This chapter tackles the evolutionary history of Theria and its two clades, Marsupalia and Eutheria. Therians are viviparous amniotes which give birth to young after a period of internal gestation. Although most therians are terrestrial, there are also therians with burrowing, aquatic, and flying forms. Thus, the evolution of these different ways of life led to corresponding diversity in anatomy, body size, and ecology, as well as many striking cases of convergent evolution. The chapter considers the diversity of Metatheria and Eutheria in relation to the key elements of therian reproductive biology and specializations for feeding and locomotion. It notes how mammals front extinction risk at the hands of humans, which includes habitat loss and the introduction of alien species.
