Temporal range: Late Jurassic–Present, 160–0Ma
|Life restoration of Juramaia sinensis, the earliest known eutherian|
Eutheria (//; 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 Early Cretaceous or perhaps the Late Jurassic. The other is the Metatheria, which includes marsupials, most of whom accommodate their neonates in pouches. Except for the Virginia opossum, which is a metatherian, all mammals indigenous to Europe, Africa, Asia, and North America north of Mexico 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).
"Eutheria" was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than "Placentalia", the term previously in use.
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.
- the joint between the first metatarsal bone and the entocuneiform bone in the foot is offset further back than the joint between the second metatarsal and middle cuneiform bones – in metatherians these joints are level with each other.
- various features of jaws and teeth.
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.
- 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.
- a wide opening at the bottom of the pelvis, which allows the birth of large, well-developed offspring. Marsupials and nonplacental eutherians have a narrower opening that allows only small, immature offspring to pass through.
- 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. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.
These are the subgroups of extant members of Eutheria:
- Boreoeutheria, e.g. badgers, rabbits, guinea pigs, dogs
- Xenarthra, e.g. armadillos, anteaters
- Afrotheria, e.g. elephants, manatees
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. If accepted, this conclusion would eliminate the need to choose between the previously proposed groupings of Boreoeutheria and Xenarthra (Exafroplacentalia), Afrotheria and Xenarthra (Atlantogenata), Afrotheria and Boreoeutheria (Epitheria). 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.
|The fossil eutherian species believed to be the oldest known is Juramaia sinensis, which lived about . Montanalestes was found in North America, while all other nonplacental eutherian fossils have been found in Asia. The earliest known placental fossils have also been found in Asia.|
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|Wikisource has the text of the 1911 Encyclopædia Britannica article Monodelphia.|