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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. 

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.

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.

Megaconus belongs to the Haramiyida lineage, one of the snipped branches. That group dates back to the Late Triassic, some 40 million to 50 million years before the appearance of true mammals and is not included in the Mammalia class.


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