Evidence is mounting that modern birds descended from gliding, feathered non-avian dinosaurs.
Two dinosaurs could be candidates for the bottom of the bird family tree, and each helps to reveal how feathers first evolved.
"The oldest known feathered dinosaurs would be Anchiornis (155 million years ago) and Epidexipteryx (between 152 million and 168 million years ago)," Yale University paleontologist Nicholas Longrich told Discovery News. "Feathers seem to have appeared initially for insulation. Basically they start out as down, and later are used to make wings."
For a study published in the latest Current Biology, Longrich and colleagues Jakob Vinther and Anthony Russell examined fossils of Anchiornis huxley and of Archaeopteryx lithographica, a Jurassic species that could be the world's oldest known bird.
"Where dinosaurs end and birds begin is a bit arbitrary," Longrich explained. "There's no clear cutoff that separates one from the other. That's the nature of evolution; things gradually change from one thing into another."
The scientists found that the wing feathers of Archaeopteryx and Anchiornis were similar, but not identical. The variations between the two appear to represent early experiments in the evolution of the wing.
Archaeopteryx had multiple layers of long flight feathers. In contrast, the dinosaur Anchiornis had an abundance of simple, strip-like feathers that overlap, somewhat similar to the feathers on penguins.
The design and arrangement of Anchiornis and Archaeopteryx wing feathers probably hindered liftoff. Multiple overlapping layers of long wing feathers would have complicated feather separation, minimizing the birds' ability to overcome drag on the upstroke. By contrast, the wings of modern flying birds typically have a single primary layer of easily separated long feathers overlain by short feathers.
"Modern birds have the ability to separate their wing feathers sort of like a Venetian blind," Longrich said. "This allows them to raise the wing rapidly, and seems to be critical to flapping flight at low speeds."
"The feather arrangement in Archaeopteryx and Anchiornis wouldn't let them do this," he added, "so it may have made takeoff from the ground and flapping at low speeds more difficult."
Gliding, however, must have been a lifesaver back in the dinosaur day, when huge terrestrial carnivores were stomping around.
"Gliding is a fast way to move from tree-to-tree. Instead of climbing down one tree and running up the next, you just glide quickly from one to the other," Longrich explained.
"I would imagine that the dinosaurian ancestors of birds were living in the trees," he noted, "probably to find food-like insects, lizards and mammals, and to avoid becoming food for other dinosaurs."
Longrich and his colleagues believe that the wing feather arrangement seen in modern birds may have evolved within a period spanning a few tens of millions of years and then remained largely unchanged for the past 130 million years.
In terms of this short versus long timescale, Longrich compared it to the evolution of human-constructed aircraft, which started with some years of experimentation before settling into a basic design that's just been fine-tuned during more recent years.
"Birds hit on a workable design about 130 million years ago, and it's been difficult to improve upon it," he said.
Birds benefited from both this and their small size 65 million years ago, when the larger non-avian dinosaurs bit the dust.
Xu Xing, one of the world's leading paleontologists, is a professor at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences. He told Discovery News that the determination about ancient versus modern feathers and wings "is definitively a significant discovery."
The study "has greatly improved our understanding of wing evolution," Xu continued. "(The authors) demonstrate that there are unusual wings near the dinosaur-bird transition, though more data is needed to confirm the claim that it is directly related to the origins of avian flight."