Size and Weight:
Although it was unmistakably a dromaeosaur (what ordinary folks call a raptor), Unenlagia has raised some puzzling issues for evolutionary biologists. This feathered dinosaur was distinguished by its very limber shoulder girdle, which gave its arms a broader range of motion than comparable raptors--so it's only a short step to imagining that Unenlagia actually flapped its feathered arms, which might well have resembled wings.
The puzzlement pertains to the fact that Unenlagia was clearly too big, about six feet long and 50 pounds, to take to the air (by way of comparison, flying pterosaurs with comparable wingspans weighed much less). This raises the prickly question: could Unenlagia have spawned a (now-extinct) line of flying, feathered descendants similar to modern birds, or was it a flightless relative of the first, genuine birds that preceded it by tens of millions of years?