Fossils may push back placental mammal origins

New fossils of a chipmunk-sized animal that lived about 85 million years ago suggest that placental mammals arose much earlier than is generally believed and thrived for millions of years alongside dinosaurs.

Paleontologists who analyzed about 45 jawbone and skull fragments of a long-snouted mammal called Kulbeckia conclude that it was an early relative of rabbits and rodents.

To date, the oldest fossils widely agreed to be those of placental mammals -- which nourish their young in the uterus until they are born fully formed -- date to 65 million years ago.

That's the same time that an asteroid impact is believed to have wiped out the dinosaurs, clearing the way for mammals' dominance.

If the interpretation of the new fossils are confirmed and they are indeed related to rabbits and rodents, that would push back the evolutionary split of rabbits and rodents from other placentals by at least 20 million years.

That, in turn, would mean separate placental lineages were well-established even as dinosaurs ruled the Earth.

Some scientists dispute the findings, saying it is far from clear where Kulbeckia fits in the evolutionary tree.

John J. Flynn, a curator at the Field Museum of Natural History in Chicago, said the American-Russian team conducted an incomplete analysis when they compared characteristics between the fossils and other early mammals.

He nonetheless called the new fossils "tantalizing."