Continuing the discussion from Executive Summary of the Most Convincing Lines of Evidence:
Your reasoning is strange to me. But first, it is important to realize that absence of fossils does not mean absence of specie. So remember it is the fossils that are absent… we are not sure about the actual animals. Secondly, the presumption of 50 my ago is the problem. Secondly, you are not clear whether the marsupials that went extinct (the fossils that have no clear living descendants) are the ones that got on the log rafts… my thinking is that the ones that survived got on the log rafts, not the ones that died.
No one ever said that placentals did not get on log rafts, but obviously if you have two of a kind, they cannot go to more than one place. Thus, some kind of separation of various kinds and types is more logical than if all types were on all islands and continents. Of course, animals could also transfer by various means from one continent to another by later log rafts, floating debris, land bridges, swimming, ice floes, etc.
The idea of efficiency is an adhoc catch-all. It is a poor explanation for the various types of diversity, as well as the extinctions. Most ecosystems have an incredible diversity, including numerous scales and diversity of efficiencies as well. One species does not throw competition at another as if competition was a survival manual. While some species follow others, or find smaller niches, probably only humans are interested in the extinction of certain species that can harm them, such as poisonous snakes, killer bees, ebola virus, and in some cases, marauding elephants, tigers, lions, grizzlies.