Give an example of a marsupial and a placental mammal that may have had very similar needs, and competed until one became extinct.

Placentals and marsupials came from a common ancestor which was Theria . It seems reasonable to say that the common ancestor was more similar to modern marsupials than they are to modern placentals.

Placentals seemed to outcompete most marsupials everywhere they existed. Australia has basically all marsupials because the placentals never existed there until humans brought them.

Explanation:

substitution is the answer in this case