Paleontologists have generally believed that dinosaurs were active only during the day, preserving their energy at night, when mammals hunted and ate. But an analysis of dinosaur eye structure shows that at least some dinosaurs functioned quite capably in the dark.
In a paper published online Thursday in the journal Science, researchers report that they deduced dinosaur eye structure from the size and shape of the eye socket and of the scleral ring, a bone that sits inside the white of the eye of dinosaurs, birds and lizards. In living nocturnal animals, the inner diameter of the scleral ring is large in relation to its external diameter and to the focal length of the eye (the distance from the lens to the retina). In animals active during the day, the pattern is reversed: a scleral ring with a smaller diameter in relation to its internal diameter and the eye’s focal length.
The scientists looked at the eye structures of 33 species of dinosaurs that lived from 230 million to 65 million years ago. Nine of the species, all small carnivores, had eyes that were characteristic of living nocturnal animals. All of the herbivores, on the other hand, were diurnal.
The lead author, Lars Schmitz, a postdoctoral researcher at the University of California, Davis, said that the pattern persisted today in mammals. “Large mammal plant eaters are active day and night,” Dr. Schmitz said. “Mammal predators, like the large cats, are mostly nocturnal.”
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