Strong yet light, beautiful and precisely structured, feathers are the most complex skin appendage ever evolved in vertebrates. Despite the fact that humans have been playing with feathers since prehistoric times, there is still a lot we don’t understand about them.
Our new research found that some of the first animals with feathers also had scaly skin, like that of reptiles.
After the debut of the first feathered dinosaur, Sinosauropteryx fineIn 1996, a wave of discoveries painted an increasingly interesting picture of feather evolution.
We now know that many dinosaurs and their flying cousins, the pterosaurs, had feathers. Feathers came in more forms in the past. For example, ribbon-like feathers with expanded tips were found in dinosaurs and extinct birds, but not in modern birds. Only a few ancient feather types are inherited by birds today.
Paleobiologists have also learned that early feathers were not designed for flight. Fossils of early feathers had simple structures and were sparsely distributed on the body, so they may have been intended for display or tactile observations. Fossils of pterosaurs suggest that they may have played a role in thermoregulation and color formation.
As fascinating as these fossils are, ancient feathers tell only part of the story of feather evolution. The rest of the action took place in the skin.
Bird skin today is soft and developed for the support, control, growth and pigmentation of feathers, unlike the scaly skin of reptiles.
Dinosaur skin fossils are more common than you think. However, so far only a handful of dinosaur skin fossils have been examined at the microscopic level. These studies, for example a 2018 study of four fossils with preserved skin, showed that the skin of early birds and their close dinosaur relatives (the coelurosaurs) was already very similar to the skin of modern-day birds. Bird-like skin evolved before bird-like dinosaurs emerged.
To understand how bird skin evolved, we need to study the dinosaurs that split off earlier in the evolutionary tree.
Our research shows that at least some feathered dinosaurs still had scaly skin, like modern-day reptiles. This evidence comes from a new copy of Psittacosaurusa horned dinosaur with brush-like feathers on its tail. Psittacosaurus lived in the early Cretaceous (about 130 million years ago), but its clan, the ornithischian dinosaurs, diverged from other dinosaurs much earlier, in the Triassic (about 240 million years ago).
In the new specimen, the soft tissues are hidden from the naked eye. However, under ultraviolet light, the flaky skin reveals itself in an orange-yellow glow. The skin is preserved on the trunk and limbs, parts of the body that had no feathers.
These luminous colors come from silica minerals that are responsible for preserving the fossil skin. During the fossil record, silica-rich fluids permeated the skin before it decayed, replicating the skin’s structure in incredible detail. Fine anatomical features are preserved, including the epidermis, skin cells and skin pigments called melanosomes.
The fossil skin cells have much in common with modern reptile skin cells. They share a similar cell size and shape and both have fused cell boundaries – a feature known only in modern reptiles.
The distribution of fossil skin pigment is identical to that in modern crocodile scales. However, the fossil skin appears relatively thin by reptilian standards. This suggests that the fossil scales in Psittacosaurus were also similar in composition to reptilian scales.
Reptile scales are hard and stiff because they are rich in a type of skin-building protein called the tough horny beta proteins. In contrast, the soft skin of birds is made of a different type of protein, the keratin, the main structural material in hair, nails, claws, hooves and our outer skin.
To provide physical protection, the thin, bare skin of Psittacosaurus must be composed of tough reptilian-style horny beta proteins. A softer bird-style skin would have been too fragile without feathers for protection.
Collectively, the new fossil evidence indicates that Psittacosaurus had reptilian skin in places where it had no feathers. The tail, which retains the feathers in some specimens, unfortunately has not retained any feathers or skin in our specimen.
However, the tail feathers of other specimens show that some bird-like skin features must have already evolved to keep the feathers in place. So our discovery suggests that early feathered animals had a mix of skin types, with bird-like skin only in the feathered parts of the body, and the rest of the skin still scaly, like in modern reptiles.
This zoned development would have ensured that the skin protected the animal against wear, dehydration and pathogens.
What now?
The next gap in knowledge that scientists need to investigate is the evolutionary transition of reptile skin Psittacosaurus on the skin of other more heavily feathered dinosaurs and early birds.
We also need more experiments to study the process of fossilization itself. There’s a lot we don’t understand about how soft tissues form fossils, meaning it’s difficult to tell which skin features in a fossil are real biological features and which are simply artifacts of fossils.
Over the past thirty years, the fossil record has surprised scientists regarding the evolution of feathers. Future discoveries of fossil feathers could help us understand how dinosaurs and their relatives evolved flight, warm-blooded metabolism and how they communicated with each other.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Zixiao Yang receives funding from the Irish Research Council and Jurassic Foundation.
Maria McNamara receives funding from the European Research Council and Science Foundation Ireland.