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The factual accuracy of this article is disputed.
Please see the relevant discussion on the talk page. (March 2008) |
Placental mammals / Eutheria
Fossil range: Early Cretaceous - Recent |
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| Scientific classification |
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| Orders[1] |
- Bobolestes
- Eomaia
- Maelestes
- Montanalestes
- Murtoilestes
- Prokennalestes
- Placentalia
- Superorder Xenarthra:
- Superorder Laurasiatheria:
- Pholidota (Scaly Anteaters)
- Soricomorpha (Moles, Shrews)
- Erinaceomorpha (Hedgehogs)
- Perissodactyla (Horses, Rhinoceroses and Tapirs)
- Cetartiodactyla (Bovids, Pigs, Cervids, Camels, Whales, Dolphins)
- Carnivora (Cats, Dogs, Bears, Seals)
- Chiroptera (Bats)
- †Meridiungulata
- †Condylarthra
- †Dinocerata
- †Mesonychia
- †Cimolesta
- †Creodonta
- Superorder Afrotheria:
- Superorder Euarchontoglires:
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Eutheria[2] is a taxon containing the placental mammals, such as humans, defined as the group consisting of all animals, living or extinct, more closely related to the crown group Placentalia than to Marsupialia. The sister group of Eutheria is Metatheria, which includes marsupials and their extinct relatives. The name Eutheria comes from the Greek words eu- "well[-developed]" and ther "beast". When Eutheria was introduced by Thomas Henry Huxley in 1880, he meant it to be broader in definition than its precursor Placentalia.
Characteristics of eutherians
All living eutherians are placental mammals. This means that a eutherian fetus is nourished during gestation by a placenta, and is a synapomorphy of the group, though it is a common misconception that only placental mammals have chorio-allantoic placentae. Eutherians are also viviparous, meaning that the offspring are carried in the mother's uterus until fully developed[3]. They are further diagnosed from other therians by their lack of the epipubic bones, relative metabolic rate, and various other synapomorphies.
These are in contrast to monotremes, for instance, lay eggs which protect developing young until they are fully developed. Marsupials give birth to partially-developed young who then migrate to a special marsupium in the mother's body in which the young continue their development.[4]
Members of Eutheria are found on all continents and in all oceans.
Earliest example
The earliest known eutherian species is the extinct Eomaia scansoria from the Lower Cretaceous of China. It is a member of Eutheria, but the hips of the animal were too narrowly built to have allowed the birth of well-developed young. Thus it is unlikely that a placenta greatly contributed to the development of E. scansoria's young before they were born.
See also
Compare and contrast
Notes and references
- ^ "Eutheria phylogeny". Mikko's Phylogeny Archive. Retrieved on 2008-07-24.
- ^ Today Placentalia is the only surviving member of the group Eutheria, which is diagnosed by a wider set of synapomorphies than Placentalia and includes specimens such as Eomaia.
- ^ Again, not only mammals are viviparous. For example, most sea snakes are oviviviparous, and some other snakes and certain sharks are fully viviparous
- ^ Some exceptions do exist. Bandicoots for instance, which are marsupials, develop small, superficial placenta structures during gestation.
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