Clade of mammals in the subclass Theria
Eutheria (; from Greek εὐ-, eú- 'good, right' and θηρίον, thēríon 'beast'; lit. 'true beasts') is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials.
Eutherians are distinguished from noneutherians by various phenotypic traits of the feet, ankles, jaws and teeth. All extant eutherians lack epipubic bones, which are present in all other living mammals (marsupials and monotremes). This allows for expansion of the abdomen during pregnancy.[1]
The oldest-known eutherian species is Juramaia sinensis, dated at 161 million years ago from the early Late Jurassic (Oxfordian) of China.[2]
Eutheria was named in 1872 by Theodore Gill; in 1880 Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia.[3]
Characteristics
Distinguishing features are:
- an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones[4]
- the joint between the first metatarsal bone and the entocuneiform bone (the innermost of the three cuneiform bones) in the foot is offset farther back than the joint between the second metatarsal and middle cuneiform bones—in metatherians these joints are level with each other[4]
- various features of jaws and teeth[4]
Evolutionary history
Eutheria contains several extinct genera as well as larger groups, many with complicated taxonomic histories still not fully understood. Members of the Adapisoriculidae, Cimolesta and Leptictida have been previously placed within the outdated placental group Insectivora, while Zhelestids have been considered primitive ungulates.[6] However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia.[7][8]
The weakly favoured cladogram favours Boreoeuthearia as a basal eutherian clade as sister to the Atlantogenata.[9][10][11]
| The fossil eutherian species believed to be the oldest known is Juramaia sinensis, which lived about 160 million years ago.[2] 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.[4]
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Simplified, non-systematic, outline of evolution of eutheria from cynodont therapsids. [4]† = extinct
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