The two new fossils belong to a pigeon-size juvenile dinosaur thought to be just a year or two old and a three- to four-year-old duck-size youth.
The younger animal’s fossil included short ribbonlike feathers. On its tail, each feather was just 1.6 inches (4 centimeters) long, while on its arms a typical feather was less than 0.8 inch (2 centimeters) long.
By contrast, the older dinosaur sported long quills, with each tail feather measuring 13.7 inches (35 centimeters) long and a typical arm feather measuring roughly 9.8 inches (25 centimeters) long.
The findings suggest feathered dinosaurs might have been undergone a flurry of changes as they matured—unlike anything seen in modern birds, said study co-author Xing Xu of the Chinese Academy of Sciences in Beijing.