What Kinds of Feathers Did Dinosaurs Have? A Sequence of Increasing Complexity
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Feathered dinosaur fossils show several distinct kinds of structure, from simple filaments to the asymmetric vanes of a flight feather. They fall into a sequence, and that sequence matches the order in which feather development proceeds in living birds.
The stages
The simplest structures are unbranched hollow filaments, present across a wide range of dinosaurs and resembling nothing more complex than a bristle. Next come tufts, where several filaments arise from a common base. Then structures with a central shaft bearing unbranched barbs along it, giving a downy appearance. Then barbs that themselves branch into barbules, which allows adjacent barbs to interlock and form a coherent surface. Then a closed vane with a stiff shaft, symmetrical on both sides. Finally an asymmetric vane, with a narrow leading edge and a broad trailing one, which is the arrangement that generates lift and is found only in flight feathers. Each stage is represented in fossils, the sequence is consistent with the order structures appear during the growth of a feather in living birds, and no stage requires the one after it to be useful.
Who had what
The distribution across the group is broader than the popular picture:
- •Simple filaments in a wide range of theropods, including some large ones, documented from several deposits
- •Comparable filaments in bird-hipped dinosaurs, in groups distantly related to birds, which implies the capacity was ancestral to dinosaurs generally
- •Similar structures in pterosaurs, which pushes the origin back further still if they are the same thing
- •Branched downy feathers in small predatory dinosaurs
- •Closed symmetrical vanes in animals with no flight capability, indicating display or insulation rather than flight
- •Asymmetric flight feathers only in animals close to or within the birds, and in a few forms that appear to have glided
What they were for before flight
Since the structures appear long before any flight, the functions must have been others and the candidates are well supported. Insulation is the standard explanation for filaments and down, and it implies an elevated metabolic rate worth insulating, which fits other evidence about dinosaur physiology. Display is strongly supported for the larger vaned feathers, particularly where they form tail fans or arm arrays in animals that could not fly, and where pigment analysis has recovered iridescent colouration, which is a display signal and nothing else. Brooding is supported by specimens preserved over clutches with the arms extended in a way that would cover eggs with the feathers. Water repellency, tactile function and use in balance during running have all been proposed. As with most successful structures, the answer is that they did several things and acquired more.
How they preserve
Feathers survive only under particular conditions and the deposits producing them are few and exceptional. Fine-grained lake sediments in oxygen-poor conditions account for most specimens, including the Chinese deposits that transformed the subject from the 1990s onward, where volcanic ash falls buried whole communities rapidly. Lithographic limestone in Germany preserved the earliest known bird with feather impressions in remarkable detail. Amber preserves feathers in three dimensions rather than as flattened films, and pieces containing a partial tail and individual feathers have supplied structural detail no compression fossil can, though the pieces are necessarily small. What survives is generally a carbonised film or an impression rather than the original material. That restriction means the absence of feathers on a specimen is weak evidence of anything, since most environments would not have preserved them in any case.
How colour was recovered
The discovery that colour could be determined changed what these fossils can be asked. Pigment in feathers is held in small structures whose shape correlates with colour in living birds, with elongated forms associated with black and grey and spherical forms with reddish brown, and those structures are preserved in some exceptionally preserved fossils. Comparing their shapes with a reference set from living birds allows a colour to be assigned statistically, and several feathered dinosaurs have been restored with patterns supported by measurement, including striped tails and iridescent sheens. The method has critics, since the correlation is imperfect, since structural colours produced by microscopic architecture rather than pigment require different evidence, and since preservation is uneven across a specimen. It nevertheless moved colour from pure invention into partial evidence, which is a substantial change for a field that had assumed it was permanently unknowable.
The takeaway
Filaments, tufts, shafted down, interlocking barbs, closed vanes and finally asymmetric flight feathers form a sequence matching the order structures appear as a feather grows. Simple filaments occur in groups distantly related to birds, so the capacity is ancestral to dinosaurs generally. Insulation, display and brooding account for them before flight, and pigment structures now allow colour to be measured.