Dinosuar-fossil hunters working in China have unearthed one of the most remarkable creatures yet discovered, a feathered meat-eater that sported not one but two pairs of wings. The spectacular find opens fresh debate on the evolution of modern bird flight.
Dr Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and colleagues report the discovery of six specimens of feathered dinosaurs in the journal, Nature. The fossilised bones, 124 to 128 million years old, were dug out of the ground in western Liaoning, China.
They revealed a striking metre-long meat-eater, close relative of a group known as the Microraptors. This new species has fully developed feathered wings on both its forelimbs andf hindlimbs, and a feather-fringed tail. "In general the leg feathers are arranged in a pattern similar to wing feathers in modern birds, suggesting the presence of a hindlimb wing," the authors write.
The discovery rekindles the debate about how flight first evolved, and adds new evidence supporting a theory that dates back to the early 1900s. It is now widely accepted that birds are close relatives of the Microraptors and other ground-dwelling dinosaurs.
All were equipped with powerful hindlimbs adapted for running. The theory holds that over time these animals also developed feathered forelimbs. When beaten these gave the dinosaur extra thrust for greater speed, allowed it to scale steep inclines to escape being eaten and eventually enabled it to achieve a running take-off.
The new species of dromaeosaur, Microraptor gui, confounds this, and supports an earlier theory that flight evolved from animals that used primitive wings to glide through the air when jumping from tree to tree.
"In a colourful and prescient paper of 1915, William Beebe proposed that avian flight evolved through a gliding, four-winged - tetrapteryx - stage with wing feathers on both the arms and the legs," writes Dr Richard O. Prum of the University of Kansas in an accompanying article in Nature.
This "arboreal" theory held that tree-dwelling animals gradually evolved wings and eventually feathers as a way to help them glide long distances between trees. This kept them out of the claws and jaws of ground-dwelling meat-eaters and over time may have facilitated wing-driven flight.
Dr Xu and colleagues described a small dinosaur "that sports four wings of fully modern, asymmetrical feathers on its forelimbs and legs, and looks as if it could have glided straight out of the pages of Beebe's notebooks".
The discovery won't end the debate about how modern birds found flight, but Dr Xu's team are in no doubt. "We suggest that basal dromaeosaurs were arboreal animals and that the ancestor of birds first learned to glide by taking advantage of gravity before flapping flight was acquired in birds," they write.