Palaeontologists now know from the fossil evidence that some dinosaurs had not the scaly skins of today’s reptiles, but coats of feathers—much more like their living descendants, the birds. This is especially common among the maniraptor group of theropods (which includes the dromaeosaurs, oviraptorids and therizinosaurs) from which birds are most likely descended. The original purpose of having feathers was possibly for insulation—to keep warmth in. This makes it likely that feathered dinosaurs were warm-blooded, like birds, but unlike other reptiles. The feathers would have also been used in mating displays by males, particularly if they were brightly coloured. Long feathers on their limbs gave the very lightest dinosaurs the ability to glide or later fly.

Feather impressions

Fossils of the Jurassic dinosaur, Archaeopteryx, known from its skeletal simliarities to be related to some theropods such as Deinonychus, show well-preserved feathers. A number of dinosaur fossils that had feather impressions were discovered preserved in fine-grained volcanic ash in Liaoning province, China, in the 1990s.

Even where feather impressions are absent, the existence of quill knobs in the fossilized arm and leg bones of a dinosaur shows that feathers must have have been present—just as they are on the skeletons of modern birds. The presence of a pygostyle, the fused bones at the tail tip which supports large tail feathers, also provides convincing evidence.

Studies of the fossils of the dinosaur Caudipteryx, show that it had fans of feathers on its hands and tail. Its body was covered in short, down-like feathers. About 1 metre (39 inches) long, Caudipteryx lived in China during the Early Cretaceous Period, around 125 million years ago.

Nesting behaviour

Evidence for feathers can also come from what we know about dinosaur behaviour. Fossils of Citipati, an oviraptorid dinosaur, have been found in brooding positions. The most famous of these is known as “Big Mamma”. It is sitting on top of its eggs, with its limbs spread either side of the nest. This posture is found today only in birds. It is clear that most of the eggs would not have been covered by Citipati's body had not its forelimbs been covered in feathers.