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Eutheria (/juːˈθɪəriə/; from Greek εὐ-, eú- 'good, right' and θηρίον, thēríon 'beast'; lit. 'true beasts') is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials. There are 20 orders of animals in this subclass, which account for about 95% of the existing mammals. Viviparous, with a true placenta.

Eutheria includes one of three major clades of mammals, the extant members of which are referred to as placentals. With more than 5000 extinct and extant genera, including 1220 extant (living) genera and over 6000 extant species arrayed in 19 extant orders, placentals (extant eutherians) are the most taxonomically diverse of the three branches of extant mammals. Eutherians appear in the fossil record by 125 Ma (million years ago). Extant orders of eutherians (placentals) do not occur in the fossil record until after dinosaur extinction 65–66 Ma. Molecular studies generally agree with these dates for the origin of placental orders but place the origin of Eutheria at approximately 190 Ma and groups related to living orders of mammals by 100 Ma. Placentals vary greatly in size (whales down to shrews), in locomotion (flying, swimming, climbing, burrowing, running), and diet (meat, leaves, fruit, termites). They have a high resting temperature (homiothermy) and produce this heat internally (endothermy). They have a chorioallantoic placenta that allows a long gestation for development. Eutherian mammals are the most ecomorphological and taxonomically diverse group of mammals. The evolutionary success of eutherian mammal is related to their mode of reproduction (euviviparity). Eutherian mammals originated in the Early Cretaceous but modern placental orders radiated in the Cenozoic, after extinction of dinosaurs. The four major groups of Placentalia are Xenarthra, Afrotheria, Laurasiatheria and Euarchontoglires. The Cretaceous stem placentals mostly retained primitive therian dental formula which was further modified in the Cenozoic crown placentals.

Scientific classification

Named by and Year:
named in 1872 by Theodore Gill
Subphylum Vertebrata
Class Mammalia


Placentalia is the most taxonomically diverse of three branches orclades of modern mammals, the other two being Marsupialia andMonotremata. The crown-group Placentalia includes all extantmammals and their most recent common ancestor. Eutheria, orPan-Placentalia, is a total clade of mammals that includes Placen-talia and the extinct stem taxa that are more related to placentalsthan to modern marsupials. In the nineteenth century, mammalswere often separated into Ptototheria (monotremes) and Eutheria(marsupials and placentals) (Gill, 1872). The term Eutheria was first used in a modern sense by Huxley (1880), who divided mam-mals by the level of organisation into Hypotheria (hypotheticalancestors), Prototheria (monotremes), Metatheria (marsupials)and Eutheria (placentals). The evolutionary high-level classica-tion of eutherians was elaborated in the rst half of the twentiethcentury (Gregory, 1910; Simpson, 1945). First classications ofeutherians based on cladistics principles appeared in the secondhalf of the twentieth century (McKenna, 1975). Development ofmolecular methods considerably reshaped phylogeny of Euthe-ria since the end of the twentieth century. See also:Mammalia;Marsupialia (Marsupials);Monotremata


The whole body is covered with hair (except for a few species), fast-moving, and thermostatic viviparous vertebrate, which is the highest-level animal group with the most complex body structure and functional behavior among vertebrates. Body coat; constant body temperature; viviparous (except monotreme) and lactation; left and right chambers of the heart are completely separated, and the left ventricle pumps blood to all parts of the body through the left arterial arch; enlarged brain and increased brain volume; middle ear has 3 ossicles; the mandible is composed of 1 dentary bone, which is articulated with the skull as tooth-squamous bones; the teeth are differentiated into incisors, canines and buccal teeth; 7 cervical vertebrae, the first and second cervical vertebrae are divided into ring vertebrae and axis vertebrae.

Viviparous, with a true placenta. The embryo develops in the mother's womb for a long time, absorbs the mother's nutrition through the placenta, and the cubs produced are fully developed and can suck milk by themselves. The mammary glands are well developed with nipples. The cerebral cortex is developed, and the corpus callosum connects the two cerebral hemispheres. The body temperature is high and constant, and the replacement of deciduous teeth and permanent teeth is obvious. The shoulder girdle is a single scapula, and the coracoid degenerates into the coracoid process on the scapula. There is no cloaca, the intestinal tube opens out of the body through the anus alone, and the excretory and reproductive ducts merge into the cloacal sinus, which opens out of the body through the cloaca.


During the Jurassic period of the Mesozoic Era, the molar tips of the trinodular dentition seen in this period have evolved from the straight arrangement of the triconical dentition to a triangular arrangement. Three branches evolved from the trinodular tooth, namely, the trident, the anti-tooth and the ancient beast. Among them, the first two lived until the end of the Jurassic and the beginning of the Cretaceous, and the third branch of the ancient beast was obtained. Flourish. The molar tips of ancient mammals are also arranged in a triangle, but the mandibular molars have a "lower calcaneus" with two cusps behind them. The ancient mammals are the ancestors of the post-animal subclass and the true animal sub-class.

Fossil History And Distribution

The earliest known possible fossils of eutherians come fromAsia. Juramaia from the late Middle Jurassic (approximately 160Ma) of China was described as the oldest eutherian mammal(Luo et al., 2011) but it might be a stem therian, originatingbefore the split of Metatheria and Eutheria. Eomaia from theEarly Cretaceous (approximately 125 Ma) of China (Ji et al.,2002) and slightly younger Prokennalestes from the Early Cre-taceous (approximately105 Ma) of Mongolia (Lopatin and Ave-rianov, 2017) are among the earliest known eutherian mammals.Prokennalestes and other Cretaceous eutherians show the typi-cal eutherian pattern of at most five upper and lower premolarsand three upper and lower molars. The last upper and lowerpremolars in the earliest eutherians as compared to metatheri-ans already show trends towards molarisation (i.e. adding extracusps found on molars). The labial (cheek side) of the uppermolars has a wide area called the stylar shelf which unlikein contemporary metatherians has few cusps. The back, lowermargin of the lower jaw, the dentary, has a projection (angu-lar process) that points backwards in eutherians but internally inmetatherians. These forms were all small, ranging in size froma shrew to an opossum. Eomaia is argued to show both scanso-rial (climbing) and arboreal (tree-living) adaptations, comparedto other Cretaceous eutherians that, when known, are terrestrialand sometimes cursorial (running). Diets were mostly carnivo-rous to insectivorous, but omnivory and probably even herbivoryoccurred in some eutherians by the time of dinosaur extinction 65Ma. Within approximately 15 million years of dinosaur extinc-tion, most of the 19 extant orders of placentals had appearedalong with some 16 other orders that are now extinct. This wasa truly explosive radiation and diversification (O’Leary et al.,2013).

North America and Eurasia are known to have served as cen-tres for much of the diversi fication of extant placental ordersthroughout much of the Tertiary. Although less is known aboutthe early radiation of extant placental orders in Africa, both cur-rent diversity on this continent and recent molecular studies ofendemic African clades indicate that this continent was also amajor centre for placental evolution. Eutherians probably did notreach South America until approximately 65 Ma. Except for pos-sibly Cingulata and Pilosa, no extant placental orders are believedto have originated in South America. This is not true of extinctorders. At least five extinct orders are endemic to South America.These mostly herbivorous taxa fiourished throughout much ofthe Tertiary, possibly rivalling the diversity among extant Africanherbivorous placentals. They ranged from rabbit- to rhino-sized.In Australia, except for bats, which reached Australia in theearly Eocene (approximately 55 Ma), eutherians are not defi-nitely known from this continent until approximately 5 Ma, whenrodents arrived. Today, bats and rats are the only placentals thatreached Australia without the aid of humans. Madagascar has anunusual placental fauna, the best known being lemurs and rela-tives, which hark back to an early Tertiary African fauna. Exceptfor bats, oceanic islands lack any nonmarine mammals, with theexception of a few species of rodents on the Galapagos. Even thelargest oceanic islands such as those comprising New Zealand,today totally lack nonmarine placentals, except for a few species of bats. See also:Diversity of Life through Time;FossilRecord; Geological Time: Principles


Based upon anatomical and developmental studies, the fossilrecord, and molecular studies, 19–20 orders of placental are nowrecognised (Figure 3). Most of the orders appear in the fossilrecord within the first 15–20 million years of the Cenozoic.Although ordinal and superordinal groupings of placentals basedon molecular studies and those based on fossil and anatomi-cal data agree in many ways (Archibald, 2003), a number ofmolecular studies have argued for greatly altering parts of thetraditional phylogeny (Murphy et al., 2001). The earliest diverg-ing major new taxon is Afrotheria, which groups together sixorders that are restricted to Africa (and Madagascar), or appearto have originated on this continent. Afrotheria includes thetraditionally recognised Paenungulata of Hyracoidea (hyraxesof Africa), Proboscidea (elephants) and Sirenia (the tropicalmarine manatees and dugongs), plus Tubulidentata (aardvarkof Africa), Macroscelidea (African elephant shrews), and also anew order, Afrosoricida (=Tenrecoidea), which includes tenrecsand golden moles, both formerly placed in Lipotyphla. A secondgroup is Xenarthra, which includes Cingulata (armadillos) andPilosa (anteaters and sloths) with a history mostly found inSouth America. Finally are two groups, Euarchontoglires andLaurasiathera, together known as Boreoeutheria, refiecting anorthern or boreal distribution earlier in their evolutionary his-tory. Euarchontoglires includes Glires (Lagomorpha, the rabbitsand pikas, as well as Rodentia), which are linked to a modifiedArchonta, including Scandentia (tree shrews), Dermoptera (theso-called fiying lemurs), and Primates but lacking Chiroptera(bats). Laurasiastheria includes Lipotyphla (Erinaceomorpha andSoricomorpha), Chiroptera, Pholidota (pangolins), Carnivora,Perissodactyla (odd-toed ungulates such as horses, rhinos andtapirs), and a new ordinal grouping. This new ordinal group-ing includes cetaceans (whales and relatives), and artiodactyls(even-toed ungulates such as deer, antelope, pigs, camels andhippos), and is called Artiodactyla or Cetartiodactyla. Unliketraditional phylogenies that link them as sister taxa, the newmolecular studies indicate that the nearest relative of Cetaceais within Artiodactyla, specifically the family Hippopotamidae. See also:Molecular Phylogeny Reconstruction