A new study by researchers at University of California-Davis finds that velociraptors had night vision that helped them stalk their prey at night.
The study reverses conventional wisdom that dinosaurs were active by day while early mammals scurried around at night, said Ryosuke Motani, professor of geology at UC Davis and co-author of the report.
“It was a surprise, but it makes sense,” he said.
Plant-eating dinosaurs also had some limited night vision, likely to satisfy their round the clock appetite, while flying dinosaurs, like birds, were active only during the day, the researchers said.
The study also sheds light into how ecology influences the evolution of animal shape and form over tens of millions of years, said Motani and collaborator Lars Schmitz, a postdoctoral researcher in the Department of Evolution and Ecology at UC Davis.
Motani and Schmitz inferred the dinosaur’s daily habits by studying their eyes.
Dinosaurs, lizards and birds all have a bony ring known as a “scleral ring” in their eye, something lacking in mammals and crocodiles. Schmitz and Motani measured the inner and outer dimensions of this ring, plus the size of the eye socket, in 33 fossils of dinosaurs, ancestral birds and pterosaurs. They also obtained the same measurements in 164 living species.
Day-active, or diurnal, animals have a small opening in the middle of the ring. In nocturnal animals, the opening is much larger, while cathemeral animals, those active both day and night, tend to be in between.
The size of these features is affected by a species’ environment as well as by ancestry. For example, two closely related animals might have a similar eye shape, something controlled by ancestry, even though one is active by day and the other by night.
By analyzing 164 living species, the researchers were able to confirm that eye measurements are quite accurate in predicting whether animals are active by day, by night or around the clock.
Next, the scientists applied the technique to fossils from plant-eating and carnivorous dinosaurs, flying reptiles called pterosaurs, and ancestral birds.
The measurements revealed that the large, plant-eating dinosaurs were active day and night, likely because they had to eat most of the time, except for the warmest hours of the day when they needed to avoid overheating.
Modern megaherbivores like elephants show the same activity pattern, Motani said.
Velociraptors and other small carnivores were night hunters, the study revealed.
The researchers were not able to study large carnivores such as Tyrannosaurus rex, because there are no fossils with sufficiently well-preserved scleral rings.
Flying creatures, including early birds and pterosaurs, were mostly day-active, although some of the pterosaurs were apparently night-active.
The ability to separate out the effects of ancestry gives researchers a new tool to understand how animals lived in their environment and how changes in the environment influenced their evolution over millions of years, Motani said.
The study was published online April 14 in the journal Science.
Image 1: Close-up of the eye socket and ring of the dinosaur Protoceratops, active by day and night. Credit: Ryosuke Motani and Lars Schmitz
Image 2: The small carnivorous dinosaur Juravenator starki was nocturnal. Credit: Ryosuke Motani and Lars Schmitz
Image 3: The pterosaur Scaphognathus crassirostris was a day-active archosaur, evidenced by its eye. Credit: Ryosuke Motani and Lars Schmitz
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