Placental mammals are a kind of mammal, they deliver babies rather than eggs, and include all mammals except marsupials—that much biologists can agree on. When the first such mammals first evolved is still subject to fierce debate. The problem comes down to how those seeking to find the definitive answer go about it. O’Leary et al claim the only proven method of dating timelines for organisms is by studying the fossil evidence—no fossils of placental mammals have been found that predate the time when the dinosaurs died out. dos Reis and others on his side shoot holes in that line of reasoning by noting that it’s eminently possible that some organisms, such as placental mammals, existed for which there is no fossil record.
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