The only order within the subclass Metathaliana is the order Marsupialia. Originating from the ancestors of ancient mammals in the Cretaceous period, it underwent significant development and became widely distributed throughout the world, competing with primitive placental mammals on a somewhat equal footing. However, with the advent of the Cenozoic era, placental mammals began to show a stronger evolutionary advantage, leading to a significant decline and eventual extinction of marsupials in most continental regions.
Tasmanian tiger
However, in both regions, marsupials continue to survive in "paradises" isolated from the rest of the world.
One region is Australia, which separated completely from the Asian continent during the Cretaceous or late Cretaceous period, before placental mammals had a chance to invade. Without competition from placental mammals, marsupials were able to fully develop and undergo extensive adaptive radiation (evolving like rays to occupy various ecological niches), giving rise to carnivorous marsupials and herbivorous marsupial rabbits and diprotodons. To this day, marsupials remain the dominant mammals in Australia; carnivores such as the Tasmanian devil and Tasmanian wolf hunt various kangaroos, playing ecological roles similar to those of wolves, badgers, and various deer and cattle in the Asian interior.
Opossum—a extant American marsupial.
Another region was South America, which was isolated from North America during the early Tertiary period due to a ruptured isthmus. Before this isolation, it sheltered a fauna including marsupials and primitive placental animals. After the isolation, because the more advanced placental animals had no opportunity to enter from North America, the marsupials continued to evolve and became strong competitors to the surviving descendants of the primitive placental animals. They lived quite successfully until the end of the Tertiary period, when South America was once again connected to North America by the isthmus, and highly evolved placental animals invaded from the north. The fierce competition led to the extinction of the vast majority of marsupials and the primitive indigenous placental animals.
Marsupials had limited development in other regions. They lived in Eurasia during the early and middle Tertiary period, but later disappeared from these areas.
