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It seems hard to believe, but it wasn't that long ago that the idea of birds evolving directly from dinosaurs seemed just a little flaky. Sure, they shared generally similar body plans — paleontologists have known that for more than a century — but that hardly constituted an airtight case. Over the past couple of decades, however, scientists have uncovered all sorts of detailed characteristics common to birds and dinosaurs: wishbones, swiveling wristbones and, most recently, proof that some dinosaurs sported feathers. There's behavioral evidence too. Some dinosaurs made nests and sat on them, and one four-winged, feathered dino evidently glided like a flying squirrel.
A flurry of new finds offers even more evidence that the pigeon you see in the park had an ancestor that ruled the earth. Writing in Nature over the past two weeks, Chinese and American paleontologists announced the discovery of one dinosaur that evidently slept curled up in a posture identical to that of a sleeping duck and another that is the first tyrannosaur ever found with feathers. The discovery of the tyrannosaur is significant because that family of dinosaurs is believed to be among the closest relatives of modern birds.
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Another team, meanwhile, recently published an analysis showing that Tyrannosaurus rex grew (and thus metabolized) at an impressively fast rate — suggesting that it might have been warm-blooded like birds. "There's now so much material [linking dinosaurs and birds] that I can't imagine anybody being able to ignore it," says paleontologist Luis Chiappe of the Natural History Museum of Los Angeles County.
Like many of the most dramatic dinosaur finds of the past few years, the new specimens were unearthed in a geological formation in northeastern China's Liaoning province that has become one of the world's most renowned fossil beds. Since the late 1990s, digs in Liaoning have produced an astonishing array of exquisitely preserved plants, insects, primitive mammals, birds and, most famously, feathered and winged dinosaurs.
Luckily for paleontologists, the beds are divided into different layers that yield different sorts of fossils. The sleeping dino, for example, was found in what Mark Norell of the American Museum of Natural History in New York City calls Liaoning's "Pompeii layer," a 10-ft.-thick stratum of ash and sand. It was deposited so quickly that, like the ash from the infamous eruption in Italy, it buried creatures alive wherever they were standing — or snoozing. This one was tiny: excluding its tail, it's about the size of a Rock Cornish hen. That some of its bones have not completely fused indicates that this particular specimen was not quite fully grown.
In addition, it's a new species, which Norell and co-discoverer Xing Xu of Beijing's Institute of Vertebrate Paleontology and Paleoanthropology named Mei long, from the Chinese for "soundly sleeping dragon." But the specimen, dating to between 128 million and 139 million years ago, is clearly an early troodontid, an evolutionary cousin of tyrannosaurs.
"Not only are troodontids very closely related to birds," says Norell, "but this particular one is in a stereotypical resting pose of birds." The sleeping dragon was found sitting on its hindlimbs, its forelimbs folded at its side, its head tucked under its left elbow and its long tail curled around its body. Experts believe modern birds sleep in a similar position to conserve heat; presumably Mei long did too, which suggests that the animal was warm-blooded. If that was the case, says Norell, it also offers an explanation for feathers: "It's likely they first evolved for insulation rather than flight." Birds simply found another use for them.
While the Pompeii layer preserved natural body postures, it was too coarse to take imprints of soft tissues and delicate structures, so there's no way of knowing whether Mei long had feathers. But other strata of the Liaoning fossil beds are much finer grained. That's where paleontologists found the feathered tyrannosaur, which Xu and Norell named Dilong paradoxus ("surprising emperor dragon"). It's one of the oldest known tyrannosaurs, and one of the emu-size specimens has unmistakable traces of primitive feathers on its tail and jaw. Those filaments, which are about three-quarters of an inch long and branched like modern feathers, are the first direct evidence that tyrannosaurs sported plumage. Because Dilong paradoxus is one of the earliest tyrannosaurs, Norell and his colleagues infer that its larger, more advanced relatives, including T. rex, must have had feathers for at least part of their life span.
That notion is reinforced, albeit indirectly, by the growth analysis Norell and a group of American and Canadian scientists published in Nature in August. By looking at growth lines — skeletal marks, analogous to tree rings, that show how much bigger a dinosaur got from year to year — the scientists were able to estimate that T. rex packed on weight at a blistering pace, sometimes as much as 5 lbs. a day. That also supports the idea of warm-bloodedness, which means baby T. rex had to have a way to retain body heat. As the dinosaur shot toward adulthood, however, it would have developed the opposite problem: shedding the excess heat pumped out by an active, 11,000-lb. body. Norell and Xu theorize that T. rex probably lost its feathers as it matured, just as growing elephants lose their body hair.
Impressive as these new discoveries are, they hardly mean that all the details of the dinosaur-bird link have been ironed out. While current thinking favors speedy predators like velociraptors as the direct ancestors of modern birds, both Chiappe and Norell argue that the birds' forebears could just as easily have been troodontids like Mei long or even oviraptors, another related type of dinosaur. (Several years ago in Mongolia, Norell and colleagues unearthed a fossil oviraptor sitting on its egg-filled nest.) And then there's also the open question of how flight evolved. Mei long, says Chiappe, was clearly sleeping on the ground. But if flight began as flying-squirrel-like glides out of trees, he wonders, "wouldn't it have been safer resting in a tree?"
Those are important points, and they may take years to work out. But the fact that paleontologists are focusing on such details makes it clear that the dinosaur-bird connection, so bitterly controversial just a few short years ago, is no longer in dispute.
- Michael D. Lemonick