146-65 million years ago


Placental mammals give birth to their young in an advanced state because of the nourishment of the fetus through the placenta.

Both placental mammals and marsupials give birth to live young, but in placental mammals the young are born in a much more advanced state. The fetus is nourished in the uterus by the formation of a chorioallantoic placenta where nutrients, gases, and wastes are exchanged between the fetus and the mother.
Male placental mammals evolved prostate glands and seminal vesicles. The testes are retained in the body cavity in monotremes, some insectivores, edentates, and elephants and descend into a muscular pouch which is not a true scrotum in many rodents, some insectivores, and hyenas. Male placentals possess a single glans penis (which is forked in marsupials). Most female placentals have a bipartite or bicornuate uterus rather than the primitive uterus duplex (with completely separate tubes). The cloaca was lost in most placentals but the ventral portion is retained in females as the vestibule.