As a regular reader of this blog, you are well aware that dinosaurs had feathers (unless you are a certain film franchise). Dinosaurs were covered in patches of fuzz, proto-floof, shook their tail feathers, and in some cases displayed full-fledged plumage. Over the last decade, exceptionally preserved fossils and intense genetic study have taught us a lot about feather evolution. But what do we know about the evolution of the skin of dinosaurs and early birds?
Vertebrate skin consists of several layers of cells making up the epidermis (the outer skin), the dermis (containing connective tissues, hair follicles and sweat glands) and the hypodermis (consisting of fat and connective tissues). As we grow, our skin continually renews itself. The inner layers generate new cells, which then slowly migrate to the outer layers to replace those tired, worn-out outer skin cells. This is called desquamation.
When we think of reptile skin, we think of smooth, scaly snake skin, or the thick and bumpy skin of crocodiles. As birds evolved from reptiles, one would expect bird skin to be quite similar to that of reptiles. However, bird skin looks rather different. With the exception of the feet, where the reptile heritage is still visible, scales are generally absent on bird skin, and the skin itself is rather thin. Moreover, in contrast to lizards and snakes, who shed their skin in large patches or in a single moult, birds shed their skin in small flakes. In these aspects, bird skin looks more like mammalian skin. Interestingly, primitive reptiles such as crocodiles and turtles also shed their skin in small patches. This suggests that shedding one’s skin in small patches is an ancient trait. How did birds evolve their skin, and more important, did dinosaurs have dandruff?
Palaeontologist Maria McNamara from University College Cork in Ireland and her team had the opportunity to take a good look at the fossilised skins of a series of non-avian dinosaurs and early birds from the Early Cretaceous Jehol biota from north-eastern China. The Jehol biota is famously known for its exquisitely preserved fossil birds and dinosaurs, which often display detailed soft-tissue preservation. The specimens studied – the bird Confuciusornis and the maniraptoran dinosaurs Beipiaosaurus, Sinornithosaurus and Microraptor – all display fossil feathers across their bodies. From areas covered by fossil feathers, McNamara then removed small patches of the specimens’ surface to study under a high-resolution scanning electron microscope.
Under the microscope’s powerful beam of electrons, tiny, flat polygonal structures were visible. In places where the plane of splitting of the fossil slab passed through the patch of tissue, a fibrous internal structure was exposed. Given their small size and irregular yet flattened shape, McNamara and her team argue these structures are unlike the fish and reptile scales or shell fragments that have also been found in the Jehol sediments. Nor are they patches of actual fossilised skin, as the patches in question appear only one cell layer thin, whereas vertebrate skin’s outermost layer is generally five to 20 cells thick. Instead these structures represent shed corneocytes, cells from the outermost layer of the skin. In other words, fossilised dandruff.
So yes, dinosaurs appear to have had dandruff, but what does actually tell us about the evolution of dinosaur and bird skin? (Apart from the assumption that they probably also worried about it when wearing dark colours.) In contrast to popular opinion, not all dinosaur skin was scaly and reptile-like. But exactly when and how the skin of reptiles evolved into that of dinosaurs and birds remains unclear. These new data show that certain non-avian dinosaurs and early birds shed their skin in ways very similar to that of modern birds and mammals. Feathered dinosaurs thus had already evolved some of the features of modern bird skin by the Middle Jurassic (c125m years ago).
Interestingly, while basal birds and some dinosaurs had already evolved certain distinctively modern skin features, these animals were not capable of active flight. This suggests that the evolution of these features was linked to something other than the evolution of active flight.
The fibres seen inside the fossil corneocytes are identified as mineralised keratin fibres (the same durable material that constitutes our hair and nails). In modern birds, keratin fibres are loosely dispersed throughout the cells and interspersed with droplets of lipid. This is linked to the ability of cooling through evaporation in response to heat production during active flight. When you’re flapping up a sweat, it is important to stay cool to prevent overheating.
The fossil fibres, however, form a sturdy scaffolding network that links adjacent cells together, and are much more densely packed than in modern bird skin cells. In these fossil animals the density of keratin fibres in skin cells was thus much higher than in living birds. This suggests that these early birds and dinosaurs had less need for evaporative cooling, possibly because they were not active flyers and less prone to overheating through vigorous flapping. This is in line with other evidence for low metabolic rates in non-avian dinosaurs and early birds and confirms the growing body of evidence that suggests feathers originally arose for functions other than powered flight.
References
McNamara et al., 2018. Fossilised skin reveals coevolution with feathers and metabolism in feathered dinosaurs and early birds. Nature Communications 9.
