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.