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As a biological structure feathers serve multiple purposes. They insulate, give aerodynamic stability, have waterproofing properties and are used in inter and intra species communication. They are the most complicated and identifiable integumentary structures in birds. While feathers are the defining feature of Aves (Linnaeus, 1758), their evolutionary origin remains the subject of intense debate (Stettenheim, 2000). The fossilised remains of feathers have been found on several non-avian members of Theropoda (Marsh, 1881). These feathers were of a basic structure with no flight function (Xu et al., 2009). There are many hypotheses as to the original function of feathers, whether they evolved for flight or were co-opted for flight having evolved for an entirely different purpose. What the purpose of the first feather prototypes was is still the subject of much conjecture.
Structure of Feathers
Figure The structure of feathers after PRUM, R. O. & BRUSH, A. H. 2003. Which came first, the feather or the bird? Scientific American, 288, 84-93.Feathers serve several functions and come in many shapes and sizes, from vaned feathers which cover the exterior, and may be symmetrical or asymmetrical, to down feathers which lie beneath the vaned feathers. These two fundamental types of feathers (figure 1, below) are pennaceous feathers which are stiff and fundamental for flight and plumulaceous feathers which are pliable functioning in insulation and crests. They all share the same basic plan: a rachis, barbs which branch off from the rachis and barbules which branch off from the barbs, these barbs interlock forming the vanes in pennaceous feathers while remaining loose in plumulaceous feathers. The rachis expands at its base forming the hollow calamus which inserts into the skin at a follicle (Prum and Brush, 2003). C:\Users\Carin\AppData\Local\Temp\msohtmlclip1\01\clip_image001.png
Evidence and form of early feathers
The non-avian theropod, Sinosauropteryx prima (Ji & Ji, 1996) found in the Jianshangou bed of theÂ Yixian FormationÂ inÂ LiaoningÂ Province,Â China, displays tufts projecting from the skin, these tufts bear a resemblance to how early feathers may have appeared, extending only a few millimetres, hollow without barbules, similar to down ~20mm in length (Currie and Chen, 2001). Beipiaosaurus inexpectusÂ (Xu, Tang & Wang, 1999), also found in the Yixian Formation appear to possess more basal feather forms, consisting of a single filament. These feathers appear only on the tail, neck and head and were ~12cm long (Xu et al., 2009).
Fossils from the Dromaeosauroidea (MatthewÂ &Â Brown,Â 1922), such as, the Sinornithosaurus
(Xu,Â WangÂ &Â Wu,Â 1999), which are more closely related to Aves, show long pinnate tail and wing feathers, which appear to have the same structure as extant birds. Other specimens of dromaeosaurid, such as, Microraptor (XuÂ et al., 2000) possessed pennaceous feathers on their hindlimbs (Figure 2, below) (Clarke and Middleton, 2006, Hone et al., 2010).
File:Microraptor gui holotype under UV light.png
Figure The holotype of Microraptor gui, IVPP V 13352 under UV light. Different filters were employed for parts A and B, hence the difference in colour and appearance. A also is labeled to indicate the preserved feathers (grey arrows) and the 'halo' around the Â specimen where they appear to be absent (black arrows) as well as phosphatised tissues (white arrows). Scale bars are 5 cm in both A and B.
Hone DWE, Tischlinger H, Xu X, Zhang F (2010) The Extent of the Preserved Feathers on the Four-Winged DinosaurÂ Microraptor guiÂ under Ultraviolet Light. PLoS ONE 5(2): e9223. doi:10.1371/journal.pone.0009223.
Origin of feathers
As reptiles are the evolutionary ancestors of birds and extinct and extant reptiles possess a scaled integument, early theories regarding the evolutionary origin of feathers suggested that feathers had developed from scales. That scales extended and developed a fringed edge, which this fringe developing into barbs and eventually hooks and grooves forming. This theory does not account for the difference in structural origin of feathers and scales, feathers emerge from tubular follicles while scales are flat (Prum and Brush, 2002). Feathers and scales both form over thickened epidermal areas (placodes). When a feather forms the feather placode develops on the epidermis, from this a feather bud forms. The feather follicle then forms by the invagination of tissue at the base of the papilla (Prum, 1999).
The evidence from fossils suggests that the evolution of feathers occurred over the course of a progression of intermediary stages (Prum, 1999, Prum and Brush, 2002). The hollow unbranched cylindrical feathers of Beipiaosaurus inexpectus developed from the lengthening of a placode as a tube. This elongation in the form of a tube meant that the feathers could grow without the necessity of the skin itself increasing in size, the growth of scales requires an increase in the area of the skin (Prum, 2005).
Figure Proposed second stage in the evolution of feathers after PRUM, R. O. & BRUSH, A. H. 2003. Which came first, the feather or the bird? Scientific American, 288, 84-93.While both scales and feathers develop from placodes, early in the evolution of feathers the nature of the placode from which feathers develop changed. The formation of follicles from which feathers develop require dermal condensations, the occurrence of these condensations with placodes required genetic changes to occur allowing morphological advancement (Prum, 2005). Following on from the evolution of this initial feather follicle, the formation of barbs from the follicle collar would have led to a feather with barbs extending from the calamus (Figure 3) (Prum and Brush, 2003). Fossils of Sinornithosaurus milleniiÂ (Xu, Wang, & Wu, 1999)Â display feathers which appear structurally similar to this hypothesised developmental stage (Xu et al., 2001).C:\Users\Carin\AppData\Local\Temp\msohtmlclip1\01\clip_image001.png
Feathers preserved in amber (~100Myr) show the barbs partially fused to form the rachis this may represent the next step in feather evolution (Perrichot et al., 2008). Following from the development of the rachis, early feathers would have resembled bird feathers but there would have been no vanes and as such they would not have had a flight or gliding function. This open pennaceous feather was an intermediate stage in the evolution of feathers following this, the evolution of barbules on the barbs is to be expected, and these initial barbules are unlikely to have possessed hooks or grooves. It was the eventual evolution of hooks and grooves on these barbules that led to the closed feather (Prum and Brush, 2003).
Functions of early feathers
The feathers visible on fossils of Beipiaosaurus inexpectusÂ which are comparable to those feathers hypothesised as the first evolutionary step in the evolution of feathers, single filaments ~12 cm in length are unlikely to have served an insulation function given their location on the body, head, neck and tail. Xu et al., (2009) suggest rather that they functioned in communication.
Asymmetrical wings are required for flight, Caudipteryx zouiÂ (JiÂ et al., 1998), had symmetrical closed pennaceous feathers, on its tail and forearms, its arms were moderately short precluding the possibility that they were flyers. Clarke and Middleton, (2006), have suggested that this indicate that they used their 'wings' in wing assisted incline running (WAIR) which is one of the hypotheses on the origin of flight. This WAIR would have increased hind limb grip on inclines.
Recent fossil findings have allowed researchers to reassess the various early theories about the origin of feathers. New developmental biology evidence casts doubt on the theory that elongate scales evolved into feathers. Under this theory, scales first elongated, then grew fringed edges, and ultimately producing hooked and grooved barbules evolved into feathers. But feathers are tubes, the two sides of the vane are created by the inside and outside of the tube. In contrast, the two sides of a scale grow from the top and bottom of the original epidermal outgrowth that forms the scale.
The new evidence also casts doubt on the theory that feathers evolved originally for flight. Only extremely evolved feather shapes, the asymmetrical feather with closed vanes, which occurred late in evolution, could have been used for flight. It appears that feathers took on an aerodynamic function only after the evolution of significant developmental and structural complexity. That is, they evolved for an initial purpose and were later used for a different purpose.
There are many other possible early functions of feathers which remain possible, including insulation, water repellence, courtship, camouflage and defence. While there is a profusion of new paleontological data, it appears unlikely that adequate insight will ever exist into the biology of the specific ancestry in which feathers evolved to make a distinction among these theories. Rather the theory that feathers evolved by a series of developmental improvements, each of which may have evolved for a different original function appears most plausible. It does appear that feathers appeared only after a tubular feather germ and follicle originated in the dermis of some species. The survival of this first tubular appendage and the resultant evolution of the first feathers was because it conferred a survival advantage.
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