Following on the heels of the discovery of a new dinosaur species, another interesting piece of research has come out about these prehistoric monsters: Many carnivorous species were nocturnal.
The study, published in the journal Science, casts doubt upon the idea that hundreds millions of years ago (up until about 65 million years ago), most dinosaurs were active only during the day, leading mammals to hide from them in the shade. In fact, several carnivorous dinosaur species were probably sleeping during the day, and would hunt at night, new research suggests.
"It gives us a new view of how to reconstruct the dinosaur era and how the environment in the Mesozoic, the dinosaur era, was actually used," said Lars Schmitz, a postdoctoral researcher in the Department of Evolution and Ecology at the University of California, Davis, and co-author of the study. "That's a totally new component of paleontology."
One such species that the researchers studied is the velociraptor, a feathered predator that lived about 71 million to 75 million years ago.
Nocturnal species would be expected to have eyes that function well in really dim light, Schmitz says. There's a key eye structure that we mammals lack, but that dinosaurs, lizards and birds all share:Â the scleral ring. Researchers wanted to find out how big this bony ring was, and generally the size of the eye socket, in living species and their ancestors. They examined 33 fossils of extinct creatures (dinosaurs, ancestral birds and pterosaurs) and 164 species that are alive today.
In the living species, the researchers found a clear pattern: Animals that are active during the day have a small opening in the center of the scleral ring, while the nocturnal ones have a much larger opening to collect as much light as possible. Those that are active during both time periods have a ring opening that's between the average size of those two.
For example, most geckos have large scleral ring openings and are active at night; monitor lizards, on the other hand, are active in the daytime and have much smaller scleral ring openings. Small carnivores in the study that were found to be nocturnal have scleral rings with a wide opening, Schmitz said.
The researchers then applied their insights about living species to the extinct ancestors of today's animals. Their findings suggest that predators were likely to be active during the night, and that large plant eaters were likely to be active both day and night. The mix of day-active, night-active, and day-and-night-active was comparable to what we see today, he said.
"It actually fits well with how the velociraptor was reconstructed in 'Jurassic Park,' " Schmitz said. "The night attack - actually, we can confirm that now by looking at its eye structure."
At some point down the line, this research may have something to say about extinction itself: Do nocturnal and diurnal species have different extinction patterns? This is an open question, Schmitz says.