Considering the drifting of continents, how do you explain the abundance of marsupials and the absence of native placental mammals in Australia

this phenomenon is a natural consequence of evolutionary processes and continental drift  

Explanation:

Marsupials are mammals in which newborn offspring are carried in a pouch of the female (mother) body. On the other hand, placentals are mammals that complete embryo development in the mother's body, in the placenta. All the extant marsupials are endemic to Australia and the Americas. It has been shown that marsupials diverged (evolved) in South America, Australia and Antarctica when these landmasses were joined. In Australia, marsupials evolved to fit an ecological niche, which can be attributed to geographic isolation due to continental drift, while placental mammals are relatively recent and more scarce (mainly bats and rodents) within this region.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.