Ancestors of today’s placental mammals may never have shared the Earth with dinosaurs

New family tree suggests the predecessors of rodents, horses, and humans did not emerge until after the dino extinction

By Erin Wayman, 14:49 PM February 7, 2013

Tyrannosaurus rex may never have had the chance to terrorize the grandfathers of rodents, rabbits or primates. A new family tree using both anatomical and genetic data indicates that the lineages of modern placental mammals — those that give birth to well-developed young — arose after the dinosaurs went extinct 65 million years ago.

The new study, published in the Feb. 8 Science, adds to a debate over the emergence of a diverse group that includes whales, cats, bats, horses and humans. Sin...

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