Did the pterosaurs have archetypes of feathers? The debate on this important question of evolutionary history is now picking up speed: In a publication, researchers contradict an earlier study that found feather-like structures in pterosaurs around 160 million years old. According to the critics, it is only a matter of “frayed” structures that have arisen when fibers in the animals’ flight skin break down. But the study’s authors promptly defend their results and reiterate their interpretation that the pterosaurs had protofeathers.
For a long time the dinosaurs were shown naked – but now numerous finds show that many species were feathered. The fine structures warmed the animals or possibly already played a role in courtship behavior. At some point, some representatives developed wings that they could lift into the air – the ancestors of birds evolved from the dinosaurs. Even the fine construction of the dinosaur feathers could be seen well in some finds. Most dinosaurs therefore had more down-like structures. What differentiated these protofeathers from hair was their branching structure.
When was the spring created?
Evidence of these elements, however, raised the question of whether the dinosaurs were the “inventors” of the feather or whether the history of this important structure goes back further in evolution. This is where the pterosaurs come into play: They were not representatives of the dinosaurs, but only more distant relatives – the lines of development of the two groups separated more than 250 million years ago. Even if the pterosaurs had feather-like structures, it is therefore obvious that this development goes back to the common ancestors of pterosaurs and dinosaurs. That would mean that the very first feather-like elements are much older than assumed.
This is exactly what came out two years ago from the results of the researchers working with Zixiao Yang from Nanjing University. Previously, only hair-like structures on the body surface were known of pterosaurs – so-called pycno fibers. But Yang and his colleagues have identified branched versions of the pycno fibers in two 160-million-year-old pterosaur fossils, which they interpreted as proto-feathers. As they explained, the features of these structures were similar to those of proto-feathered versions in the bird basin dinosaurs and theropods. Similar to these, they also found indications of the building material keratin as well as remnants of melanosomes, which give feathers color, inside the structures.
Questioned
But these groundbreaking results are now being questioned by David Unwin from the University of Leicester and David Martill from the University of Portsmouth in a publication in the journal Nature Ecology and Evolution. The paleontologists see no traces of proto-feathers in the formations. According to them, they are not even remnants of the hair-like pycno fibers. As they explain, it seems more likely that it is remnants of fibers that were part of the internal structure of the wing membranes of the pterosaurs examined. The branching structures were therefore only the result of the disintegration of these so-called actinofibrils. “Evidence from studies of other pterosaur fossils suggests that the branched morphologies presented by Yang and his colleagues, including brush-like and tuft-like structures, represent artifacts of conservation,” write the two paleontologists.
Even the detection of melanosomes and keratin does not convince Unwin and Martill, as they explain in their publication with the clear title “No proto-feathers on pterosaurs”. “Both elements occur in many surface structures of pterosaurs – they are not unique to pycno fibers,” write the paleontologists. According to them, it could therefore be a contamination from skin tissues. “Extraordinary claims require extraordinary evidence – we have the former, but not the latter,” comments Unwin.
Criticism rejected
But Yang and his colleagues still don’t see it that way: They published a reply in the same issue of the magazine in which they respond to the criticisms. They also put forward convincing arguments for their interpretations. They show that the structures they examined have features that do not fit the explanation that they are disintegrated fiber structures from the wing membranes. They also emphasize that they have discovered slightly different versions of the elements in different parts of the fossil body, which they interpret as proto-feathers. “It seems unlikely that a disintegration process of fibers has led to such differences in different parts of the body,” the scientists write.
As for the keratin and melanosomes, they emphasize that they discovered these elements specifically on the structures called proto-springs – they are absent in the areas immediately around them. This speaks against the explanatory approach that it is contamination from skin tissue. “The points of criticism and alternative explanations put forward by Unwin and Martill do not match our findings,” defend themselves Yang and his colleagues. “Against this background, we stick to our original conclusion that it is remnants of spring structures,” sum up the paleontologists.
So it will be interesting to see how this apparently unusually controversial topic will continue in paleontology in the future.
Source: University of Portsmouth, Nature Ecology and Evolution, doi: 10.1038 / s41559-018-0728-7 and 10.1038 / s41559-020-01308-9