Eutheria

Further information: Evolution of mammals

Eutherians Temporal range: Late Jurassic–Holocene,
Juramaia, the oldest known eutherian
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	Theria
Clade:	Eutheria Gill, 1872 Huxley, 1880
Subgroups
†Acristatherium †Adapisoriculidae †Asioryctitheria †Bobolestes †Cimolesta †Didymoconidae †Deltatherium †Endotherium †Juramaia †Leptictida †Montanalestes †Murtoilestes †Prokennalestes? †Zalambdalestidae †Zhelestidae Placentalia (placental mammals)

Eutheria (/juːˈθɪəriə/; from Greek εὐ-, eu- "true/good" and θηρίον, thēríon "beast" hence "true beasts") is one of two mammalian clades with extant members that diverged in the Early Cretaceous or perhaps the Late Jurassic. Except for the Virginia opossum, from North America, 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 members of Placentalia.

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.

The oldest known eutherian species is Juramaia sinensis, dated at 160 million years ago from the Jurassic in China.

Eutheria was named in 1872 by Theodore Gill; in 1880 Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia.

Characteristics

The entocuneiform bone

Distinguishing features 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 (the outermost 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.
• various features of jaws and teeth.

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 out-dated placental group Insectivora, while Zhelestids have been considered primitive ungulates. However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia.

The fossil eutherian species believed to be the oldest known is Juramaia sinensis, which lived about 160 million years ago. 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.

Cynodonts † Other Cynodonts † Tritylodontids Mammaliaforms † Other mammaliaforms † Hadrocodium Crown-group mammals † Other crown-group mammals Australosphenids † Other Australosphenids Monotremes Theria † Other Theria Metatheria † Other Metatheria Marsupials Eutheria † Other Eutheria Placentals Cynodonts Tritylodontids Mammaliformes Other mammaliformes Hadrocodium Mammals Australosphenids Other Australosphenids Monotremes Theria Metatheria Eutheria Simplified, non-systematic, outline of evolution of eutheria from cynodont therapsids. † = extinct