Whether non-avian dinosaurs had hair or feathers is a common question. They did not possess true hair, as this is a unique feature of mammals. Dinosaur body coverings were far more diverse and complex than the scales of a lizard, ranging from tough, reptilian skin to structures that are essentially primitive feathers. Research over the past few decades has transformed our understanding, moving the appearance of many dinosaurs away from the traditional image of giant, scaly reptiles. This shift is driven by incredible fossil finds that reveal soft tissue impressions, offering a new view of dinosaur biology.
The Biological Difference Between Hair and Feathers
Hair and feathers represent two distinct evolutionary paths for integumentary structures. Hair is a keratinous filament that grows from a follicle deep in the dermis, or middle layer of the skin, and is exclusively found in mammals. Hair cells are rapidly keratinized to form a shaft, and the structure is typically made of alpha-keratin.
Feathers, in contrast, are made of beta-keratin and develop differently from a raised papilla in the epidermis, or outer layer of the skin. This distinction in development and keratin type highlights that feathers and hair are not interchangeable, despite both being filamentous structures. Feathers are complex, often featuring a central shaft (rachis) and interlocking barbs and barbules. They are fundamentally unique to birds and their dinosaur ancestors.
The Initial Understanding: Scales and Reptilian Skin
For over a century, the public and scientific community pictured all dinosaurs as possessing tough, scaly, reptilian skin. This traditional view stemmed from early fossil discoveries, such as Iguanodon in the 1820s, which offered little information on skin texture. Later, fossilized skin impressions from massive dinosaurs like Edmontosaurus and Tyrannosaurus showed a covering of non-overlapping, polygonal scales.
These rare skin impressions are a type of fossilization where the texture of the skin is pressed into the surrounding sediment. The resulting fossils preserve a surface texture reminiscent of a crocodile or large lizard, leading to the assumption that all non-avian dinosaurs were uniformly scaled. The immense size of many species reinforced this idea, suggesting a need for thick, protective skin rather than an insulating coat.
Evidence for Feathered Dinosaurs
The traditional scaly-dinosaur paradigm was challenged by discoveries in the Liaoning Province of China, beginning in the mid-1990s. Exceptional preservation conditions in this region, part of the Early Cretaceous Jehol Biota, allowed for the fossilization of soft tissues, including integument. The first definitive evidence came with the discovery of Sinosauropteryx, a small theropod covered in simple, filament-like structures.
These structures are understood to be primitive feathers, often called protofeathers. Unlike the complex flight feathers of modern birds, protofeathers on dinosaurs like Sinosauropteryx were simple filaments, likely serving primarily for insulation and display. The discovery of melanosomes within these structures allowed scientists to infer color patterns, showing that Sinosauropteryx had countershading and a banded tail. More advanced feathered dinosaurs, such as Microraptor and relatives of Velociraptor, possessed complex feathers with vanes and barbs, definitively establishing the link between birds and non-avian dinosaurs.
Integument of Non-Avian Species
While feathered dinosaurs are widely accepted for many theropods, a significant portion of the dinosaur family tree remained covered in scales and thick skin. Massive herbivorous dinosaurs, including the long-necked sauropods and heavily-armored ankylosaurs, show little evidence of feathers. Species like Ankylosaurus and ceratopsians such as Chasmosaurus possessed thick, protective skin with large, patterned scales.
Many of these huge dinosaurs also incorporated bony armor plates called osteoderms embedded within their skin. This suggests that the evolutionary development of feathers was concentrated within certain lineages, particularly the coelurosaurian theropods. The enormous body size of sauropods and ceratopsians likely made an insulating feather coat unnecessary, and perhaps even detrimental, due to the challenge of dissipating body heat.