rhynchocyon chrysopygus-J Smit white background.jpg|50 px]]
            |label2=[[Tubulidentata]]
            |2=aardvarks [[File:Aardvark2 (PSF) colourised.png|70 px]]
          }}
          |label2=[[Paenungulata]]
          |sublabel2=("not&nbsp;quite<br/>ungulates")
          |2={{clade
            |label1=[[Hyracoidea]]
            |1=hyraxes [[File:DendrohyraxEminiSmit white background.jpg|50 px]]
            |label2=[[Proboscidea]]
            |2=elephants, mammoths, mastodons [[File:Indian elephant white background.jpg|70 px]]
            |label3=[[Sirenia]]
            |3=manatees, dugongs [[File:Manatee white background.jpg|70 px]]
          }}
        }}
      }}
    }}
    |label2=[[Boreoeutheria]]
    |sublabel2=("northern true /<br/>placental&nbsp;mammals")
    |2={{clade
      |label1=[[Laurasiatheria]]
      |1={{clade
        |label1=[[Cetartiodactyla]]
        |1=camels and llamas, pigs and peccaries, ruminants, whales and hippos [[File:Walia ibex illustration white background.png|70 px]]
        |label2=[[Eulipotyphla]]
        |2=shrews, hedgehogs, gymnures, moles and solenodons [[File:Mole white background.jpg|50 px]]
        |label3=[[Pegasoferae]]
        |3={{clade
          |label1=[[Carnivora]]
          |1=cats, dogs, bears, seals [[File:Cynailurus guttata - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg|70 px]]
          |label2=[[Chiroptera]]
          |2=bats [[File:Vampire bat white background.jpg|50 px]]
          |label3=[[Perissodactyla]]
          |3=horses, rhinos, tapirs [[File:Equus quagga (white background).jpg|70 px]]
          |label4=[[Pholidota]]
          |4=pangolins [[File:FMIB 46859 Pangolin a grosse queue white background.jpeg|60 px]]
        }}
      }}
      |label2=[[Euarchontoglires]]
      |2={{clade
        |label1=[[Glires]]
        |1={{clade
          |label1=[[Lagomorpha]]
          |1=[[rabbit]]s, [[hare]]s, [[pika]]s [[File:Lepus timidus - 1700-1880 - Print - Iconographia Zoologica -(white background).jpg|50 px]]
          |label2=[[Rodent]]ia
          |2=mice and rats, cavys and porcupines, squirrels [[File:Ruskea rotta.png|50 px]]
          |sublabel2=(late [[Paleocene]])
        }}
        |label2=[[Euarchonta]]
        |2={{clade
          |label1=Dermoptera
          |1=[[colugo]]s 
          |label2=[[Scandentia]]
          |2=[[treeshrew|tree shrews]] 
          |label3=[[Primate]]s
          |3=tarsiers, lemurs, [[monkey]]s, [[ape]]s (including [[human]]s) [[File:Die Säugthiere in Abbildungen nach der Natur, mit Beschreibungen (Plate 8) (white background).jpg|50 px]]
        }}
      }}
    }}
  }}
}}

Here are the most significant of the differences between this family tree and the one familiar to paleontologists:
* The top-level division is between Atlantogenata and Boreoeutheria, instead of between Xenarthra and the rest. However, analysis of [[transposable element]] insertions supports a three-way top-level split between Xenarthra, Afrotheria and Boreoeutheria<ref>{{cite journal | vauthors = Nishihara H, Maruyama S, 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 of the United States of America | volume = 106 | issue = 13 | pages = 5235–40 | date = March 2009 | pmid = 19286970 | pmc = 2655268 | doi = 10.1073/pnas.0809297106 | bibcode = 2009PNAS..106.5235N | doi-access = free }}</ref><ref>{{cite journal | vauthors = Churakov G, Kriegs JO, Baertsch R, Zemann A, Brosius J, Schmitz J | title = Mosaic retroposon insertion patterns in placental mammals | journal = Genome Research | volume = 19 | issue = 5 | pages = 868–75 | date = May 2009 | pmid = 19261842 | pmc = 2675975 | doi = 10.1101/gr.090647.108 }}</ref> and the Atlantogenata clade does not receive significant support in recent distance-based molecular phylogenetics.<ref name="Meredith 2011">{{cite journal | vauthors = Meredith RW, Janečka JE, Gatesy J, Ryder OA, Fisher CA, Teeling EC, Goodbla A, Eizirik E, Simão TL, Stadler T, Rabosky DL, Honeycutt RL, Flynn JJ, Ingram CM, Steiner C, Williams TL, Robinson TJ, Burk-Herrick A, Westerman M, Ayoub NA, Springer MS, Murphy WJ | display-authors = 6 | title = Impacts of the Cretaceous Terrestrial Revolution and KPg extinction on mammal diversification | journal = Science | volume = 334 | issue = 6055 | pages = 521–4 | date = October 2011 | pmid = 21940861 | doi = 10.1126/science.1211028 | bibcode = 2011Sci...334..521M | s2cid = 38120449 }}</ref>
*Afrotheria contains several groups that are only distantly related according to the paleontologists' version: Afroinsectiphilia ("African insectivores"), Tubulidentata (aardvarks, which paleontologists regard as much closer to odd-toed ungulates than to other members of Afrotheria), Macroscelidea (elephant shrews, usually regarded as close to rabbits and rodents). The only members of Afrotheria that paleontologists would regard as closely related are Hyracoidea (hyraxes), Proboscidea (elephants) and Sirenia (manatees, dugongs).
* Members of the defunct [[order (biology)|order]] of [[Insectivora|Insectivores]] are divided among three [[clade]]s: One clade is part of Afrotheria, and the other two clades are distinct sub-groups within Boreoeutheria.
* Bats are closer to Carnivora and odd-toed ungulates than to Primates and Dermoptera (colugos).
* Perissodactyla (odd-toed ungulates) are closer to Carnivora and bats than to Artiodactyla (even-toed ungulates).

The grouping together of the Afrotheria has some geological justification: All surviving members of the Afrotheria originate from South American or (mainly) African lineages — even the [[Indian elephant]], which diverged from an African lineage about {{ma|7.6}}.<ref>{{Cite web
| url=http://news.bbc.co.uk/2/hi/science/nature/6913934.stm
| title=Scientists map elephant evolution | access-date=2008-08-11
| work=BBC News | date=2007-07-24}}</ref> As [[Pangaea]] broke up, Africa and South America separated from the other continents less than 150M years ago, and from each other between 100M and 80M years ago.<ref>{{Cite web |url=http://pubs.usgs.gov/gip/dynamic/historical.html |title=Historical perspective (the Dynamic Earth, USGS) |access-date=2007-06-24 |archive-date=2018-07-27 |archive-url=https://web.archive.org/web/20180727100546/https://pubs.usgs.gov/gip/dynamic/historical.html |url-status=live }}</ref><ref>{{cite web |url=http://www.scotese.com/cretaceo.htm |title=Cretaceous map |access-date=2007-04-18 |archive-date=2007-04-10 |archive-url=https://web.archive.org/web/20070410061346/http://www.scotese.com/cretaceo.htm |url-status=live }}</ref> So it would not be surprising if the earliest eutherian immigrants into Africa and South America were isolated there and [[Adaptive radiation|radiated]] into all the available [[ecological niche]]s.

Nevertheless, these proposals have been controversial. Paleontologists naturally insist that fossil evidence must take priority over deductions from samples of the DNA of modern animals. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly:<ref>{{cite web |url = http://www.palaeos.com/Vertebrates/Units/460Insectivora/460.000.html |series=Insectivora |title=Overview |website=Palaeos |archive-url=https://web.archive.org/web/20070715124547/http://www.palaeos.com/Vertebrates/Units/460Insectivora/460.000.html |archive-date=2007-07-15 }}</ref>
* [[Mitochondrial DNA]]'s mutation rate in mammals varies from region to region — some parts hardly ever change and some change extremely quickly and even show large variations between individuals within the same species.<ref>{{cite journal | vauthors = Springer MS, Douzery E | title = Secondary structure and patterns of evolution among mammalian mitochondrial 12S rRNA molecules | journal = Journal of Molecular Evolution | volume = 43 | issue = 4 | pages = 357–73 | date = October 1996 | pmid = 8798341 | doi = 10.1007/BF02339010 | bibcode = 1996JMolE..43..357S | s2cid = 12317859 }}</ref><ref>{{cite journal | vauthors = Springer MS, Hollar LJ, Burk A | title = Compensatory substitutions and the evolution of the mitochondrial 12S rRNA gene in mammals | journal = Molecular Biology and Evolution | volume = 12 | issue = 6 | pages = 1138–50 | date = November 1995 | pmid = 8524047 | doi = 10.1093/oxfordjournals.molbev.a040288 | doi-access = free }}</ref>
* Mammalian mitochondrial DNA mutates so fast that it causes a problem called "saturation", where random noise drowns out any information that may be present. If a particular piece of mitochondrial DNA mutates randomly every few million years, it will have changed several times in the 60 to 75M years since the major groups of placental mammals diverged.<ref>{{Cite book | vauthors = Li WH | title=Molecular Evolution | publisher=Sinauer Associates | year=1997 | isbn=978-0-87893-266-5 }}</ref>

===Timing of placental evolution===
Recent molecular phylogenetic studies suggest that most placental [[Order (biology)|orders]] diverged late in the Cretaceous period, about 100 to 85 million years ago, but that modern families first appeared later, in the late Eocene and early [[Miocene]] epochs of the [[Cenozoic]] period.<ref>{{cite journal |last1=Bininda-Emonds |first1=Olaf R. P. |last2=Cardillo |first2=Marcel |last3=Jones |first3=Kate E. |last4=MacPhee |first4=Ross D. E. |last5=Beck |first5=Robin M. D. |last6=Grenyer |first6=Richard |last7=Price |first7=Samantha A. |last8=Vos |first8=Rutger A. |last9=Gittleman |first9=John L. |last10=Purvis |first10=Andy |title=The delayed rise of present-day mammals |journal=Nature |date=29 March 2007 |volume=446 |issue=7135 |pages=507–512 |doi=10.1038/nature05634 |pmid=17392779 |bibcode=2007Natur.446..507B |s2cid=4314965 }}</ref><ref>{{cite journal |last1=Tarver |first1=James E. |last2=dos Reis |first2=Mario |last3=Mirarab |first3=Siavash |last4=Moran |first4=Raymond J. |last5=Parker |first5=Sean |last6=O'Reilly |first6=Joseph E. |last7=King |first7=Benjamin L. |last8=O'Connell |first8=Mary J. |last9=Asher |first9=Robert J. |last10=Warnow |first10=Tandy |last11=Peterson |first11=Kevin J. |last12=Donoghue |first12=Philip C.J. |last13=Pisani |first13=Davide |title=The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference |journal=Genome Biology and Evolution |date=February 2016 |volume=8 |issue=2 |pages=330–344 |doi=10.1093/gbe/evv261 |pmid=26733575 |pmc=4779606 }}</ref> Fossil-based analyses, on the contrary, limit the placentals to the Cenozoic.<ref>{{cite journal |last1=Wible |first1=J. R. |last2=Rougier |first2=G. W. |last3=Novacek |first3=M. J. |last4=Asher |first4=R. J. |title=Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary |journal=Nature |date=June 2007 |volume=447 |issue=7147 |pages=1003–1006 |doi=10.1038/nature05854 |pmid=17581585 |bibcode=2007Natur.447.1003W |s2cid=4334424 }}</ref> Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds, and mammals, but not the modern forms of mammals. It is possible that they simply did not exist, and that the [[molecular clock]] runs fast during major [[evolutionary radiation]]s.<ref>{{cite journal |last1=Benton |first1=Michael J. |title=Early origins of modern birds and mammals: molecules vs. morphology |journal=BioEssays |date=1999 |volume=21 |issue=12 |pages=1043–1051 |doi=10.1002/(SICI)1521-1878(199912)22:1<1043::AID-BIES8>3.0.CO;2-B |pmid=10580989 }}</ref> On the other hand, there is fossil evidence from {{ma|85}} of [[hoof]]ed mammals that may be ancestors of modern [[ungulate]]s.<ref>{{cite journal |last1=Archibald |first1=J. D. |title=Fossil Evidence for a Late Cretaceous Origin of 'Hoofed' Mammals |journal=Science |date=24 May 1996 |volume=272 |issue=5265 |pages=1150–1153 |doi=10.1126/science.272.5265.1150 |pmid=8662448 |bibcode=1996Sci...272.1150A |s2cid=33510594 }}</ref>

Fossils of the earliest members of most modern groups date from the [[Paleocene]], a few date from later and very few from the Cretaceous, before the extinction of the dinosaurs. But some paleontologists, influenced by molecular phylogenetic studies, have used statistical methods to [[Extrapolation|extrapolate]] ''backwards'' from fossils of members of modern groups and concluded that [[primates]] arose in the late Cretaceous.<ref>{{cite journal |last1=Martin |first1=Robert D. |last2=Soligo |first2=Christophe |last3=Tavaré |first3=Simon |title=Primate Origins: Implications of a Cretaceous Ancestry |journal=Folia Primatologica |date=2007 |volume=78 |issue=5–6 |pages=277–296 |doi=10.1159/000105145 |pmid=17855783 |s2cid=6651479 }}</ref><ref>{{cite book |doi=10.1007/978-0-387-33507-0_2 |chapter=New Light on the Dates of Primate Origins and Divergence |citeseerx=10.1.1.552.2941 |year=2007 |last1=Soligo |first1=C. |last2=Will |first2=O. A. |last3=Tavaré |first3=S. |last4=Marshall |first4=C. R. |last5=Martin |first5=R. D. |title=PRIMATE ORIGINS: Adaptations and Evolution |pages=29–49 |isbn=978-0-387-30335-2 |editor1-first=Matthew J. |editor1-last=Ravosa |editor2-first=Marian |editor2-last=Dagosto }}</ref> However, statistical studies of the fossil record confirm that mammals were restricted in size and diversity right to the end of the Cretaceous, and rapidly grew in size and diversity during the Early Paleocene.<ref>{{cite journal |last1=Alroy |first1=John |title=The Fossil Record of North American Mammals: Evidence for a Paleocene Evolutionary Radiation |journal=Systematic Biology |date=1 March 1999 |volume=48 |issue=1 |pages=107–118 |doi=10.1080/106351599260472 |pmid=12078635 |doi-access=free }}</ref><ref>{{cite journal |last1=Archibald |first1=J. David |last2=Deutschman |first2=Douglas H. |title=Quantitative Analysis of the Timing of the Origin and Diversification of Extant Placental Orders |journal=Journal of Mammalian Evolution |date=1 June 2001 |volume=8 |issue=2 |pages=107–124 |doi=10.1023/A:1011317930838 |s2cid=15581162 }}</ref>

==Evolution of mammalian features==

===Jaws and middle ears===
{{See also|Evolution of mammalian auditory ossicles}}

''[[Hadrocodium]]'', whose fossils date from the early Jurassic, provides the first clear evidence of fully mammalian jaw joints and middle ears, in which the jaw joint is formed by the [[dentary]] and [[squamosal]] bones while the [[articular]] and [[Quadrate bone|quadrate]] move to the middle ear, where they are known as the [[incus]] and [[malleus]].

One analysis of the monotreme ''[[Teinolophos]]'' suggested that this animal had a pre-mammalian jaw joint formed by the [[Angular bone|angular]] and quadrate bones and that the definitive mammalian middle ear evolved twice independently, in [[monotremes]] and in [[theria]]n mammals, but this idea has been disputed.<ref name="RichEtAl2005IndepOriginsOfMidEarBones">{{cite journal | vauthors = Rich TH, Hopson JA, Musser AM, Flannery TF, Vickers-Rich P | title = Independent origins of middle ear bones in monotremes and therians | journal = Science | volume = 307 | issue = 5711 | pages = 910–4 | date = February 2005 | pmid = 15705848 | doi = 10.1126/science.1105717 | bibcode = 2005Sci...307..910R | s2cid = 3048437  }} For other opinions see "Technical comments" linked from same Web page</ref> In fact, two of the suggestion's authors co-authored a later paper that reinterpreted the same features as evidence that ''Teinolophos'' was a full-fledged [[platypus]], which means it would have had a mammalian jaw joint and middle ear.<ref name="RoweRichRichWoodburne2008"/>

===Lactation===
{{Main|Lactation#Evolution}}

It has been suggested that lactation's original function was to keep eggs moist. Much of the argument is based on [[monotremes]] (egg-laying mammals):<ref>{{cite journal |last1=Oftedal |first1=Olav T. |title=The Mammary Gland and Its Origin During Synapsid Evolution |journal=Journal of Mammary Gland Biology and Neoplasia |date=July 2002 |volume=7 |issue=3 |pages=225–252 |doi=10.1023/A:1022896515287 |pmid=12751889 |s2cid=25806501 }}</ref><ref>{{cite journal |last1=Oftedal |first1=Olav T. |title=The Origin of Lactation as a Water Source for Parchment-Shelled Eggs |journal=Journal of Mammary Gland Biology and Neoplasia |date=July 2002 |volume=7 |issue=3 |pages=253–266 |doi=10.1023/A:1022848632125 |pmid=12751890 |s2cid=8319185 }}</ref><ref>[http://nationalzoo.si.edu/ConservationAndScience/SpotlightOnScience/oftedalolav20030714.cfm Lactating on Eggs] {{webarchive |url=https://web.archive.org/web/20090414083919/http://nationalzoo.si.edu/ConservationAndScience/SpotlightOnScience/oftedalolav20030714.cfm |date=April 14, 2009 }}</ref>
*While the amniote egg is usually described as able to evolve away from water, most reptile eggs actually need moisture if they are not to dry out.
*Monotremes do not have nipples, but secrete milk from a hairy patch on their bellies.
*During incubation, monotreme eggs are covered in a sticky substance whose origin is not known. Before the eggs are laid, their shells have only three layers. Afterwards, a fourth layer appears with a composition different from that of the original three. The sticky substance and the fourth layer may be produced by the mammary glands.
*If so, that may explain why the patches from which monotremes secrete milk are hairy. It is easier to spread moisture and other substances over the egg from a broad, hairy area than from a small, bare nipple.

Later research demonstrated that [[casein]]s already appeared in the common mammalian ancestor approximately 200–310 million years ago.<ref name=Brawand_2008>{{cite journal | vauthors = Borders R, Robertson WO | title = Washington's pesticide panel: process and product | journal = Veterinary and Human Toxicology | volume = 35 | issue = 3 | pages = 258–9 | date = June 1993 | pmid = 8351802 }}</ref> The question of whether secretions of a substance to keep eggs moist translated into actual lactation in therapsids is open. A small mammaliomorph called ''[[Sinocodon]]'', generally assumed to be the sister group of all later mammals, had front teeth in even the smallest individuals. Combined with a poorly ossified jaw, they very probably did not suckle.<ref>Crompton, Alfred W.; Luo, Zhexi (1993). Mammal Phylogeny. New York, NY: Springer New York. pp. 30–44. doi:10.1007/978-1-4615-7381-4_4. {{ISBN|9781461573838}}.</ref> Thus suckling may have evolved right at the pre-mammal/mammal transition. However, [[Tritylodontidae|tritylodontids]], generally assumed to be more basal, show evidence of suckling.<ref>{{cite journal | vauthors = Hu Y, Meng J, Clark JM | year = 2009| title = A New Tritylodontid from the Upper Jurassic of Xinjiang, China | journal = Acta Palaeontologica Polonica | volume = 54 | issue = 3| pages = 385–391 | doi = 10.4202/app.2008.0053 | s2cid = 53964916| doi-access =free  }}</ref> [[Morganucodonta]]ns, also assumed to be basal [[Mammaliaformes]], also show evidence of lactation.<ref>{{cite journal | vauthors = Panciroli E, Benson RB, Walsh S | year = 2017 | title = The dentary of Wareolestes rex (Megazostrodontidae): a new specimen from Scotland and implications for morganucodontan tooth replacement | url = https://ora.ox.ac.uk/objects/uuid:6a862377-ad73-4435-9bc7-b372c59d5be1| journal = Papers in Palaeontology | volume =  3| issue = 3| pages =  373–386| doi = 10.1002/spp2.1079 | bibcode = 2017PPal....3..373P | s2cid = 90894840 | url-access = subscription }}</ref>

===Digestive system===
The evolution of the digestive system has formed a significant influence in mammal evolution. With the emergence of mammals, the digestive system was modified in a variety of ways depending on the animal's diet. For example, cats and most carnivores have simple large intestines, while the horse as a herbivore has a voluminous large intestine.<ref>{{Cite web|url=https://campus.murraystate.edu/faculty/tderting/cva_atlases/CAT/catdigestiveevolution.htm|title=Evolutionary Trends In The Mammalian Digestive System|website=campus.murraystate.edu|access-date=2019-09-30}}</ref> An ancestral feature of [[ruminant]]s is their multi-chambered (usually four-chambered) stomach, which evolved about 50 million years ago.<ref>{{cite journal | vauthors = Hackmann TJ, Spain JN | title = Invited review: ruminant ecology and evolution: perspectives useful to ruminant livestock research and production | journal = Journal of Dairy Science | volume = 93 | issue = 4 | pages = 1320–34 | date = April 2010 | pmid = 20338409 | doi = 10.3168/jds.2009-2071 | doi-access = free }}</ref> Along with morphology of the gut, [[gastric acid]]ity has been proposed as a key factor shaping the diversity and composition of microbial communities found in the vertebrate gut. Comparisons of stomach acidity across [[Trophic level|trophic groups]] in mammal and bird taxa show that scavengers and carnivores have significantly higher stomach acidities compared to herbivores or carnivores feeding on phylogenetically distant prey such as insects or fish.<ref>{{cite journal | vauthors = Beasley DE, Koltz AM, Lambert JE, Fierer N, Dunn RR | title = The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome | journal = PLOS ONE | volume = 10 | issue = 7 | pages = e0134116 | date = 2015-07-29 | pmid = 26222383 | pmc = 4519257 | doi = 10.1371/journal.pone.0134116 | bibcode = 2015PLoSO..1034116B | doi-access = free }}</ref>

Despite the lack of fossilization of the gut, microbial evolution of the gut can be inferred from the interrelationships of existing animals, microbes and probable foodstuffs.<ref>{{cite book |vauthors = Hume ID, Warner AC |chapter=Evolution of microbial digestion in mammals|date=1980|work=Digestive Physiology and Metabolism in Ruminants: Proceedings of the 5th International Symposium on Ruminant Physiology, held at Clermont — Ferrand, on 3rd–7th September, 1979|pages=665–684| veditors = Ruckebusch Y, Thivend P |publisher=Springer Netherlands|language=en|doi=10.1007/978-94-011-8067-2_32|isbn=9789401180672|title=Digestive Physiology and Metabolism in Ruminants}}</ref> Mammals are [[Metagenomics|metagenomic]], in that they are composed of not only their own genes, but also those of all of their associated microbes.<ref>{{cite journal | vauthors = Moeller AH, Suzuki TA, Phifer-Rixey M, Nachman MW | title = Transmission modes of the mammalian gut microbiota | journal = Science | volume = 362 | issue = 6413 | pages = 453–457 | date = October 2018 | pmid = 30361372 | doi = 10.1126/science.aat7164 | bibcode = 2018Sci...362..453M | doi-access = free }}</ref> [[Human gastrointestinal microbiota|Gut microbiota]] has co-diversified as mammalian species have evolved. Recent studies indicate that adaptive divergence between mammalian species is shaped in part by changes in the gut microbiota.<ref>{{cite journal | vauthors = Moeller AH, Gomes-Neto JC, Mantz S, Kittana H, Segura Munoz RR, Schmaltz RJ, Ramer-Tait AE, Nachman MW | display-authors = 6 | title = Experimental Evidence for Adaptation to Species-Specific Gut Microbiota in House Mice | journal = mSphere | volume = 4 | issue = 4 | pages = e00387–19 | date = July 2019 | pmid = 31292233 | pmc = 6620377 | doi = 10.1128/mSphere.00387-19 }}</ref><ref>{{cite journal | vauthors = Sharpton TJ | title = Role of the Gut Microbiome in Vertebrate Evolution | journal = mSystems | volume = 3 | issue = 2 | pages = e00174–17 | date = 2018-04-24 | pmid = 29629413 | pmc = 5881020 | doi = 10.1128/mSystems.00174-17 }}</ref> The house mouse may have evolved not only with, but also in response to, the unique bacteria inhabiting its gut.<ref>{{Cite web|url=https://phys.org/news/2019-09-gut-bacteria-mammalian-evolution.html|title=Study suggests gut bacteria helped shape mammalian evolution|website=phys.org|language=en-us|access-date=2019-09-30}}</ref>

===Hair and fur===
{{See also|Evolution of hair}}
The first clear evidence of hair or fur is in fossils of ''[[Castorocauda]]'' and ''[[Megaconus]]'', from 164M years ago in the mid-Jurassic.<ref name="JiLuoYuanTabrumCastorocauda" /> As both mammals ''Megaconus'' and ''Castorocauda'' have a double coat of hair, with both guard hairs and an undercoat, it may be assumed that their last common ancestor did as well.<ref name=Luo2007/> More recently, the discovery of hair remnants in Permian coprolites pushes back the origin of mammalian hair much further back in the synapsid line to [[Paleozoic]] therapsids.<ref name="Microbiota and food residues includ">{{cite journal |last1=Bajdek |first1=Piotr |last2=Qvarnström |first2=Martin |last3=Owocki |first3=Krzysztof |last4=Sulej |first4=Tomasz |last5=Sennikov |first5=Andrey G. |last6=Golubev |first6=Valeriy K. |last7=Niedźwiedzki |first7=Grzegorz |title=Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia |journal=Lethaia |date=October 2016 |volume=49 |issue=4 |pages=455–477 |doi=10.1111/let.12156 |bibcode=2016Letha..49..455B }}</ref>

In the mid-1950s, some scientists interpreted the foramina (passages) in the [[maxilla]]e (upper jaws) and [[premaxilla]]e (small bones in front of the maxillae) of [[cynodont]]s as channels that supplied blood vessels and nerves to vibrissae ([[whiskers]]) and suggested that this was evidence of hair or fur.<ref name=Brink>{{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=Kemp>{{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}}</ref> It was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae; the modern lizard ''[[Tupinambis]]'' has foramina that are almost identical to those found in the non-mammalian cynodont ''[[Thrinaxodon]]''.<ref name=Bennet_Ruben/><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 |volume=125|url=https://www.biodiversitylibrary.org/page/2807770}}</ref> Popular sources, nevertheless, continue to attribute whiskers to ''Thrinaxodon''.<ref>{{cite web |url=http://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=February 14, 2009 |title=''Thrinaxodon:'' The Emerging Mammal |date=February 11, 2009 |publisher=National Geographic Daily News |access-date=August 26, 2012}}</ref> A [[trace fossil]] from the Lower Triassic had been erroneously regarded as a cynodont footprint showing hair,<ref>{{cite journal |last1=Ellenberger |first1=Paul |year=1976 |title=Une piste avec traces de soies épaisses dans le Trias inférieur a moyen de Lodève (Hérault, France): ''Cynodontipus polythrix nov. gen., nov. sp.'' les Cynodontes en France |journal=Geobios |volume=9 |issue=6 |pages=769–787 | doi = 10.1016/S0016-6995(76)80078-2 |bibcode=1976Geobi...9..769E }}</ref> but this interpretation has been refuted.<ref>{{cite conference |last1=Olsen |first1=Paul E. |date=March 2012 |url=http://gsa.confex.com/gsa/2012NE/finalprogram/abstract_200184.htm |title=''Cynodontipus'': A procolophonid burrow – not a hairy cynodont track (Middle-Late Triassic: Europe, Morocco, Eastern North America) |archive-url=https://web.archive.org/web/20160309232947/https://gsa.confex.com/gsa/2012NE/finalprogram/abstract_200184.htm |archive-date=9 March 2016 |conference=Northeastern Section – 47th Annual Meeting |location=Hartford, Connecticut |publisher=Geological Society of America }}</ref> A study of cranial openings for facial nerves connected whiskers in extant mammals indicate the [[Prozostrodontia]], small immediate ancestors of mammals, presented whiskers similar to mammals, but that less advanced therapsids would either have immobile whiskers or no whisker at all.<ref name=APP>{{cite journal | vauthors = Benoit J, Manger PR, Rubidge BS | title = Palaeoneurological clues to the evolution of defining mammalian soft tissue traits | journal = Scientific Reports | volume = 6 | pages = 25604 | date = May 2016 | pmid = 27157809 | pmc = 4860582 | doi = 10.1038/srep25604 | bibcode = 2016NatSR...625604B }}</ref> Fur may have evolved from whiskers.<ref>{{cite news |last1=Black |first1=Riley |title=Getting to the Root of Fur |url=https://www.nationalgeographic.com/science/article/getting-to-the-root-of-fur |archive-url=https://web.archive.org/web/20210308001402/https://www.nationalgeographic.com/science/article/getting-to-the-root-of-fur |url-status=dead |archive-date=March 8, 2021 |work=National Geographic |date=10 October 2014 }}</ref> Whiskers themselves may have evolved as a response to nocturnal and/or burrowing lifestyle.

Ruben & Jones (2000) note that the [[Harderian gland]]s, which secrete [[lipid]]s for coating the fur, were present in the earliest mammals like ''[[Morganucodon]]'', but were absent in near-mammalian therapsids like ''Thrinaxodon''.<ref name="RubenJones2000FurAndFeathers"/> The [[Msx2]] gene associated with hair follicle maintenance is also linked to the closure of the [[parietal eye]] in mammals, indicating that fur and lack of pineal eye is linked. The pineal eye is present in ''Thrinaxodon'', but absent in more advanced cynognaths (the [[Probainognathia]]).<ref name=APP/>

Insulation is the "cheapest" way to maintain a fairly constant body temperature, without consuming energy to produce more body heat. Therefore, the possession of hair or fur would be good evidence of homeothermy, but would not be such strong evidence of a high metabolic rate.<ref>{{Cite book | vauthors = Schmidt-Nielsen K | title=Animal physiology: Adaptation and environment | publisher=Cambridge University Press | year=1975 | location=Cambridge | page=[https://archive.org/details/animalphysiology00schm_0/page/699 699] | isbn=978-0-521-38196-3 | url=https://archive.org/details/animalphysiology00schm_0/page/699 }}</ref><ref>{{Cite book| vauthors = Withers PC | title=Comparative Animal Physiology | publisher=Saunders College | year=1992| location=Fort Worth |page=949| isbn=978-0-03-012847-9}}</ref>

===Erect limbs===
Understanding of the evolution of erect limbs in mammals is incomplete — living and fossil [[monotreme]]s have sprawling limbs. Some scientists think that the parasagittal (non-sprawling) limb posture is limited to the [[Mammal classification#Luo, Kielan-Jaworowska, and Cifelli classification|Boreosphenida]], a group that contains the therians but not, for example, the multituberculates. In particular, they attribute a parasagittal stance to the therians ''Sinodelphys'' and ''Eomaia'', which means that the stance had arisen by 125 million years ago, in the [[Early Cretaceous]]. However, they also discuss that earlier mammals had more erect forelimbs as opposed to the more sprawling hindlimbs, a trend still continued to some extent in modern placentals and marsupials.<ref>{{cite journal |last1=Kielan-Jaworowska |first1=Zofia |last2=Hurum |first2=Jørn H. |title=Limb posture in early mammals: Sprawling or parasagittal |journal=Acta Palaeontologica Polonica |date=2006 |volume=51 |issue=3 |pages=393–406 |url=https://www.app.pan.pl/article/item/app51-393.html }}</ref>

===Warm-bloodedness===
"[[Warm-blooded]]ness" is a complex and rather ambiguous term, because it includes some or all of the following:
*'''[[Endothermy]]''', the ability to generate heat internally rather than via behaviors such as basking or muscular activity.
*'''[[Homeothermy]]''', maintaining a fairly constant body temperature. Most [[enzyme]]s have an optimum operating temperature; efficiency drops rapidly outside the preferred range. A homeothermic organism needs only to possess enzymes that function well in a small range of temperatures.
*'''[[Tachymetabolism]]''', maintaining a high metabolic rate, particularly when at rest. This requires a fairly high and stable body temperature because of the [[Q10 (temperature coefficient)|Q<sub>10</sub> effect]]: biochemical processes run about half as fast if an animal's temperature drops by 10&nbsp;°C.

Since scientists cannot know much about the internal mechanisms of extinct creatures, most discussion focuses on homeothermy and tachymetabolism. However, it is generally agreed that endothermy first evolved in non-mammalian synapsids such as [[dicynodont]]s, which possess body proportions associated with heat retention,<ref>{{cite book |doi=10.1007/978-3-642-87810-7_21 |chapter=Experimental and Fossil Evidence for the Evolution of Tetrapod Bioenergetics |title=Perspectives of Biophysical Ecology |series=Ecological Studies |year=1975 |last1=Bakker |first1=Robert T. |volume=12 |pages=365–399 |isbn=978-3-642-87812-1 }}</ref> high vascularised bones with [[Haversian canal]]s,<ref>{{cite journal |last1=Botha-Brink |first1=Jennifer |last2=Angielczyk |first2=Kenneth D. |title=Do extraordinarily high growth rates in Permo-Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction? |journal=Zoological Journal of the Linnean Society |date=26 July 2010 |volume=160 |issue=2 |pages=341–365 |doi=10.1111/j.1096-3642.2009.00601.x |doi-access=free }}</ref> and possibly hair.<ref name="Microbiota and food residues includ"/> More recently, it has been suggested that endothermy evolved as far back as ''[[Ophiacodon]]''.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2015/10/151029134252.htm |title=Ancestry of mammalian 'warm-bloodedness' revealed |date=October 29, 2015 |website=www.sciencedaily.com |publisher=Society of Vertebrate Paleontology |access-date=October 29, 2015}}</ref>

Modern [[monotremes]] have a low body temperature compared to marsupials and placental mammals, around {{cvt|32|°C}}.<ref>{{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 | url=https://archive.org/details/predatorydinosau00paul/page/464 }}</ref> [[Phylogenetic bracketing]] suggests that the body temperatures of early crown-group mammals were not less than that of extant monotremes. There is [[Cell biology|cytological]] evidence that the low metabolism of monotremes is a secondarily evolved trait.<ref name="temp">{{cite journal |last1=Watson |first1=JM |last2=Graves |first2=JAM |title=Monotreme Cell-Cycles and the Evolution of Homeothermy |journal=Australian Journal of Zoology |date=1988 |volume=36 |issue=5 |pages=573 |doi=10.1071/ZO9880573 }}</ref>

====Respiratory turbinates====
Modern mammals have respiratory turbinates, convoluted structures of thin bone in the nasal cavity. These are lined with [[mucous membrane]]s that warm and moisten inhaled air and extract heat and moisture from exhaled air. An animal with respiratory turbinates can maintain a high rate of breathing without the danger of drying its lungs out, and therefore may have a fast metabolism. Unfortunately these bones are very delicate and therefore have not yet been found in fossils. But rudimentary ridges like those that support respiratory turbinates have been found in advanced Triassic [[cynodont]]s, such as ''[[Thrinaxodon]]'' and ''[[Eucynodontia|Diademodon]]'', which suggests that they may have had fairly high metabolic rates.<ref name=Brink/><ref>{{cite journal |last1=Hillenius |first1=Willem J. |title=The evolution of nasal turbinates and mammalian endothermy |journal=Paleobiology |date=1992 |volume=18 |issue=1 |pages=17–29 |doi=10.1017/S0094837300012197 |bibcode=1992Pbio...18...17H |s2cid=89393753 }}</ref><ref>{{cite journal |last1=Ruben |first1=John |title=The Evolution of Endothermy in Mammals and Birds: From Physiology to Fossils |journal=Annual Review of Physiology |date=October 1995 |volume=57 |issue=1 |pages=69–95 |doi=10.1146/annurev.ph.57.030195.000441 |pmid=7778882 }}</ref>

====Bony secondary palate====
Mammals have a secondary bony palate, which separates the respiratory passage from the mouth, allowing them to eat and breathe at the same time. Secondary bony palates have been found in the more advanced cynodonts and have been used as evidence of high metabolic rates.<ref name=Brink/><ref name=Kemp/><ref>{{cite journal |last1=McNab |first1=Brian K. |title=The Evolution of Endothermy in the Phylogeny of Mammals |journal=The American Naturalist |date=January 1978 |volume=112 |issue=983 |pages=1–21 |doi=10.1086/283249 |bibcode=1978ANat..112....1M |s2cid=84070652 }}</ref> But some cold-blooded vertebrates have secondary bony palates (crocodilians and some lizards), while birds, which are warm-blooded, do not.<ref name=Bennet_Ruben/>

====Diaphragm====
A muscular [[Thoracic diaphragm|diaphragm]] helps mammals to breathe, especially during strenuous activity. For a diaphragm to work, the ribs must not restrict the abdomen, so that expansion of the chest can be compensated for by reduction in the volume of the abdomen and ''vice versa''. Diaphragms are known in [[Caseidae|caseid]] pelycosaurs, indicating an early origin within synapsids, though they were still fairly inefficient and likely required support from other muscle groups and limb motion.<ref>{{cite journal |last1=Lambertz |first1=Markus |last2=Shelton |first2=Christen D. |last3=Spindler |first3=Frederik |last4=Perry |first4=Steven F. |title=A caseian point for the evolution of a diaphragm homologue among the earliest synapsids: Early synapsid origin of a diaphragm homologue |journal=Annals of the New York Academy of Sciences |date=December 2016 |volume=1385 |issue=1 |pages=3–20 |doi=10.1111/nyas.13264 |pmid=27859325 |bibcode=2016NYASA1385....3L |s2cid=24680688 }}</ref>

The advanced cynodonts have very mammal-like rib cages, with greatly reduced lumbar ribs. This suggests that these animals had more developed diaphragms, were capable of strenuous activity for fairly long periods and therefore had high metabolic rates.<ref name=Brink/><ref name=Kemp/> On the other hand, these mammal-like rib cages may have evolved to increase agility.<ref name= Bennet_Ruben/> However, the movement of even advanced therapsids was "like a wheelbarrow", with the hindlimbs providing all the thrust while the forelimbs only steered the animal, in other words advanced therapsids were not as agile as either modern mammals or the early dinosaurs.<ref name="cowen" /> So the idea that the main function of these mammal-like rib cages was to increase agility is doubtful.

====Limb posture====
The [[therapsids]] had sprawling forelimbs and semi-erect hindlimbs.<ref name=Kemp/><ref>{{Cite journal |id={{NAID|20001686481}} | vauthors = Jenkins FA Jr | title=The postcranial skeleton of African cynodonts | journal=Bulletin of the Peabody Museum of Natural History | issue=36 | pages=1–216 |year=1971 }}</ref> This suggests that [[Carrier's constraint]] would have made it rather difficult for them to move and breathe at the same time, but not as difficult as it is for animals such as lizards, which have completely sprawling limbs.<ref>{{Cite book | vauthors = Pough FH, Heiser JS, McFarland WN | title=Vertebrate Life | publisher=Prentice-Hall | year=1996 | location=New Jersey | page=[https://archive.org/details/vertebratelife00poug/page/798 798] | isbn=978-0-02-396370-4 | url=https://archive.org/details/vertebratelife00poug/page/798 }}</ref> Advanced therapsids may therefore have been significantly less active than modern mammals of similar size and so may have had slower metabolisms overall or else been [[Bradymetabolism|bradymetabolic]] (lower metabolism when at rest).

===Brain===
Mammals are noted for their large brain size relative to body size, compared to other animal groups. Recent findings suggest that the first brain area to expand was that involved in smell.<ref name="Gill 2011">{{cite news |title=Mammals' large brains evolved for smell |author=Victoria Gill |url=https://www.bbc.co.uk/nature/13448202 |newspaper=BBC News |date=20 May 2011 |access-date=22 May 2011}}</ref> Scientists scanned the skulls of early mammal species dating back to 190–200 million years ago and compared the brain case shapes to earlier pre-mammal species; they found that the brain area involved in the sense of smell was the first to enlarge.<ref name="Gill 2011" /> This change may have allowed these early mammals to hunt insects at night when dinosaurs were not active.<ref name="Gill 2011" />

After the extinction of the dinosaurs 66 million years ago, mammals began to increase in body size as new niches became available, but their brain lagged behind their bodies for the first ten million years. Relative to body size the brain of Paleocene mammal was relatively smaller than that of Mesozoic mammals. It was not until the Eocene that the mammalian brains began to catch up with their bodies, particularly in certain areas associated with their senses.<ref>{{cite news |last1=Gramling |first1=Carolyn |title=Mammals' bodies outpaced their brains right after the dinosaurs died |url=https://www.sciencenews.org/article/mammal-big-bodies-small-brains-dinosaur-extinction-fossils |work=Science News |date=31 March 2022 }}</ref>

===Testicular descent===
{{excerpt|Evolution of descended testes in mammals}}

== Sexual selection ==
{{excerpt|Sexual selection in mammals}}
== See also ==
* [[History of life]]
* [[Evolution of primates]]
* [[Evolution of ungulates]]
** [[Evolution of even-toed ungulates]]
** [[Odd-toed ungulate#Evolutionary history|Evolution of odd-toed ungulates]]
* [[Genome diversity and karyotype evolution of mammals]]
* [[List of examples of convergent evolution#Mammals|List of examples of convergent evolution in mammals]]
* ''[[Juramaia]]''

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

== Further reading ==
{{Refbegin}}
* [[Robert L. Carroll]], ''Vertebrate Paleontology and Evolution'', W. H. Freeman and Company, New York, 1988 {{ISBN|0-7167-1822-7}}. Chapters XVII through XXI
* Nicholas Hotton III, Paul D. MacLean, Jan J. Roth, and E. Carol Roth, editors, ''The Ecology and Biology of Mammal-like Reptiles'', Smithsonian Institution Press, Washington and London, 1986 {{ISBN|0-87474-524-1}}
* T. S. Kemp, ''The Origin and Evolution of Mammals'', Oxford University Press, New York, 2005 {{ISBN|0-19-850760-7}}
* {{cite book |last1=Kielan-Jaworowska |first1=Zofia |authorlink1=Zofia Kielan-Jaworowska |last2=Cifelli |first2=Richard |last3=Luo |first3=Zhe-Xi |title=Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure |date=2004 |publisher=Columbia University Press |isbn=978-0-231-50927-5 |oclc=61160163 |doi=10.7312/kiel11918 |jstor=10.7312/kiel11918 }}
* {{cite journal |last1=Weil |first1=Anne |title=Upwards and onwards |journal=Nature |date=April 2002 |volume=416 |issue=6883 |pages=798–799 |doi=10.1038/416798a |pmid=11976661 |bibcode=2002Natur.416..798W |s2cid=4332049 }}
* {{cite journal |last1=Whitfield |first1=John |title=Tree-climbing with dinosaurs |journal=Nature |date=25 April 2002 |doi=10.1038/news020422-15 }}
{{Refend}}

== External links ==
*[http://www.nasmus.co.za/PALAEO/jbotha/the_cynodontia.htm The Cynodontia] {{Webarchive|url=https://web.archive.org/web/20081012005104/http://www.nasmus.co.za/PALAEO/jbotha/the_cynodontia.htm |date=2008-10-12 }} covers several aspects of the evolution of cynodonts into mammals, with plenty of references.
*[https://www.bbc.co.uk/programmes/p003k9ds Mammals], BBC Radio 4 discussion with Richard Corfield, Steve Jones & Jane Francis (''In Our Time'', Oct. 13, 2005)

{{Evolution}}
{{Mammals}}
{{Authority control}}

{{DEFAULTSORT:Evolution Of Mammals}}
[[Category:Evolution of mammals| ]]