By Erin Wayman
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. Since the 1990s, some scientists have concluded from family trees based largely on molecular evidence that at least some lineages of modern placental mammals originated as early as 100 million years ago, during the Cretaceous period. But paleontologists have been skeptical because they have found no fossils resembling these mammals that are older than 65 million years.
“What the [new] analyses do is vindicate the fossil record,” says Ken Rose, a paleontologist at the Johns Hopkins University School of Medicine.