A STUNNING set of six fossils discovered in China looks set to overturn our ideas about how birds first took to the sky. The fossils show a small dinosaur that had flight feathers covering its legs, as well as its tail and arms, forming an extra pair of wings never before seen by palaeontologists.
The announcement comes days after a scientist in the US published details showing that young partridges flap their wings to improve their grip on steep slopes, which may explain why wings evolved. Together, the two discoveries may represent a turning point in the contentious study of avian evolution.
鈥淲e need to be prepared to change some cherished notions,鈥� palaeontologist Larry Witmer of Ohio University told 快猫短视频. Traditionally, experts have been split between two opposing theories: either flight began with small, fleet predatory dinosaurs leaping from the ground into the air, or by different animals jumping to earth from trees. The new studies suggest the picture is more complex, with the ancestors of birds potentially finding numerous ways to take to the sky.
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The new specimens 鈥渁re potentially as important as Archaeopteryx鈥�, the famous feathered fossil discovered in the 1860s that alerted scientists to the link between dinosaurs and birds, says Kevin Padian of the University of California at Berkeley. They are of a small dinosaur belonging to the Microraptor genus, the most primitive of the two-legged predatory dinosaurs called dromaeosaurs, which are known to be closely related to birds. Earlier Microraptor fossils lacked feathers, but the Chinese specimens, which appear to belong to a new species, have the most extensive coat of feathers ever seen on a dinosaur.
The best-preserved skeleton is just 77 centimetres from the nose to the tip of its tail, making it light enough to fly. There is 鈥渘o doubt the new animal is a flying animal鈥�, says Xing Xu from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, who details the new fossils in Nature (vol 421, p 335).
Xu describes the new Microraptor as a 鈥渇our-winged dinosaur鈥�, which he says was not capable of powered flight. Instead it used all four limbs to climb trees, and then glided back down. But Microraptor gui, as it has been called, is unusual because the feathers at the ends of its arms and legs are twice as long as those close to its body. In most creatures, gliding surfaces are narrower the further they are from the body.
The feathers on M. gui鈥檚 legs are particularly baffling. Xu thinks M. gui spread its back legs to generate lift, but other dinosaurs could not do this. 鈥淭his is so far out of the box that we need to sit back and figure out how this can work,鈥� says Witmer.
Palaeontologists have had more time to digest the baby partridge findings, which Ken Dial of the University of Montana had presented at meetings before publishing the details in Science (vol 299, p 402). Dial sought to answer a central question about the origin of flight: why did the ancestors of birds evolve wings that would initially have been too small to use for flight?
Dial had a hunch. One-day-old chukar partridges are able to run to avoid predators, and Dial鈥檚 son had noticed the chicks rapidly flapped their wings as they ran up bales of hay. Could this be helping them climb, Dial wondered?
He clipped or plucked the wing feathers of some birds, then used high-speed photography to see how they fared climbing slopes of different inclines compared with birds with intact wings. He found that even chicks with intact wings ran rather than flew up the slope. While doing so they flapped their wings from head to foot, a stroke that is 90掳 out from the back-to-belly stroke of normal flight.
Dial found that the bigger the chicks鈥� wings, the steeper the slope they could climb. The wing motion was helping to push the birds down, so their claws grasped rough surfaces more firmly. The birds run so fast no one had noticed the flapping before.
The earliest feathered dinosaurs may have behaved in a similar way, Dial says. He suggests that baby dinosaurs tried to escape predators by running to higher ground, seeking refuge up trees for instance. Even the smallest wings would have provided an edge in this race for survival. Further increases in wing size could then have led to gliding and powered flight.
Luis Chiappe of the Los Angeles County Museum of Natural History says Dial鈥檚 experiments are 鈥渋mpeccable鈥�. But he points out we may never know if baby dinosaurs behaved like partridges.
The ancestry of the first birds also remains an open question. M. gui lived around 130 million years ago, at least 20 million years after Archaeopteryx, the first known bird. So was it a living fossil in its time, a throwback to a common ancestor that never developed powered flight, or a bizarre evolutionary experiment, a dead-end twig on the complex family tree of dinosaurs and birds? Perhaps it was something else altogether. Palaeontologists are still scratching their heads.