The late Cretaceous map looks much like our map, but chopped up. In each continent, different mammals evolved. I'd been looking at the Laurasia group this morning, injecting that into a post thought up last night.
As of 100 million years ago, Africa was an island continent and so were both Americas. The Afrotheres include, today, elephants, rhinos, and hippopotamoi. Not us monkeys and lemurs: we postLaurasians rafted from Europe, etched out a fully arboreal and nocturnal niche there, and thence - well into Cenozoic - rafted to America. Africa's Carnivora likely got there from Europe too.
(South) America, for its part, did get placentals... but for whatever reason, they stagnated. Armadillos and sloths. The slow class, quite literally - which is why they're barely even warmblooded. Seriously, you can catch some nasty bacteria from them leprosy not least.
In America, these placentals shared space with marsupials. Their marsupials rafted to Australia, an even MORE isolated and stagnant land. These continents may as well be Hollow World.
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