Discover Magazine: Paleontologists have made strides in understanding the circadian rhythms and sleep habits of dinosaurs.

in 2011, paleontologists Lars Schmitz and Ryosuke Motani suggested that various non-avian dinosaurs were active at different times of the day, too. Schmitz and Motani investigated delicate eye bones of various dinosaurs to see how those structures, called scleral rings, related to when the reptiles might have been active. The bones not only outline how large the eyes were, but how much light they let in. Following this logic, then, the researchers found that large, plant-eating dinosaurs like the duckbilled Corythosaurus were likely stomping around during morning and evening, while predators such as the infamous Velociraptor stalked the night.

But dinosaurs eventually wound down from their daily business, no matter whether they were active at night or during the day. A growing collection of fossils is revealing how they did so.When a drowsy raptor, for example, set about going to sleep, the dinosaur probably took a familiar pose. Rare skeletons and trace fossils — or impressions made by once-alive dinosaurs — indicate that at least some dinosaurs shuffled their feet beneath them, folded their arms, and rested their heads on their backs just like some slumbering birds today. Almost a century ago, paleontologist Charles Lewis Camp described the bones of a small, meat-eating dinosaur called Segisaurus found with its arms and legs tucked beneath it.

Multiple other finds have popped up since then. In the Jurassic rock of southern Utah there’s a body impression of a large, Dilophosaurus-like dinosaur that sat down to rest, shuffled forward, and settled in. From the way the dinosaur sat to how it held its hands, this carnivore acted in a very bird-like way despite living over 40 million years before the first birds evolved. Better yet, paleontologists have also described the fossil of a small, raptor-like dinosaur named Mei long that was asleep — curled up and snugly — as ash buried the unfortunate animal. Its name translates to “sleeping dragon.”

To date, most resting dinosaur finds have been of strange, parrot-like dinosaurs called oviraptorosaurs. “We have way more resting oviraptorids than any other group of dinosaurs,” says University of Edinburgh paleontologist Greg Funston, noting that there are more than half a dozen published specimens. Just last year, Funston and colleagues published another example — youngsters of a new oviraptorosaur named Oksoko that were found in the classic resting pose.

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