Mammals are today represented by three ma jor groups: monotremes, marsupials, and placentals. The reptile-to-mammal transition oc curred about 200 million years ago. At that time or earlier, two broad stem lines of present-day mammals emerged with the therian line, giving rise to the marsupials and placentals about 100 million years ago (Dawson, 1983). Marsupials originated in North America, and their early radiation was more extensive than placentals and encompassed the Americas, Europe, and eventually Antarctica and Australia but not apparently Asia or Africa. Subsequently the placental radiation eclipsed the marsupial one in North America and Europe and later in South America with the rise of placental herbivores. Because of its isolation with the break-up of Gondwana, the Australian radiation suffered no such decline, and thus today Australia and off shore islands such as New Guinea have the most diverse marsupial fauna with seven superfami lies. There are also three families in South and Central America and the Didelphis virginiana in North America (see accompanying table).