Czerkas, S.A., and Ji, Q. (2002). A new rhamphorhynchoid with a headcrest and complex integumentary structures.
"A new rhamphorhynchoid is described with a headcrest that is unprecedented among the long-tailed pterosaurs. The preservation of the headcrest presents significant implications regarding the physical appearance and aerodynamics of all pterosaurs. Also, "hair-like" [pycnofibre] integumentary structures of this pterosaur are shown to be complex multi-strand structures which presents evidence on the origin of feathers and the possibility of a remarkably early ancestral relationship between pterosaurs and birds."
The soft tissue preserved in the holotype of J. ningchengensis indicates that the wing membrane is attached to the body until reaching the ankle. It also concurs with the general notion that the plagiopatagium can be divided into two distinct functional parts: the more distal actinopatagium that contains extensive actinofibrils and a softer, perhaps more flexible, proximal tenopatagium. The Chinese specimen further shows that the plagiopatagium of this pterosaur is formed by an external epidermis, followed by several layers (at least three) with closely packed actinofibrils. Part of the plagiopatagium, particularly the region closer to the body (the tenopatagium), was extensively covered by elongated and thick fibres here called pycnofibres. Individual pycnofibres are formed by fibrils of a different diameter, the nature of which is unknown. Regarding other pterosaur specimens, at least S. pilosus has a similar extensive integumental covering as noted in the original description.
At least some pterosaurs had hair-like filaments known as pycnofibres on the head and body, similar to, but not homologous (sharing a common structure) with, mammalian hair. Though a fuzzy "integument" (natural covering/outer coat) "was first reported in 1831" by Goldfuss, recent pterosaur finds and the technology for histological and ultraviolet examination of pterosaur specimens have provided incontrovertible proof: pterosaurs had pycnofibre coats. Pycnofibres were not true hair as seen in mammals, but a unique structure that developed a similar appearance. Although, in some cases, actinofibrils (internal structural fibres) in the wing membrane have been mistaken for pycnofibres or true hair, some fossils such as those of Sordes pilosus (which translates as "hairy demon") and Jeholopterus ninchengensis do show the unmistakable imprints of pycnofibres on the head and body, not unlike modern-day bats, another example of convergent evolution. The head-coats do not cover the pterosaur's large jaws in many of the specimens found so far.
Some (Czerkas and Ji, 2002) have speculated that pycnofibers were an antecedent of proto-feathers, but the available impressions of pterosaur integuments are not like the "quills" found on many of the bird-like maniraptoran specimens in the fossil record.Pterosaur pycnofibers were structured similarly to theropod proto-feathers. Pycnofibers were flexible, short filaments, "only 5-7mm in some specimens" and rather simple, "apparently lacking any internal detail aside from a central canal". Pterosaur "pelts" found "preserved in concentrated, dense mats of fibers, similar to those found surrounding fossilized mammals" suggest coats with a thickness comparable to many Mesozoic mammals, at least on the parts of the pterosaur covered in pycnofibers. The coat thickness, and surface area covered, definitely varied by pterosaur species.
The presence of pycnofibres (and the demands of flight) imply that pterosaurs were endothermic (warm-blooded). The absence of pycnofibres on pterosaur wings suggests that the coat didn't have an aerodynamic function, lending support to the idea that pycnofibres evolved to aid pterosaur thermoregulation, as is common in warm-blooded animals, insulation being necessary to conserve the heat created by an endothemic metabolism.Pterosaur "hair" was so unique, so obviously distinct from mammalian fur and other animal integuments, it required a new, separate name. The term "pycnofibre", meaning "dense filament", was first coined in a paper on the soft tissue impressions of Jeholopterus by palaeontologist Alexander W.A. Kellner and colleagues in 2009.
Two other Chinese specimens were reported with integumental covering, coming from the same stratum (the Daohugou Bed) as Jeholopterus. So far we have not had the opportunity to examine this material. The first one is a small unnamed anurognathid with extensive preservation of soft tissue, including fibres that have been interpreted as protofeathers (Ji & Yuan 2002). The published pictures show that the soft tissue interpreted as protofeathers is of the same nature as the pycnofibres of Jeholopterus.http://en.wikipedia.org/wiki/Sordes
Sordes was a small basal pterosaur from the Late Jurassic (Oxfordian - Kimmeridgian) Karabastau Svita of Kazakhstan.The genus is based on holotype PIN 2585/3, a crushed relatively complete skeleton on a slab. It was found in the sixties at the foothills of the Karatau in Kazakhstan. The fossil shows remains of the soft parts, such as membranes and hair. This was the first unequivocal proof that pterosaurs had a layer of fur [pycnofibres]. The integument served as insulation, an indication the group was warm-blooded, and provided a streamlined flight profile. The hairlike structures (pycnofibres) are present in two main types: longer at the extreme part of the wing membrane and shorter near the body. In the 1990s, David Unwin argued that both types were essentially not hairs but reinforcing fibres of the flight membranes. Later he emphasized that "hair" in the form of fur was indeed present on the body, after the find of new specimens clearly showing this.
In 1998, the discovery of one specimen assigned to P. kochi shed light on the life appearance of Pterodactylus, as it preserved unique soft-tissue traits not present in previous fossil skeletons, including long, bristly pycnofibres (a fur-like body covering known only in pterosaurs) on the neck, details of an urpatagium (hind wing membrane between the legs and tail) that also stretched between the toes as webbing, and a pelican-like throat pouch. An additional specimen, studied using ultra-violet light, revealed even more information on the soft anatomy of Pterodactylus. This specimen (catalog number JME SOS 4784) showed that like many other pterosaurs, Pterodactylus had a striated soft-tissue crest on the skull. Soft tissue impressions also showed unusually long, sharp, and recurved keratin sheaths on its claws. This specimen was also covered in hair-like pycnofibres, with unusually long pycnofibres covering the back of its neck. The remains of a small, hooked beak were preserved at the tips of the jaws between its upper and lower front teeth.
It is likely that feathers evolved from a conical shaped tubercle rather than a plate-like structure. Although the morphology of the presumably most primitive feather is unknown, minimal conditions for its production include the cellular capacity to synthesize feather proteins (=ϕ-keratin) which provides the molecular phenotype, and a follicular mechanism for production and assembly of molecular and gross structure. Once the minimal structural element, presumably recognizable as a barb, existed, a variety of phenotypes followed rapidly. A tubercular growth center of appropriate size could produce a simple barb-like element, with cortex and medulla. This might be recognized externally as a bristle, but need never existed as a separate morphological unit. Rather, if individual placodes gave rise to multiple barb ridges that fused proximally, a structure resembling natal down would have resulted. Subsequent differentiation is controlled by the follicular symmetry, and the feather shape is regulated by barb length. Barb length is directly related to growth period. As feathers appear to grow at roughly similar, size independent rates, shape is determined by individual barb growth periods. The simple fusion of individual proto-barbs would produce a morphology identifiable as natal down. Although this might be the simplest feather structure, others could emerge quickly, perhaps simultaneously, a consequence of the same redundant processing. Once the machinery existed, broad phenotypic plasticity was possible. I constructed a feather phylogram based on these conditions, the fossil record, and ontogeny. I organized the subsequent changes in morphology by perceived complexity. The changes are simply individual responses to similar processes that might be time (when in ontogeny) and space (where on body) dependent.
Also there were filoplumes:
Feathers are equipped with a variety of sensors which are able to detect both position and movements. There are hair-like feathers (filoplumes) associated with most feathers which play a special role as sensory "hairs". Interestingly the information of these sensors is transmitted directly to the cerebellum of the brain which is very important for the control of locomotion.
There are two basic types of feather: vaned feathers which cover the exterior of the body, and down feathers which are underneath the vaned feathers. The pennaceous feathers are vaned feathers. Also called contour feathers, pennaceous feathers arise from tracts and cover the whole body.A third rarer type of feather, the filoplume, is hairlike and (if present in a bird) grows along the fluffy down feathers. In some passerines, filoplumes arise exposed beyond the contour feathers on the neck.
Filoplumes are always situated beside other feathers. They are simple, hairlike structures that grow in circles around the base of contour or down feathers. They usually stand up like hairs, and are made up of a thin rachis with a few short barbs of barbules at the tip. Filoplumes are generally smaller than semiplumes and are on half to three fourths of the length of the covering contour feathers.http://en.wikipedia.org/wiki/Patagium
The origins of filoplumes is currently under debate. Some ornithologists disagree with the theory that filoplumes are degenerate contour feathers and believe instead that they are sensitive structures that assist in the nerve endings in the follicle. It is therefore quite possible the filoplumes play a key role in keeping contours in place during preening, display, and flight.
Similarly the fleshy pad that houses the follicles of the remiges (primary and secondary feathers) caudal to the hand and the ulna is also often referred to as a patagium.