Placentalia

Placentalia is the mammalian clade whose members give birth to live young after development in the uterus, supported by a placenta. It includes nearly all living mammals except marsupials and monotremes.

Mammal: A vertebrate whose females produce milk for their young and whose members have hair at some stage of life. Placentalia is one of the three major living mammal lineages.
Uterus: A muscular reproductive organ in which an embryo or fetus can develop during pregnancy. Placentals develop their young inside the uterus.
Cretaceous–Paleogene extinction event: The mass extinction about 66 million years ago that eliminated non-avian dinosaurs and many other organisms. Placental mammals diversified extensively after this ecological upheaval.
Cetacea: The mammalian order comprising whales, dolphins, and porpoises. Cetaceans show how placental mammals adapted to fully aquatic life.
Crown group: The clade consisting of the last common ancestor of living members of a group and all its descendants. Its definition determines which fossils count as placentals rather than stem eutherians.
Theria: The mammalian group comprising marsupials and placentals, excluding monotremes. Placentalia belongs to Theria alongside Marsupialia.
Chorioallantoic placenta: A placenta formed from the chorion and allantois, connecting fetal tissues with the maternal uterus. This is the characteristic placental interface in most placental mammals.
Eomaia: An Early Cretaceous fossil mammal from China, known from an exceptionally preserved skeleton. Its anatomy preserves traits associated with early eutherian mammals.
Chiroptera: The mammalian order comprising bats, the only mammals capable of sustained powered flight. Bats exemplify a distinctive placental adaptation to powered flight.
Molecular clock: A method that estimates evolutionary divergence times from genetic differences and calibrated mutation rates. Molecular estimates of placental origins have sometimes differed from fossil-based estimates.

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