What are the origins of wings and tails in birds? This is one of the key questions in the evolution of animals. It has long been accepted that their evolution began in feathered dinosaurs.

Some of these dinosaurs had feathers on the tails and small wing-like feathers on their forelimbs. These small wing-like structures called ‘proto-wings’ are composed of special feathers known as pennaceous feathers—the stiff feathers found in the wings and tails of birds.

The ancient form of these feathers first emerged in dinosaurs during the Jurassic Period, and these dinosaurs, called Pennaraptorans, had proto-wings made of pennaceous feathers. However, it has been known that these proto-wings were too small for powered flight. Because we cannot time-travel to observe their behavior, what dinosaurs did and how they behaved remains unanswered.

Various functions of proto-wings and tail feathers in the ancestors of birds have been considered since John Harold Ostrom proposed the first idea 50 years ago that proto-wings were used to knock down insect prey by small predatory dinosaurs living on the ground and following their prey. However, how the small ‘proto-wings’ and feathered tails helped the dinosaurian ancestors of birds in their lives has not been resolved.