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A second specimen of the troodontid Mei, preserved in a bird-like sleeping position. From Gao et al., 2012.

Bone by bone and study by study, paleontologists are learning more than ever before about dinosaurs. But there are still many aspects about prehistoric biology that we know little about. In fact, some of the simplest facets of dinosaur lives remain elusive.

For one thing, we don’t know much at all about how dinosaurs slept. Did Apatosaurus doze standing up or kneel down to rest? Did tyrannosaurs use their tiny, muscular arms to push themselves off the ground after a nap? And, given the discovery of so many enfluffled dinosaurs, did fuzzy dinosaurs ever cuddle up together to stay warm on chilly Mesozoic nights?

Since we can’t observe living non-avian dinosaurs directly, some of these questions have to remain in the realm of speculation. But a handful of fossils have shown us that at least some dinosaurs curled up just like birds. In 2004, Xing Xu and Mark Norell described the tiny, early Cretaceous dinosaur Mei long–a feathery troodontid dinosaur with big eyes and a little switchblade claw on each foot. What made Mei special, though, was the way the dinosaur was preserved.

Many articulated dinosaur skeletons are found in the classic dinosaur death pose, with their tails tilted up and their necks thrown over their backs. The nearly-complete skeleton of Mei was different. The foot-long dinosaur rested its head over its folded arms, and its tail wrapped around the dinosaur’s torso. Mei died sleeping in a roosting position similar to that of modern birds. The dinosaur’s name, which means “sleeping dragon,” is a tribute to the behavior.

Now another Mei specimen has confirmed that the first find was not a fluke. Last week, paleontologist Chunling Gao, of the Dalian Natural History Museum in China, and colleagues described a second, slightly smaller Mei that was preserved in a nearly identical sleeping position. Much like the first, this Mei probably died in a prehistoric ashfall that both killed and preserved the dinosaur in delicate detail without jarring the snoozing troodontid out of position. Some feathery, non-avian dinosaurs not only looked like birds, but they slept like them, too.

The two Mei specimens aren’t the only dinosaurs found in such positions. Gao and colleagues also point out that a specimen of another troodontid found in the Cretaceous rock of Mongolia, Sinornithoides youngi, was found in the same sort of sleeping position. And while not mentioned by the authors of the new study, the sleeping positions of Mei and Sinornithoides remind me of the early Jurassic dinosaur Segisaurus. Described in 1936, the partial skeleton of Segisaurus was found with its legs tucked beneath its body and arms apparently in a resting position. Perhaps this dinosaur, too, died while dozing, and records an even older record of how dinosaurs rested. Such glimpses are rare, but they help fill in some of the most elusive moments in Mesozoic history.

[Check out artist Julius Csotonyi's blog for a lovely new illustration of the second Mei specimen.]

Reference:

Gao C, Morschhauser EM, Varricchio DJ, Liu J, Zhao B (2012). A Second Soundly Sleeping Dragon: New Anatomical Details of the Chinese Troodontid Mei long with Implications for Phylogeny and Taphonomy. PLOS One DOI: 10.1371/journal.pone.0045203