Temporal range: 165–0 Ma Mid Jurassic – Recent
Fossil of Eomaia, an early eutherian Scientific classification Kingdom: Animalia Phylum: Chordata Class: Mammalia Subclass: Theria Infraclass: Eutheria
Thomas Henry Huxley, 1880
Superorders and genera
Eutheria (pronounced /juːˈθɪəriə/; Greek ευθήριον, pronounced euthérion and meaning "true/good beasts") is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They 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 (monotremes and marsupials).
The oldest known eutherian species is Juramaia sinensis, dated at Jurassic in China. The previously earliest known fossil eutherian, Eomaia scansoria, was also from China and is dated to the Early Cretaceous period, about .from the
Eutheria was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than its precursor Placentalia.
Eutherians are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos).
There are no living nonplacental eutherians, and so knowledge of their synapomorphies ("defining features") is entirely based on a few fossils, which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining Eutheria. 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 mesocuneiform bones – in metatherians these joints are level with each other.
- various features of jaws and teeth.
Reproductive features are also of no use in identifying fossil placental mammals, which 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 have and nonplacental eutherians had 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 mammaliformes – and even in the cynodont therapsids 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, dolphins
- Xenarthra, e.g. armadillos, anteaters
- Afrotheria, e.g. elephants, manatees
Analysis of transposable element insertions around the time of divergence of Boreoeutheria, Afrotheria, and Xenarthra strongly support a near-concomitant origin (trifurcation) of these three superorders, making further subdivision impractical and meaningless. These observations eliminate the need to choose between the previously proposed groupings of Boreoeutheria and Xenarthra (Exafroplacentalia), Afrotheria and Xenarthra (Atlantogenata), Afrotheria and Boreoeutheria (Epitheria).
The fossil eutherian species believed to be the oldest known is Juramaia sinensis, said to be about  Another early eutherian mammal is the nonplacental Eomaia scansoria from the Lower Cretaceous of China, dated to about . Some of its fossils show thick fur. 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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Extant mammal orders by infraclassKingdom Animalia · Phylum Chordata · Subphylum Vertebrata · (unranked) Amniota Australosphenida Metatheria
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