"[T]he most primitive theropods had five digits or fingers. Remains from theropods that are more recent, evolutionarily speaking, show these dinosaurs had three fingers. And until now, scientists have suspected those digits were the inner three (starting with the thumb with respect to our hands). But embryos of living birds suggest the wings are made up of the middle digits, with the inner- and outer-most digits missing. So there's a discrepancy.
"The digit arrangement of L. inextricabilis, which Clark says is an evolutionary intermediate between the most primitive five-fingered dinosaurs and the three-fingered ones, matches up with birds' wings. The specimen had the three most central fingers (considered the second, third and fourth digits on a five-digit hand, counting the thumb as the first digit) while an inner, first finger or thumb was reduced to what Clark called "a nub."
"With the new discovery, the transition from a five-fingered forelimb to three digits in birds' wings makes sense: Basically, the middle three digits persisted and became modified as part of the wings of modern birds."