Everything about Placental Mammals totally explained
Eutheria is a
taxon containing the
placental
mammals, such as
humans. The
sister group of Eutheria is
Metatheria, which includes
marsupials and their extinct relatives.
Origin of the word
The name Eutheria comes from the
Greek words
eu- "well[-developed]" and
ther "beast". When Eutheria was introduced by
Thomas Henry Huxley in
1880, he meant it to be broader in definition than its precursor
Placentalia. Some use Eutheria as a
total group which includes the
crown group Placentalia and extinct mammals which are closer to Placentalia than to
Marsupialia.
Characteristics of eutherians
Nevertheless, all living eutherians are placental mammals. This means that a eutherian
fetus is nourished during gestation by a
placenta. Eutherians are also
viviparous, meaning that the offspring are carried in the mother's
uterus until fully developed.
Differences from other mammals
Because of this, eutherians are different from other mammal groups such as
monotremes and
marsupials which are not placental. Monotremes, for instance, lay
eggs which protect developing young until they're fully developed. Marsupials give birth to partially-developed young who then migrate to a special
pouch in the mother's body in which the young continue their development. (Some exceptions do exist.
Bandicoots for instance, which are marsupials, develop small placenta-like structures during gestation.)
Earliest example
The earliest known eutherian species is the extinct
Eomaia scansoria from the Lower
Cretaceous of
China. It is a member of Eutheria, but the hips of the animal were too narrowly built to have allowed the birth of well-developed young. Thus it's unlikely that a placenta greatly contributed to the development of
E. scansoria's young before they were born.
Members of Eutheria are found on all continents and in all oceans.
Further Information
Get more info on 'Placental Mammals'.
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