Pterosaur
Relation with feathers

Pterosaur filaments could share a common origin with feathers, as speculated in 2002 by Czerkas and Ji. In 2009, Kellner concluded that pycnofibers were structured similarly to theropod proto-feathers. Others were unconvinced, considering the difference with the "quills" found on many of the bird-like maniraptoran specimens too fundamental.

A 2018 study of the remains of two small Jurassic -age pterosaurs from Inner Mongolia, China, found that pterosaurs had a wide array of pycnofiber shapes and structures, as opposed to the homogeneous structures that had generally been assumed to cover them. Some of these had frayed ends, very similar in structure to four different feather types known from birds or other dinosaurs but almost never known from pterosaurs prior to the study, suggesting homology. A response to this study was published in 2020, where it was suggested that the structures seen on the anurognathids were actually a result of the decomposition of aktinofibrils: a type of fibre used to strengthen and stiffen the wing. However, in a response to this, the authors of the 2018 paper point to the fact that the presence of the structures extend past the patagium, and the presence of both aktinofibrils and filaments on Jeholopterus ningchengensis and Sordes pilosus. The various forms of filament structure present on the anurognathids in the 2018 study would also require a form of decomposition that would cause the different 'filament' forms seen. They therefore conclude that the most parsimonious interpretation of the structures is that they are filamentous protofeathers. But Liliana D'Alba points out that the description of the preserved integumentary structures on the two anurognathid specimens is still based upon gross morphology. She also points out that Pterorhynchus was described to have feathers to support the claim that feathers had a common origin with Ornithodirans but was argued against by several authors. The only method to assure if it was homologous to feathers is to use a scanning electron microscope.

In 2022, a new fossil of Tupandactylus cf. imperator was found to have melanosomes in forms that signal an earlier than anticipated development of the patterns found in extant feathers than previously thought. In these fossils, it appears as though the feather melanosomes took on a more complex form than the melanosome organization in scales that near relatives of Tupandactylus had. This discovery is one of many that leads us away from many previous theories of feathers evolving directly from scales in reptiles, given the significant distinction of melanosome organization and content between the two. This indicates a distinct form of melanosomes within feather structures at the time, different from other contemporary feathers that did not carry this formation. The feather fossils obtained from this specimen also suggested the presence of Stage IIIa feathers, a new discovery which may also suggest that more complex feather structures were present at this time. Previously, no Stage III feather forms had been discovered in this time. This study contains multiple indications about the development of feather forms. These include a more precise estimate for the development of avian feather forms, as well as a more ancient ancestor that contained the origins of feather-specific melanosome signaling found in extant birds.
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