Exceptionally well preserved dinosaur fossils uncovered in north-eastern China display the earliest known feathers.
The creatures are all more than 150 million years old.
The new finds are indisputably older than Archaeopteryx, the "oldest bird" recognised by science.
Professor Xu Xing and colleagues tell the journal Nature that this represents the final proof that dinosaurs were ancestral to birds.
The theory that birds evolved from dinosaurs has always been troubled by the absence of feathers more ancient than those on the famous Archaeopteryx.
This has given critics room to question the idea.
But the new fossils, which come from two separate locations, are in most cases about 10 million years older than the primitive Archaeopteryx discovered in the late 19th Century.
One of the new dinosaur specimens, named Anchiornis huxleyi, is spectacular in its preservation.
It has extensive plumage covering its arms and tail, and also its feet - a "four-winged" arrangement, says Professor Xu from the Chinese Academy of Science in Beijing.
"The first specimen we discovered earlier this year was incomplete," he told BBC News.
"Based on that specimen, we named it Anchiornis; and we thought it was a close relative of birds. But then we got a second specimen, which was very complete - beautifully preserved.
"All over the skeleton, you see feathers.
"Based on this second specimen, we realised that this was a much more important species, and definitely one of the most important species for our understanding of the origin of birds and of their flight."
Professor Xu believes the four-winged shape may have been a very important stage in the evolutionary transition from dinosaurs to birds.
Details of the latest discoveries have been presented this week at the annual meeting of the Society of Vertebrate Paleontologists, being held this year at the University of Bristol, UK.
The renowned Bristol palaeontologist Michael Benton said the announcement was immensely exciting.
"Drawing the tree of life, it's fairly obvious that feathers arose before Archaeopteryx appears in the fossil record," he told BBC News.
"Now these fantastic new discoveries by Professor Xu Xing prove that once and for all.
"These new discoveries are maybe 10 million years older than Archaeopteryx."