An infraclass of mammals containing the marsupials. The female bears an abdominal pouch (marsupium) into which the newly born young, which are in a very immature state, move to complete their development. They obtain nourishment from the mother’s mammary teats. Modern marsupials are restricted to Australasia (where they include the kangaroos, koala bears, phalangers, and bandicoots) and the Americas (the opossums). Common ancestors of marsupials and placental mammals probably evolved during the early to mid-Cretaceous period, 140–125 mya, and marsupials diverged from placental mammals about 90 mya. In Australia, where the marsupials have been isolated for millions of years, they show the greatest diversity of form, having undergone adaptive radiation to many of the niches occupied by placental mammals elsewhere. Compare eutheria; prototheria.