Dinosaurs have been considered to be daytime animals who were in hibernation at night, but a new study published in the Science journal suggests that the commonly-held assumption no longer applies.

Looking at 33 dinosaur species from the Mesozoic era, researchers discovered that 5 of the 33 species were active at night. The way that researchers were able to determine this pertains to the scleral ossicles, a bone that surrounds the iris (located in the eye) of each dinosaur. Dinosaurs who hunted during the day possessed the light needed to find food, so the width of the eye socket, iris, and the scleral ossicle (the eye bone) were smaller than other animals. Those who hunted at dawn or dusk needed more light from their eyes to see and hunt for food (since dawn and dusk contain less external light than daytime), so animals who hunted during this time must have larger scleral ossicles than daytime dinosaurs.

Interestingly enough, there was a third category of dinosaur hunters found in the study. Whereas some hunted during the day, and others either at dawn or at dusk, a third category hunted periodically throughout the day – hunting every few hours after some sort of biological “nap.” A large number of herbivores (plant-eating dinosaurs) were awake in something akin to “around the clock fashion,” and large mammals tended to stay awake because of their large eating habits (dinosaurs, like humans, could not inhale all their food at once).

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Humboldt University of Berlin evolutionary biologist Christian Kammerer praised the new findings as something of a breakthrough for common assumptions about the land-roaming mammals: “Mammals are thought to be ancestrally nocturnal, but this study demonstrates that this was not some crucial novelty in mammalian evolution: There had been nocturnal members of the mammalian stem lineage for 100 million years.”

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While Kammerer does not suggest that dinosaur daytime exhibits be revised just yet, he does state that the study shows that dinosaurs, as with all mammals, cannot be placed in one broad, extended category of daytime hunters.