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What did the ancestor to most mammals look like?

An artist’s rendering of the hypothetical placental ancestor, a small, insect-eating animal. The research team reconstructed the anatomy of the animal by mapping traits onto the evolutionary tree most strongly supported by the combined phenomic and genomic data and comparing the features in placental mammals with those seen in their closest relatives.
[Image courtesy of Carl Buell]

A tiny, furry-tailed creature was the earliest ancestor of the placental mammals, according to a new study. That is, it was the ancestor of all the mammals except the marsupials and the small handful of mammals that lay eggs. The findings also help answer a decades-old debate about when the placental mammals first evolved.

Maureen O'Leary of Stony Brook University and colleagues analyzed a wide assortment of fossil and living mammalian species and compared thousands of physical features, or "traits," among them.

Combining these results with DNA evidence, they produced a family tree showing that the placental mammals arose after the Cretaceous-Paleogene extinction event about 66 million years ago – in other words, after the non-avian dinosaurs and other large reptiles went extinct.

The researchers also used this comparison to figure out how the earliest placental mammal probably looked and behaved. 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.

Scientists have debated for years about when placental mammals evolved. First, fossil evidence indicated that these mammals arose after the extinction of the dinosaurs. According to this picture, this extinction event opened up lots of new opportunities for mammals to adapt to new conditions and diversify into new species.

Starting in the 1990s, however, studies based on DNA evidence suggested that placental mammalian lineages were much older, and that their diversification was related to the breakup of the continents before the end of the Cretaceous. The new study by Dr. O'Leary's team challenges this model and instead agrees with the fossil-based studies.

The research appears in the 8 February issue of the journal Science.