Mammals’ Rise Began Before Dinosaurs’ Fall, Study Finds

 It was long believed that mammals began to diversify and flourish only after dinosaurs died out in the mass extinction at the end of the Cretaceous period, about 66 million years ago. But a new study in the journal Nature suggests that some mammals diversified well before that.

“The story appears more complex,” said an author of the study, Gregory Wilson, a vertebrate paleontologist at the University of Washington. Using 3-D imaging and CT scanning, he and his colleagues studied the teeth of multituberculates, a group of rodentlike mammals that lived from 165 million years ago to about 35 million years ago. Some of the teeth were tiny: as small as four-hundredths of an inch across.

The researchers found that over time, the mammals’ teeth evolved to have more patches, or bumps. “In modern mammals, the greater number of patches you have, the more likely you are to have a diet composed of high-fiber or plant materials,” Dr. Wilson said.

From that, the researchers inferred that the multituberculates diversified at least 20 million years before dinosaurs disappeared. “They were able to expand their range pretty dramatically, and the teeth complexity implies they ate plants,” Dr. Wilson said.

The multituberculates started out the size of mice, he added, but “by the time they started to get more complex teeth, they started to increase in body size such that they were up to the size of porcupines and beavers.”