In 2000, paleontologists in China discovered the fossil remains of a four-winged dinosaur with fully developed, modern feathers on both the forelimbs and hind limbs. The six specimens were excavated from the rich fossil beds of Liaoning Province in northeastern China. They are dated at between 128 to 124 million years old (Early Cretaceous). The new species, Microraptor gui, provides more evidence that birds evolved from dinosaurs, and could go a long way to answering a question scientists have puzzled over for close to 100 years: How did a group of ground-dwelling flightless dinosaurs evolve to a feathered animal capable of flying?
    Xu Xing, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, and colleagues suggest in the January 23 issue of the journal Nature that the species is an early ancestor of birds that probably used its feathered limbs, along with a long, feather-fringed tail, to glide from tree to tree. They argue that the animal represents an intermediate stage in the evolution of flight, from gliding much as flying squirrels do today to the active wing flapping of modern birds.
    "To have fully formed flight feathers on the hind legs is fascinating," said James Clark, Ronald Weintraub Associate Professor of Biology at George Washington University, Washington, D.C.  "There were some interesting speculations about 90 years ago that birds might have had four feathered limbs, but no one has suggested it in recent times, since all living birds use only their forelimbs," he said. "This find broadens the whole scope of thinking about the origins of flight."
    Much fossil evidence has been uncovered supporting the idea that birds evolved from a group of bipedal carnivorous dinosaurs called theropods. Within the theropod group, birds are most closely related to dromaeosaurids. Velociraptor, a star in the movie Jurassic Park, is probably the most famous of dromaeosaurs.
    Earlier finds in Liaoning suggest that the earliest dromaeosaurs were small, feathered animals with forelimbs similar to those of Archaeopteryx, the oldest known bird at around 150 million years old, and feet with features comparable to modern tree-living birds.
     This species provides another link in the emerging transition from small, meat-eating dinosaurs to birds," said Hans-Dieter Sues, curator of vertebrate paleontology and associate director for science and collections at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. "These fossils fill in a blank in the fossil record."
    Although the M. gui fossils are about 25 million years younger than Archaeopteryx, the four-winged dinosaur is a more primitive form derived from a very early evolutionary branch of dromaeosaurs. The Chinese scientists suggest that the four-winged dinosaur is the most recent known common relative shared by both birds and dinosaurs.

Microraptor zhaoianus

Scientific American - Kate Wong May 2001
    An exquisitely complete feathered dinosaur has emerged from the famed fossil beds of northeastern China's Liaoning Province. The new discovery, announced today in the journal Nature, gives further weight to the argument that birds evolved from dinosaurs and provides the strongest evidence yet that feathers pre-date the origin of flight.
    Earlier finds from Liaoning had hinted at the presence of featherlike structures on several dinosaur specimens, but critics charged that the structures were instead fibers of the protein collagen or that the fossils represented not dinosaurs but flightless birds. Opponents of the bird-dinosaur connection also noted that no feathers were known from dromeosaurs-a group of small- to medium-size theropod dinosaurs that exhibit numerous traits in common with birds and are therefore widely held to be their closest relatives.
   The new fossil, however, appears to answer both of those arguments. Paleontologist Mark Norell of the American Museum of Natural History in New York City and his colleagues report that the 130-million-year-old specimen represents a dromeosaur covered with filamentous structures that exhibit a branching pattern unique to feathers.
The presence of featherlike structures on such a creature indicates that feathers must have evolved for some purpose other than flight-perhaps to help the animal keep warm. Indeed, for modern birds, which are warm-blooded, feathers provide critical insulation. Thus, Norell says, non-avian dinosaurs may have developed primitive feathers as they developed warm-bloodedness.
   The dinosaur, Microraptor zhaoianus, is also touted as the smallest non-avian dinosaur yet discovered, and may also give key clues to the relationships between dromaeosaurs, troodontids, and modern birds. It was officially named by Xu and two colleagues in the December 7th issue of the journal Nature.
    Measuring in at 39 centimeters (nearly 16 inches) and possessing a tail of 24 centimeters, Microraptor was a bipedal dinosaur that may have been adapted to live in trees. A small section of intermentary fuzz, what scientists believe to be the precursor to feathers, was also found on the specimen. Although it lived some 20 million years after Archaeopteryx, the first known bird, Microraptor is being called one of the most-bird like dinosaurs known, and many paleontologists believe it may play a key role in proving once and for all that birds evolved from small meat-eating dinosaurs.
    "(Microraptor) shows a number of modifications to the hips, tail and teeth which are in some ways intermediate between those of advanced meat-eating dinosaurs and birds. There also appears to have been feathers, adding more evidence to the view that feathers and feather-like structures predated the origin of birds," Dr. Paul Barrett, a professor at Oxford University said.  "It might represent the most bird-like dinosaur, " Xu told reporters.