Birds Evolved From Dinosaurs Slowly—Then Took Off

These fossils found in northeastern China show the diversity and small sizes of some of the feathered dinosaurs. Many avian traits evolved in dinosaurs long before birds themselves appeared.

An 80-million-year transition was capped with a burst of feathered diversity.

Birds evolved from dinosaurs in patchwork fashion over tens of millions of years before finally taking to the skies some 150 million years ago, paleontologists report.

Birds are defined by a plethora of traits that are unique to them, such as feathers, hollow bones, a wishbone, and beaks. Paleontologists once supposed that the earliest bird, 150-million-year-old Archaeopteryx, represented a great evolutionary leap from dinosaurs. But over the past two decades, new discoveries have revealed that many of its avian traits had evolved in dinosaurs long before.

The Current Biology journal report released on Thursday confirms this new picture, finding that the dinosaur forebears of birds began gradually evolving avian traits almost as soon as dinosaurs appeared on Earth some 230 million years ago.

The new study also supports a view proposed by the American Museum of Natural History paleontologist George Gaylord Simpson in 1944. He suggested that evolutionary novelty, flight in this case, can lead to rapid diversification among species exploiting new environmental niches.

Epidexipteryx is the first example in the fossil record of a dinosaur flaunting feathers for display.

The Current Biology paper shows that about 80 million years of gradual evolution culminated in a burst of bird diversity after Archeopteryx took off, albeit clumsily. "Once the whole body plan finally came together, then something was unlocked and they started evolving really fast," says paleontologist Stephen Brusatte of the University of Edinburgh in Scotland, lead author on the study.

"This is statistical confirmation of a view about bird evolution that paleontologists have described for a while," says paleontologist Roger Benson of the University of Oxford in the United Kingdom. "Scientifically, it would have been crazier if they had shown birds appearing from dinosaurs all of a sudden out of nowhere."

Dinosaur Database

In the study, Brusatte and colleagues looked at a database of 152 carnivorous, two-legged dinosaur species in the family that led to both Tyrannosaurus rex and birds. The records allowed statistical comparison among 853 traits on the creatures, looking at everything from the presence or absence of feathers to the size of the gap between their wrist bones.

Feathered dinosaur discoveries have come at "astounding" rates in the past two decades, according to Benson, making such close looks at the ancient roots of birds possible. "We really do have a strong fossil record for birds now," he says.

In August, a research team led by Michael Lee of the South Australian Museum in Adelaide reported in Science magazine that bird ancestors decreased in weight from about 359 pounds (163 kilograms) to 1.8 pounds (0.8 kilograms) over 50 million years to reach the size of Archaeopteryx.

This illustration of a Guanlong, an ancestor of Tyrannosaurus rex that lived 160 million years ago in present-day China, shows the feathered crest that gave it its name, which means "crowned dragon" in Chinese. A primitive form of feathers probably covered its body, and the head crest may have been used for species recognition or courtship displays.

That miniaturization was accompanied, according to the new study, by the steady acquisition of traits that are characteristic of birds today. After Archaeopteryx, which may only have been able to glide, the variety of birds expanded much more quickly in just a few million years, and the new species included some proficient fliers.

"Birds became more and more 'birdy' gradually," Brusatte says. "There was no big jump from non-bird to bird among dinosaurs, just a seamless transition."

Unlocking fight as an evolutionary niche in a new way may also have been what allowed birds to escape the extinction of other dinosaurs some 66 million years ago, Brusatte suggests. "Small dinosaurs that flew had a lot of advantages over other ones," he says.