Dangerous dinos came out after dark

Scientists could see it in their eyes — or rather, the bone around them

As if velociraptors weren’t scary enough. A new analysis of dinosaur-era skulls suggests that at least one of these birdlike predators, along with many of its ancient brethren, hunted by night. 

FLY NOT BY NIGHT Like modern day birds of prey, the flying archosaur Scaphognathus crassirostris probably took to the skies mostly during the day. Scientists inferred this ancient beast’s activity patterns by measuring its eye socket, or orbit, and scleral ring (in purple). Lars Schmitz

Not all extinct archosaurs — dinosaurs and their close relatives — loved to bask in the sun. Like other animal groups, these ancient creatures were up at all hours, researchers from the University of California, Davis report online April 14 in Science. And just like modern mammals, predatory dinos seem to have preferred prowling the night, while herbivores grazed during the day and into the evening.

“We shouldn’t be surprised that there were predatory dinosaurs skulking around the shadows,” says Lawrence Witmer, a paleobiologist with Ohio University in Athens. “The surprising thing about this study is we could tell.”

The UC Davis team could tell nocturnal from daytime dinos by looking deep into their eyes. In living animals, that’s easy. Nocturnal critters’ eyes usually have wide pupils, for instance, but are relatively shallow. That means a cat’s peepers let in lots of light, but can’t take detailed pictures. Dino fossils, however, don’t come with mushy eye tissue. So researchers have the much harder task of inferring visual prowess from bones.

Study coauthors Lars Schmitz, an evolutionary biologist, and Ryosuke Motani, a paleobiologist, started with living animals — lizards and birds. They looked at the sizes of eye sockets and scleral rings — thin, bony layers that sit, in these creatures, roughly where the whites of a mammal’s eyes would be. Taken together, these structures appeared to predict activity patterns.

Then Schmitz and Motani inspected skulls of 33 archosaurs, spotting eight dinos that seemed to prefer the sun and nine with eyes better suited to dim light. Others appeared to do well in a mix of sunlight and moonlight. And these ancient beasts seem to fall in line with the same patterns that organize many living vertebrates. Like most birds, flying archosaurs, including the original early bird Archaeopteryx lithographica, tended to soar by day. Herbivores probably dined both during the day and in dim light, much like modern cows. And in true horror movie style, many predators, such as Velociraptor mongoliensis of Jurassic Park fame, seem to have stalked their prey at night. “The velociraptor was nocturnal in that movie, and it actually fits,” Schmitz says.

Evolutionary biologist Margaret Hall has doubts about whether scientists can infer nocturnal behavior from the kind of bony evidence used by the UC Davis team. But she would not be surprised if dinosaurs skulked in the shadows. Every living animal group has night and day ambassadors, says Hall, of Midwestern University in Glendale, Ariz. Why should archosaurs be any different?

The new study supports current appreciations of dino diversity, says Tony Fiorillo, curator of paleontology at the Dallas Museum of Natural History. Many species don’t fit the old notion that dinosaurs moved slowly and depended entirely on the sun for warmth. He’s confident that perceptions will evolve even further. “The field of dinosaur paleontology is so active and engaged right now that we won’t have to wait 10 years,” Fiorillo says. “At some levels, it changes from year to year.”

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