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 non-placental 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 the 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 non-placental eutherians had a narrower opening that allows only small, immature offspring to pass through.

• the absence of epipubic bones extending forwards from the pelvis, which are not found in any placental, but are found in all other mammals ??? non-placental 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. humans, badgers, rabbits, guinea pigs, dogs, dolphins

• * Euarchontoglires, e.g. monkeys, rats, hares

• * Laurasiatheria, e.g. cattle, whales, bats, cats

• Atlantogenata, e.g. sloths, hyraxes, aardvarks, tenrecs, armadillos, anteaters

• * Xenarthra, e.g. armadillos, anteaters

• * Afrotheria, e.g. elephants, manatees

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

• Cynodonts

• *??? Other Cynodonts

• *??? Tritylodontids

• *Mammaliformes

• **??? Other mammaliformes

• **??? ''Hadrocodium''

• **Mammals

• ***??? Other mammals

• ***Australosphenids

• ****??? Other Australosphenids

• ****Monotremes

• ***Theria

• ****??? Other Theria

• ****Metatheria

• *****??? Other Metatheria

• *****Marsupials

• ****Eutheria

• *****??? Other Eutheria

• *****Placentals

Simplified, non-systematic, outline of evolution of eutheria from cynodont therapsids.
??? = extinct

The earliest known eutherian species is the extinct non-placental ''Eomaia|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 non-placental eutherian fossils have been found in Asia. The earliest known placental fossils have also been found in Asia.

• Goloboff, P.A.; Catalano, S.A.; Mirande, J.M.; Szumik, C.A.; Arias, J.S.; K??llersj??, M & Farris, J.S. 2009. Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups. Cladistics 25 (3): 211-230