Microraptor
"The evolution of powered flight in birds from theropod dinosaurs is recognized as the key adaptive breakthrough that contributed to the biological success of this group...  Hundreds of small, exquisitely preserved, feathered theropods were discovered in the Early Cretaceous Jehol Group of northeastern China as they died some 125 million years ago, smothered in the "Cretaceous Pompeii"... Both anatomy and phylogeny strongly suggest that these theropods... offer new insights into the origins of feathers and flight, favoring the arboreal ('trees-down') over the cursorial ('ground-up') hypothesis".