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The monotremes split off about 166m years ago and the marsupials parted company with the placentals 148m years ago.
ECONOMIST: Rewriting the history of the mammals
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Today, 90% of all mammals, which include humans of course, are placentals.
BBC: Fossil redefines mammal history
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Nothing much then happened for 50m years when, all of a sudden, the placentals (or, at least, those with surviving descendants) split into four so-called superorders.
ECONOMIST: Rewriting the history of the mammals
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Modern mammals divide into three overarching groups: the placentals (most species, including man), the marsupials (kangaroos and their like) and the monotremes (the egg-laying platypus and echidnas).
ECONOMIST: Rewriting the history of the mammals
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Another interesting aspect of the discovery is what the fossil can tell us about the lifestyles of the early placentals, and it seems they were pretty adept at climbing.
BBC: Fossil redefines mammal history