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Researchers Give a Face and Age to Mammalian Ancestor

A tiny, furry-tailed creature was the earliest ancestor of the placental mammals—a group excluding marsupials and egg-laying mammals—and lived after the extinction of the dinosaurs, according to a study in the 8 February Science.

The origins and early evolution of placental mammals have long been a matter of debate.

First, fossil evidence indicated that these mammals arose after the Cretaceous-Paleogene extinction event about 66 million years ago. According to this “explosive model,” placental mammalian lineages emerged and diversified to fill ecological niches left vacant after the demise of the non-avian dinosaurs and other large reptiles.

Starting in the 1990s, however, studies based on genetic diversity suggested that mammalian lineages were far older and that their diversification was related to the breakup of the continents before the end of the Cretaceous.

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Maureen O’Leary of Stony Brook University and colleagues have now analyzed thousands of physical characteristics across a wide assortment of fossil and living mammalian species. Combining these results with molecular sequences, they produced a family tree showing that placental mammals arose after the Cretaceous, consistent with the explosive model.

They also reconstructed the physical traits of the hypothetical placental ancestor. This four-legged creature probably weighed less than half a pound, ate insects, and was more adapted for general scampering than for more specific types of movement, they say.

“Discovering the tree of life is like piecing together a crime scene—it is a story that happened in the past that you can’t repeat,” O’Leary said. “Just like with a crime scene, the new tools of DNA add important information, but so do other physical clues like a body or, in the scientific realm, fossils, and anatomy. Combining all the evidence produces the most informed reconstruction of a past event.”

Read the abstract, “The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals,” by Maureen O’Leary and colleagues.