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Placental

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Placental

"Placental" redirects here. For the organ interfacing between a mammalian mother and a fetus, see Placenta.
Further information: Evolution of mammals
Eutheria
Temporal range: Late Jurassic–Present, 160–0Ma
Life restoration of Juramaia sinensis, the earliest known eutherian
Subgroups

Eutheria (/juːˈθɪəriə/; from Greek ευ-, eu- "true/good" and θηρίον, thērion "beast" hence "true beasts") is one of two mammalian clades with extant members that diverged in the Late Jurassic. The other is the Metatheria, which includes kangaroos and the other mammals (most of the marsupials) that accommodate their neonates in pouches. Except for the opossum, which is a metatherian, all mammals indigenous to North America, Europe, Africa, and Asia are eutherians. Extant eutherians, their last common ancestor, and all extinct descendants of that ancestor are placentals, in the infraclass Placentalia.

Eutherians are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil and living mammals (marsupials and monotremes).

The oldest known eutherian species is Juramaia sinensis, dated at

"Eutheria" was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than "Placentalia", the term previously in use.

Characteristics

The features of Eutheria that distinguish them from metatherians, a group that includes modern marsupials, are:

  • an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones.[3]
  • the joint between the first
  • various features of jaws and teeth.[3]

Placental mammals are distinguished from other eutherians by:

  • the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.[3]
  • a complete mortise and tenon upper ankle joint, where the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula.[3]
  • a wide opening at the bottom of the pelvis, which allows the birth of large, well-developed offspring. Marsupials have and nonplacental eutherians had a narrower opening that allows only small, immature offspring to pass through.[4]
  • the absence of epipubic bones extending forward from the pelvis, which are not found in any placental, but are found in all other mammals – nonplacental eutherians, marsupials, monotremes, and earlier mammaliaforms as well as in other cynodonts that are closest to mammals. Their function is to stiffen the body during locomotion.[5] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.[6]

Subgroups

These are the subgroups of extant members of Eutheria:

These groups together make up the crown group Placentalia (placental mammals). Eutheria also includes now extinct lineages that lie outside of Placentalia (see below).[7]

Another now extinct clade - the Meridiungulata - a set of ungulates from South America appear to group with the Afrotheria and the Xenarthra.

An analysis of transposable element insertions around the time of divergence of Boreoeutheria, Afrotheria, and Xenarthra has supported a near-concomitant origin (trifurcation) of these three superorders.[8][9] If accepted, this conclusion would eliminate the need to choose between the previously proposed groupings of Boreoeutheria and Xenarthra (Exafroplacentalia),[10] Afrotheria and Xenarthra (Atlantogenata),[11][12][13] Afrotheria and Boreoeutheria (Epitheria).[14][15] The debate continues, however; a 2013 paper identifying Protungulatum donnae as one of the first placental mammals supports the Epitheria hypothesis using an analysis that combines molecular and phenomic considerations.[16][17]

Evolutionary history

The fossil eutherian species believed to be the oldest known is Juramaia sinensis, which lived about
Millions of years ago
= Placentals      = Other eutheria
Origin
of
eutheria
    = Asian fossils         = N American fossils
    = Period when placental classes diverged according to molecular phylogenetics estimates
Murtoilestes
Prokennalestes
Montanalestes
Ukhaatherium
Asioryctes
Daulestes
Aspanestes
Eoungulatum
Gypsonictops
Fossil record of Cretaceous eutheria[3]
Simplified, non-systematic, outline of evolution of eutheria from cynodont therapsids.[3]
† = extinct

References

Further reading

External links

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