how the fluffy pterosaurs reignited the debate

When the fossils of the oldest known bird, Archeopteryx, were first discovered nearly 160 years ago, the find created a puzzle that has troubled paleontologists ever since.

These fossils were celebrated for their chimera-like combination of seemingly reptilian features (such as a bony tail and toothed jaws) and those seemingly unique to birds, especially feathers. They helped show that birds actually evolved from dinosaurs.

But they also presented a serious evolutionary problem. Prehistoric feathers were now indistinguishable from those of birds. So it wasn’t clear how or when the feathers evolved and into what kind of ancient beasts.

Spectacular fossil discoveries from China in the mid-1990s overturned our notions of feather evolution, as they revealed that feathers are not, in fact, unique to birds, but have also occurred in many dinosaurs. Over the past 30 years, additional fossil records have revealed remarkable details about the evolution of feathers and flight.

Today, more recent discoveries of what appear to be feathered fossils of pterosaurs, the flying cousins ​​of dinosaurs, have led to the theory that feathers first evolved even earlier with the ancestors of all these creatures. But not everyone is convinced and the debate on the origins of feathers continues.

Feathered dinosaurs

Dinosaurs had far more types of feathers than we see in birds today. Some dinosaurs had four wings. Some species have given up their wings entirely and glided using large flaps of skin.) At least some dinosaurs had colorful feathers, which were used for camouflage and mating.

And with the evolution of feathers, the skin of dinosaurs and birds also evolved, even starting to produce dandruff. But still, for many years, feathers were only known to maniraptor dinosaurs (the group of species that actually includes birds).

There were hints that the evolution of feathers wasn’t that simple. Feather-like structures, also called “protopfeathers”, have been reported in ornithischian dinosaurs. Theoretical models predict that early feathers would resemble hair-like filaments. The simple hair-like shape of the fossil filaments, however, led some workers to question whether they were really feathers, rather than the degraded remnants of some other material, such as skin collagen.

In 2014, a Jurassic Ornithischian dinosaur from Siberia known as Kulindadromeus it was found that they had both simple monofilaments and more complex feathers emerging from her skin. This dinosaur confirmed that feathers are not just a feature of maniraptor dinosaurs, but they likely originated before major dinosaur groups split apart.

Clearly, the ability to grow feathers evolved with dinosaurs, although some groups of dinosaurs, particularly large sauropsids and ankylosaurs and armored stegosaurs, may have lost this ability later on. But having skin (hair) growths and losing them later is well known in mammals, including whales and elephants.

The question is not whether feathers are unique to birds, but whether they are unique to dinosaurs as well. Hairy, hair-like fibers reminiscent of dinosaur “protopfeathers” have long been known in pterosaurs. Pterosaur filaments were traditionally called “picnofibers” and were considered distinct from feathers in shape and evolution.

But in 2018 we discovered simple filaments and, surprisingly, three types of branching feathers preserved in pterosaurs from the Yanliao Biota fossil deposits of the Middle Jurassic era, located in China. Although the branching structure is not quite the same as in today’s birds, feathers are rich in keratin, the protein commonly found in feathers and hair, and contain dye melanosomes.

This finding strongly suggests that the fuzzy picnofibers of other pterosaurs are also primitive feathers. This probably means that the ability to grow feathers evolved once, about 100 million years earlier Archeopteryx, and has been passed down to various groups of species.

Unsurprisingly, this notion of feathered pterosaurs has proved controversial and other researchers have challenged our ideas. The debate focuses on a few key issues, with questions regarding the conservation of feathers in the front and center.

Dave Unwin of Leicester University and Dave Martill of Portsmouth University argue that pterosaur structures may be too degraded to be certain that they are feathers and that they may actually be degraded skin fibers. But the characteristics of the feathers are not consistent with the degradation and disintegration of the composite fibers. They are also sinuous and lacking in the spatial organization of skin fibers and contain melanosomes, which are not incorporated into skin collagen.

Unwin and Martill also point out that the keratin and other chemical evidence we found could be contamination. But this seems unlikely because it was only found in the feathers and not the surrounding tissue.

Another problem is that other pterosaur fossils have only simple hair-like filaments and no branching structures. But birds today have many different types of feathers, so these filaments could be a different form of feather or early, simple, an idea supported by theoretical models.

Debate in progress

It’s always a good idea to question interpretations of new fossils, especially where the evolutionary implications are far-reaching, even if we believe the evidence for pterosaur feathers is there in the fossils. Clearly, however, there is still a lot to do and we are currently conducting more fossil testing to better understand the chemical composition and structure of feathers.

Ultimately, if we’re right, it appears that the earliest feathers will be found in the ancestors of pterosaurs and dinosaurs in the Lower Triassic era, some 252 million to 247 million years ago. Unfortunately, we have no fossils showing soft tissue conservation from this time period.

But if we have learned anything from the fossil record of feathers, it is to be expected that more will be discovered. Over the years we have had to repeatedly expand our search for fossils with feathers and what ancient feathers looked like. Who knows what insights the future fossils will bring.