Eutheria
Eutheria is the taxonomical name for the main group of living mammals.
This taxon contains the placental mammals, of which humans are one species.
Eutheria was introduced by Thomas Henry Huxley in 1880. Members of Eutheria are now found on all continents and in all oceans.
The terms 'Eutheria and 'Placental' do not mean quite the same thing. A few early eutherians in the Lower Cretaceous were not placentals. Eomaia is the earliest example.
All living Eutherians are placental mammals. This means that a Eutherian fetus is fed during gestation by a placenta. The offspring of Eutherians are carried in the mother's uterus until fully developed.
Eutherians are different from other mammal groups such as monotremes and marsupials which (like the earliest eutherians) are not placental.
Monotremes, for example, lay eggs which protect the young until they are fully developed. Marsupials give birth to young who are not completely developed. Their young then move to a special pouch in the mother's body to continue their development.
The earliest known eutherian species is the extinct Eomaia scansoria from the Lower Cretaceous in China.
Relevant pages
Mammal for classification.
Theria
Marsupial
Monotreme
Notes and references
Mammals