by Strong yet light, beautiful and precisely structured, feathers are the most complex skin appendage ever to have evolved in vertebrates. Despite the fact that people have been playing with feathers since prehistoric times, there is still a lot we don’t understand about them.
Our new study found that some of the first animals with feathers had scaly skin similar to reptiles.
After the debut of the first feathered dinosaur, Sinosauropteryx primain 1996, an increase in discoveries has created a more interesting picture of feather evolution.
We now know that many dinosaurs and their flying cousins, the pterosaurs, had feathers. Feathers have evolved into more shapes in the past – for example, ribbon-like feathers with enlarged tips have been found in dinosaurs and extinct birds but not in modern birds. Only a few ancient feather types are inherited by birds today.
Paleobiologists also learned that early feathers were not made for flight. Early feather fossils had simple structures and sparse body distributions, so they may have been displayed or tactilely sensed. Pterosaur fossils suggest that they may have played a role in thermoregulation and color patterning.
As amazing as these fossils are, old feathers only tell part of the story of feather evolution. The rest of the action took place in the skin.
The skin of birds today is soft and changed for support, control, growth and color of feathers, unlike the scaly skin of reptiles.
Fossils of dinosaur skin are more common than you think. To date, however, only a handful of dinosaur skin fossils have been examined at a microscopic level. These studies, such as a 2018 study of four fossils with preserved skin, have shown that the skin of early birds and their close dinosaur relatives (the coelurosaurs) was very similar to the skin of birds today. Bird-like skin evolved before bird-like dinosaurs came around.
To understand how bird-like skin evolved, we need to study the dinosaurs that evolved earlier in the evolutionary tree.
Our study shows that some feathered dinosaurs, like today’s reptiles, still had scaly skin. This evidence comes from a new specimen Psittacosaurusa horned dinosaur with bristle-like feathers on its tail. Psittacosaurus lived in the early Cretaceous period (about 130 million years ago), but his clan, the ornithischian dinosaurs, differed from other dinosaurs much earlier, in the Triassic period (about 240 million years ago).
In the new specimen, the soft tissues are hidden from the naked eye. Under ultraviolet light, however, scaly skin reveals itself in an orange-yellow glow. The skin is kept on the torso and limbs which are parts of the body that did not have feathers.
These luminous colors are from silica minerals responsible for preserving the fossilized skin. During fossilization, silica-rich fluids seeped through the skin before decaying, replicating the skin’s structure in incredible detail. Fine anatomical features are preserved, including the epidermis, skin cells and skin pigments known as melanomas.
The fossil skin cells have much in common with the skin cells of modern reptiles. They have a similar cell size and shape and both have fused cell borders – a feature only known in modern reptiles.
The distribution of fossil skin pigment is identical to that of modern crocodile scales. The fossilized skin, however, appears to be relatively thin by reptilian standards. This indicates the fossil scales i Psittacosaurus also in the composition of reptile scales.
Reptile scales are hard and rigid because they are rich in a type of skin-building protein, the tough beta keratin proteins. In contrast, the soft skin of birds is made of a different type of protein, the keratins, which are the main structural material in hair, nails, hooves, hooves and later our outer skin.
To provide physical protection, the skin must be thin, exposed Psittacosaurus they must have consisted of tough reptilian-style corneal beta proteins. A softer bird-style skin would be too fragile without feathers for protection.
Taken together, the new fossil evidence indicates that Psittacosaurus he had reptile-style skin in areas where there were no feathers. Unfortunately the tail, which preserves feathers in some specimens, did not preserve any feathers or skin in our specimen.
However, the tail feathers on other specimens show that some features of bird skin must have already evolved to hold feathers in place. So our discovery suggests that early feathered animals had a mixture of skin types, with bird-like skin only in the feathered regions of the body, and the rest of the skin still scaly, like modern reptiles.
This zoned development would ensure that the skin would protect the animal from scratching, dehydration and pathogens.
What else?
The next knowledge gap for scientists to explore is the evolutionary transition from reptile skin to Psittacosaurus with the skin of dinosaurs and other more feathered early birds.
We also need more experiments studying the fossilization process itself. There is much we do not understand about how soft tissues fossilize, which means that it is difficult to tell which skin features in a fossil are genuine biological features and which are just fossil artefacts.
For the past 30 years, the fossil record has puzzled scientists about feather evolution. Future discoveries of fossil feathers may help us understand how dinosaurs and their relatives developed flight, a hot metabolism, and how they communicated with each other.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Zixiao Yang receives funding from the Research Council of Ireland and the Jurassic Foundation.
Maria McNamara receives funding from the European Research Council and Science Foundation Ireland.
