Most of the living mammals are in the subclass of placental mammals. Although there are 19 orders of placentals, most mammals belong to one of six orders, of which the largest is the rodents. One order has only one species, the aardvark. A simplified family tree of placental mammals, based on molecular evidence, is shown in Figure 14.
In this introductory course, the monotremes and marsupials are given particular focus. Although they are far less diverse than the placental mammals, their adaptations – particularly in the area of reproduction – are fascinating and informative.