A boy looks on at the skull of a dinosaur fossil in Taipei, Taiwan. Photo: Chiang Ying-ying / AP

Mammals were primarily nocturnal until about 66 million years ago — about the time when dinosaurs were wiped out. Then, they became diurnal creatures active during the daytime. The two events may be linked, according to a new study published in Nature Ecology & Evolution.

The big picture: Scientists have a long-held hypothesis that ancient mammals were nocturnal so they could avoid dinosaurs — predators who were active in the daytime. This study's analysis of 2,415 different mammal species showed that the switch from being active in the nighttime to the daytime occurred after dinosaurs were killed off. "Though scientists can't say that one caused the other, the study provides fresh circumstantial evidence," Amina Khan wrote in the LA Times.

But, but, but: Scientists found some species that are cathemeral — meaning they are out and about both during the day and the night — held those patterns of living about 9 million years before dinosaurs became extinct. This could be because dinosaurs may have been dying out before an asteroid hit Earth, allowing opportunities for mammals to come out during the day.

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