Palaeontology: Scales tip against dinosaur feathers

Fur continues to fly in the scientific controversy over whether most dinosaurs were covered with scales or feathers. Recent discoveries of feathers or filamentary “proto-feathers” in a range of dino fossils have given the impression, contrary to 20th-century assumptions, that feathers were the rule rather than the exception. Artists’ impressions of dinosaurs are making a transition from brown scaly monsters to brightly coloured feathered beasts.

But scientists at London’s Natural History Museum, the Royal Ontario Museum in Canada and Uppsala University in Sweden are urging caution on the feathered-dino enthusiasts. Their latest research — the most comprehensive study so far of the fossil record of dinosaur skin — suggests that most dinosaurs were covered in scales, like the reptiles alive today.

“Using a comprehensive database of dinosaur skin impressions, we attempted to reconstruct and interpret the evolutionary history of dinosaur scales and feathers,” says Paul Barrett of the Natural History Museum. “Most of our analyses provide no support for the appearance of feathers in the majority of non-avian dinosaurs. Although many meat-eating dinosaurs were feathered, the majority of other dinosaurs, including the ancestor of all dinosaurs [about 230 million years ago], were probably scaly.”

The spectacularly preserved fossils of feathered dinosaurs discovered over the past two decades, particularly in China, represent ancestors of birds and carnivores such as the raptor sinornithosaurus. Although the most striking evolutionary role of feathers was eventually to give birds the power of flight, they probably arose originally in small dinosaurs for insulation, signalling or display.

There is no evidence for proper feathers on the lumbering vegetarian giants such as brontosaurus. Some plant-eating dinosaur groups do show signs of quills and filament-like proto-feathers, which may have given some of them a fuzzy-looking appearance. But the new study, published in the journal Biology Letters, suggests these were “evolutionary experiments” that arose independently of the origins of true feathers.

However, the fossil record of well-preserved dinosaur skin — either scaly or feathery — remains sparse, even with recent discoveries, and further evidence could tilt the balance of the debate.

“The origin of the direct filamentous precursors to feathers remains difficult to pinpoint,” says the Royal Ontario Museum’s David Evans. “Whether or not the first dinosaurs had true ‘proto-feathers’ will only be resolved with the discovery of more fossils, particularly from early in dinosaur evolutionary history.”

Photographs: De Agostini/Natural History Museum; The Geological Museum of China