Status of Modern Marsupials D on't add marsupial futures to your retirement portfolio. Possibly the reason modern marsupials exist at all is that the southern continents became isolated at just the right time and remained so for 70 or 80 million years. Alternately, assuming metatherians and eutherians did share common ancestry, the marsupials have had the same time and niche opportunity as the placentals, and they didn't make much of these possibilities. Isolation of the southern continents may have made things too easy at a time when a bit of competition would have made them better players. Lots of good lessons in biological capitalism here.
Reasonable arguments can be advanced to both viewpoints.
There were marsupials in the northern continents during early days, but more effectively evolving eutherians ate their lunch. South American marsupials not only were the source for their Antarctic and Australian cousins, but they also diversified quite satisfactorily to fill many niches in the home continent. That's the good news. The bad news is that marsupials were not the only mammals on South America. Most of these eutherians were ungulates , and ungulates have a habit of getting large.