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A higher-level taxon that includes all mammals except monotremes and marsupials. Eutheria (Placentalia) is variously ranked as an infraclass or cohort within Mammalia. Eutheria includes over 4000 living species arranged in 18 orders; another 12 orders are known only from fossils. An ecologically diverse group, Eutheria includes primates, insectivores, bats, rodents, carnivores, elephants, ungulates, and whales. Like other mammals, eutherians are generally fur-covered and produce milk to nourish their young. In part because they can make their own body heat and regulate their body temperature, eutherians are widely distributed over most continents and occur in all oceans.

Eutherians, often called placental mammals, have a unique reproductive system in which unborn young are nourished for an extended period via a placenta. This system permits retention of the young in the protective environment of the uterus during most of early development. Fetal survival rates are high under most conditions. Young are born in a relatively advanced state of development and are never sheltered in a pouch after birth. Gestation time ranges from 20 days (for example, shrews and hamsters) to 22 months (elephants). Many eutherians have only one or two young per pregnancy, but as many as 20 offspring may be produced at a single birth in some species.

Eutherians range in size from insectivores and bats that weigh only a few grams to blue whales that can weigh over 190,000 kg (420,000 lb). All have a relatively large brain and exhibit complex behavior, with many living in social groups. Eutherians exhibit more variation in ecology than any other group of vertebrates, and these differences are reflected in their morphological specializations.

The fossil record of Eutheria extends back at least into the Cretaceous Period. Several differences in the skull and dentition distinguish fossil eutherians from early members of other mammal lineages (for example, marsupials). The earliest eutherians were apparently small, nocturnal mammals that may have resembled some modern insectivores. Although Cretaceous eutherians are known from most continents, diversification of the modern orders apparently did not occur until the Paleocene and Eocene. See Cetacea, Chiroptera, Mammalia, Theria

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



an infraclass of viviparous mammals having the highest organization and the greatest ecological and morphological diversity. The brain characteristically has large, highly developed hemispheres, which are connected by means of the corpus callosum. Embryonic development involves the formation of a placenta. The marsupial bones characteristic of marsupials, the second infraclass of viviparous mammals, are absent. The dental formula is:

The Eutheria include 14 extinct and 17 extant orders. They have been traced to the Early Cretaceous.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


(vertebrate zoology)
An infraclass of therian mammals including all living forms except the monotremes and marsupials.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Care was taken to choose taxa that sampled the Eutheria or Metatheria as broadly as possible.
Patterns of Development within the Metatheria and Eutheria
The first step distinguishes between the characters with states that are conserved across Theria, those that are conserved within the Metatheria (but differ from the Eutheria), those that are conserved within the Eutheria (and are different from the Metatheria), and those that vary, either across all Theria or within one of the superorders.
Because the Eutheria and Metatheria are both represented by an array of taxa that do not reflect higher order branching (i.e., none of the species in the sample shares close relation to another species in the sample) and that cover the diversity within the superorder fairly broadly (i.e., the families and orders represented are among the basal radiations of the groups), it is reasonably likely that the patterns that emerge are representative of marsupials and placentals.
These characters were those element pairs that exhibited consistent patterns within but distinct patterns between the Eutheria and Metatheria.
Because data are not as yet available on taxa beyond the Eutheria and the Metatheria, it is impossible to infer the primitive condition.
In Metatheria (Cisternas & Armati, 1999) and Eutheria (Hartwig & Hartwig, 1985) the splenic structure is similar.
The armadillos are an ancient eutheria group appeared in the limit between the Cretacic and the Tertiary, that has been isolated since their origins to the beginning of the Eocene (Delsuc et al., 2001).