An infraclass of mammals in which the embryos are retained in a uterus in the mother’s body and nourished by a placenta. The young are thus fully protected during their embryonic development and kept at a constant temperature. Modern placentals are a highly diverse group that occupy all types of habitat in all parts of the world. They include the orders Artiodactyla, Carnivora, Cetacea, Chiroptera, Insectivora, Perissodactyla, Primates, Proboscidea, and Rodentia. Evidence about when placental mammals evolved is contradictory. Studies of the fossil record indicate an origin around 66 million years ago (mya), whereas molecular data suggest a date of around 100 mya, during the Cretaceous period. Compare metatheria; prototheria.