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When did mammals evolve? Two new papers disagree.

Two papers published in Nature disagree on when mammals evolved but also broaden the portrait of mammalian evolution. 

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Megaconus, depicted here in an artist's illustration, is not a true mammal, but had distinctly mammalian features.

April Isch/University of Chicago

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Once, somewhere in what is now China, lived an animal with the fur and teeth – and poisonous spur – of a mammal, but with the ears and ankles of a reptile.

Called Megaconus, the 165-million-year-old precursor to modern mammals and companion to feathered dinosaurs is a new data point in paleontologists’ search to pinpoint how and when the features of modern mammals evolved. The animal, described in a paper published in Nature, suggests that mammalian characteristics well predated the existence of actual mammals.  

But at the same time, a separate paper published in Nature has classified another fossil in the same lineage to which Megaconus belongs not as a mammal precursor, but as a true mammal, a finding that would dial back the evolution of mammals some 50 million years. The conflicting findings underscore the difficulties of piecing together the still enigmatic evolution of mammals.

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Zhe-Xi Luo, a professor of organismal biology and anatomy at The University of Chicago and an author on the first paper, likens the evolution of mammals to a “vast bush” made of about two-dozen branches, the majority of which were “pruned in extinction.” Just three of those branches were spared natural selection's gardener’s shears and now form the three lineages of modern mammals.

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