Wikipedia

Feather

For other uses, seeFeather (disambiguation).

Feathersareepidermalgrowths that form a distinctive outer covering, orplumage,on bothavian(bird) and some non-aviandinosaursand otherarchosaurs.They are the most complexintegumentarystructures found invertebrates[1][2]and an example of a complex evolutionary novelty.[3]They are among the characteristics that distinguish the extantbirdsfrom other living groups.[4]

Feather variations

Although feathers cover most of the bird's body, they arise only from certain well-defined tracts on the skin. They aid in flight, thermal insulation, and waterproofing. In addition, coloration helps in communication andprotection.[5]The study of feathers is calledplumology(orplumage science).[6][7]

People use feathers in many ways that are practical, cultural, and religious. Feathers are both soft and excellent at trappingheat;thus, they are sometimes used in high-classbedding,especiallypillows,blankets,andmattresses.They are also used as filling for winterclothingand outdoor bedding, such as quiltedcoatsandsleeping bags.Gooseandeiderdownhave greatloft,the ability to expand from a compressed, stored state to trap large amounts of compartmentalized, insulating air.[8]Feathers of large birds (most oftengeese) have been and are used to makequillpens. Historically, thehunting of birds for decorative and ornamental feathershas endangered some species and helped to contribute to the extinction of others.[9]Today, feathers used in fashion and in military headdresses and clothes are obtained as a waste product of poultry farming, includingchickens,geese,turkeys,pheasants,andostriches.These feathers are dyed and manipulated to enhance their appearance, as poultry feathers are naturally often dull in appearance compared to the feathers of wild birds.

Etymology

Parts of a feather:
  1. Vane
  2. Shaft, rachis
  3. Barb
  4. Aftershaft, afterfeather
  5. Quill, calamus

Feather derives from theOld English"feþer", which is ofGermanicorigin; related to Dutch "veer" and German "Feder", from an Indo-European root shared by Sanskrit's "patra" meaning 'wing', Latin's "penna" meaning 'feather', and Greek's "pteron", "pterux" meaning 'wing'.

Because of feathers being an integral part ofquills,which were early pens used for writing, the wordpenitself is derived from theLatinpenna,meaning feather.[10]The French wordplumecan meanfeather,quill,orpen.

Structures and characteristics

Feather microstructure showing interlocking barbules
Anterior region of a contour feather from arock dove,visualized usingscanning electron microscopy.Interlocking barbules are clearly seen in the middle image.

Feathers are among the most complexintegumentaryappendagesfound invertebratesand are formed in tiny follicles in theepidermis,or outer skin layer, that producekeratinproteins.Theβ-keratinsin feathers,beaksandclaws– and the claws,scalesandshellsofreptiles– are composed of protein strandshydrogen-bondedintoβ-pleated sheets,which are then further twisted andcrosslinkedbydisulfidebridges into structures even tougher than theα-keratinsof mammalianhair,hornsandhooves.[11][12]The exactsignalsthat induce the growth of feathers on the skin are not known, but it has been found that the transcription factor cDermo-1 induces the growth of feathers on skin and scales on the leg.[13]

Classification

See also:Pennaceous featherandDown feather
Filoplumes can be important in nuptial display; in thegreat cormorantthey are white, and produced on the head and upper neck at the start of the breeding season, and shed soon after nesting.
Feather structure of ablue-and-yellow macaw

There are two basic types of feather: vaned feathers which cover the exterior of the body, anddown featherswhich are underneath the vaned feathers. Thepennaceous feathersare vaned feathers. Also called contour feathers, pennaceous feathers arise from tracts and cover the entire body. A third rarer type of feather, thefiloplume,is hairlike and are closely associated with pennaceous feathers and are often entirely hidden by them, with one or two filoplumes attached and sprouting from near the same point of the skin as each pennaceous feather, at least on a bird's head, neck and trunk.[14][15]Filoplumes are entirely absent inratites.[16]In some passerines, filoplumes arise exposed beyond the pennaceous feathers on the neck.[1]The remiges, orflight feathersof the wing, and rectrices, or flight feathers of the tail, are the most important feathers for flight. A typical vaned feather features a main shaft, called therachis.Fused to the rachis are a series of branches, orbarbs;the barbs themselves are also branched and form thebarbules.These barbules have minute hooks calledbarbicelsfor cross-attachment. Down feathers are fluffy because they lack barbicels, so the barbules float free of each other, allowing the down to trap air and provide excellent thermal insulation. At the base of the feather, the rachis expands to form the hollow tubularcalamus(orquill) which inserts into afolliclein theskin.The basal part of the calamus is without vanes. This part is embedded within the skin follicle and has an opening at the base (proximal umbilicus) and a small opening on the side (distal umbilicus).[17]

Hatchling birds of some species have a special kind of natal down feathers (neossoptiles) which are pushed out when the normal feathers (teleoptiles) emerge.[1]

Flight feathers are stiffened so as to work against the air in the downstroke but yield in other directions. It has been observed that the orientation pattern of β-keratin fibers in the feathers of flying birds differs from that in flightless birds: the fibers are better aligned along the shaft axis direction towards the tip,[18][19]and the lateral walls of rachis region show structure of crossed fibers.[20][21]

Functions

Feathers insulate birds from water and cold temperatures. They may also be plucked to line the nest and provide insulation to the eggs and young. The individual feathers in the wings and tail play important roles in controlling flight.[20]Some species have acrestof feathers on their heads. Although feathers are light, a bird's plumage weighs two or three times more than its skeleton, since many bones are hollow and contain air sacs. Color patterns serve ascamouflageagainstpredatorsfor birds in their habitats, and serve as camouflage for predators looking for a meal. As with fish, the top and bottom colors may be different, in order to provide camouflage during flight. Striking differences in feather patterns and colors are part of thesexual dimorphismof many bird species and are particularly important in the selection of mating pairs. In some cases, there are differences in the UV reflectivity of feathers across sexes even though no differences in color are noted in the visible range.[22]The wing feathers of maleclub-winged manakinsMachaeropterus deliciosushave special structures that are used to produce sounds bystridulation.[23]

A contour feather from aGuinea fowl

Some birds have a supply ofpowder downfeathers that grow continuously, with small particles regularly breaking off from the ends of the barbules. These particles produce apowderthat sifts through the feathers on the bird's body and acts as a waterproofing agent and a featherconditioner.Powder down has evolved independently in several taxa and can be found in down as well as in pennaceous feathers. They may be scattered in plumage as in the pigeons and parrots or in localized patches on the breast, belly, or flanks, as in herons and frogmouths. Herons use their bill to break the powder down feathers and to spread them, while cockatoos may use their head as a powder puff to apply the powder.[24]Waterproofing can be lost by exposure toemulsifying agentsdue to human pollution. Feathers can then become waterlogged, causing the bird to sink. It is also very difficult to clean and rescue birds whose feathers have been fouled byoil spills.The feathers of cormorants soak up water and help to reduce buoyancy, thereby allowing the birds to swim submerged.[25]

Rictal bristles of awhite-cheeked barbet

Bristlesare stiff, tapering feathers with a large rachis but few barbs.Rictal bristlesare found around the eyes and bill. They may serve a similar purpose toeyelashesandvibrissaeinmammals.Although there is as yet no clear evidence, it has been suggested that rictal bristles have sensory functions and may help insectivorous birds to capture prey.[26]In one study, willow flycatchers (Empidonax traillii) were found to catch insects equally well before and after removal of the rictal bristles.[27]

Grebesare peculiar in their habit of ingesting their own feathers and feeding them to their young. Observations on their diet of fish and the frequency of feather eating suggest that ingesting feathers, particularly down from their flanks, aids in forming easily ejectable pellets.[28]

Distribution

Feather tracts or pterylae and their naming

Contour feathers are not uniformly distributed on the skin of the bird except in some groups such as thepenguins,ratites and screamers.[29]In most birds the feathers grow from specific tracts of skin calledpterylae;between the pterylae there are regions which are free of feathers calledapterylae(orapteria). Filoplumes and down may arise from the apterylae. The arrangement of these feather tracts, pterylosis or pterylography, varies across bird families and has been used in the past as a means for determining the evolutionary relationships of bird families.[30][31]Species that incubate their own eggs often lose their feathers on a region of their belly, forming abrooding patch.[32]

Coloration

Colors resulting from different feather pigments
Left:turacin(red) andturacoverdin(green, with some structural blueiridescenceat lower end) on the wing ofTauraco bannermani
Right:carotenoids(red) andmelanins(dark) on belly/wings ofRamphocelus bresilius

The colors of feathers are produced by pigments, by microscopic structures that canrefract,reflect, or scatter selected wavelengths of light, or by a combination of both.

Most feather pigments aremelanins(brown and beigepheomelanins,black and greyeumelanins) andcarotenoids(red, yellow, orange); other pigments occur only in certaintaxa– the yellow to redpsittacofulvins[33](found in someparrots) and the redturacinand greenturacoverdin(porphyrinpigments found only inturacos).

Structural coloration[5][34][35]is involved in the production of blue colors,iridescence,mostultravioletreflectance and in the enhancement of pigmentary colors. Structural iridescence has been reported[36]in fossil feathers dating back 40 million years. White feathers lack pigment and scatter light diffusely;albinism in birdsis caused by defective pigment production, though structural coloration will not be affected (as can be seen, for example, in blue-and-whitebudgerigars).

A feather with no pigment

The blues and bright greens of manyparrotsare produced by constructive interference of light reflecting from different layers of structures in feathers. In the case of green plumage, in addition to yellow, the specific feather structure involved is called by some the Dyck texture.[37][38]Melanin is often involved in the absorption of light; in combination with a yellow pigment, it produces a dull olive-green.

EmperorPedro II of Brazilwearing a wide collar of orange toucan feathers around his shoulders and elements of theImperial Regalia.Detail from a painting byPedro Américo

In some birds, feather colors may be created, or altered, by secretions from theuropygial gland,also called the preen gland. The yellow bill colors of many hornbills are produced by such secretions. It has been suggested that there are other color differences that may be visible only in the ultraviolet region,[24]but studies have failed to find evidence.[39]The oil secretion from the uropygial gland may also have an inhibitory effect on feather bacteria.[40]

The reds, orange and yellow colors of many feathers are caused by various carotenoids. Carotenoid-based pigments might be honest signals of fitness because they are derived from special diets and hence might be difficult to obtain,[41][42]and/or because carotenoids are required for immune function and hence sexual displays come at the expense of health.[43]

A bird's feathers undergo wear and tear and are replaced periodically during the bird's life throughmolting.New feathers, known when developing asblood, or pin feathers,depending on the stage of growth, are formed through the same follicles from which the old ones were fledged. The presence of melanin in feathers increases their resistance to abrasion.[44]One study notes that melanin based feathers were observed to degrade more quickly under bacterial action, even compared to unpigmented feathers from the same species, than those unpigmented or with carotenoid pigments.[45]However, another study the same year compared the action of bacteria on pigmentations of two song sparrow species and observed that the darker pigmented feathers were more resistant; the authors cited other research also published in 2004 that stated increased melanin provided greater resistance. They observed that the greater resistance of the darker birds confirmedGloger's rule.[46]

Although sexual selection plays a major role in the development of feathers, in particular, the color of the feathers it is not the only conclusion available. New studies are suggesting that the unique feathers of birds are also a large influence on many important aspects of avian behavior, such as the height at which different species build their nests. Since females are the prime caregivers, evolution has helped select females to display duller colors down so that they may blend into the nesting environment. The position of the nest and whether it has a greater chance of being under predation has exerted constraints on female birds' plumage.[47]A species of bird that nests on the ground, rather than the canopy of the trees, will need to have much duller colors in order not to attract attention to the nest. The height study found that birds that nest in the canopies of trees often have many more predator attacks due to the brighter color of feathers that the female displays.[47]Another influence of evolution that could play a part in why feathers of birds are so colorful and display so many patterns could be due to that birds developed their bright colors from the vegetation and flowers that thrive around them. Birds develop their bright colors from living around certain colors. Most bird species often blend into their environment, due to some degree of camouflage, so if the species habitat is full of colors and patterns, the species would eventually evolve to blend in to avoid being eaten. Birds' feathers show a large range of colors, even exceeding the variety of many plants, leaf, and flower colors.[48]

Parasites

The feather surface is the home for some ectoparasites, notably feather lice (Phthiraptera) and feather mites. Feather lice typically live on a single host and can move only from parents to chicks, between mating birds, and, occasionally, byphoresy.This life history has resulted in most of the parasite species being specific to the host and coevolving with the host, making them of interest in phylogenetic studies.[49]

Feather holesare chewing traces of lice (most probablyBrueeliaspp. lice) on the wing and tail feathers. They were described onbarn swallows,and because of easy countability, many evolutionary, ecological, and behavioral publications use them to quantify the intensity of infestation.

Parasitic cuckoos which grow up in the nests of other species also have host-specific feather lice and these seem to be transmitted only after the young cuckoos leave the host nest.[50]

Birds maintain their feather condition bypreeningand bathing in water ordust.It has been suggested that a peculiar behavior of birds,anting,in which ants are introduced into the plumage, helps to reduce parasites, but no supporting evidence has been found.[51]

Human usage

Utilitarian

Pomofully feathered basketcurated at the Jesse Peter Multicultural Museum, Santa Rosa College
Female model with feathers

Bird feathers have long been used forfletchingarrows.Colorful feathers such as those belonging topheasantshave been used to decoratefishing lures.

Feathers are also valuable in aiding the identification of species in forensic studies, particularly inbird strikesto aircraft. The ratios of hydrogen isotopes in feathers help in determining the geographic origins of birds.[52]Feathers may also be useful in the non-destructive sampling of pollutants.[53]

The poultry industry produces a large amount of feathers as waste, which, like other forms of keratin, are slow to decompose. Feather waste has been used in a number of industrial applications as a medium for culturing microbes,[54]biodegradable polymers,[55]and production of enzymes.[56]Feather proteins have been tried as an adhesive for wood board.[57]

Some groups of Native people in Alaska have used ptarmigan feathers as temper (non-plastic additives) in pottery manufacture since the first millennium BC in order to promote thermal shock resistance and strength.[58]

In religion and culture

Mexican featherworkpainting ofIsidore the Laborermade from duck, hummingbird, and canary feathers. This style of painting, popular during theNovohispanicera, integrates featherwork of pre-Hispanic origin with Christian iconography. 18th century,Museo Soumaya

Eaglefeathers have great cultural and spiritual value toNative Americansin theUnited StatesandFirst Nationspeoples inCanadaas religious objects. In the United States, the religious use of eagle andhawkfeathers is governed by theeagle feather law,a federal law limiting the possession of eagle feathers to certified and enrolled members of federally recognized Native American tribes.

In South America, brews made from the feathers ofcondorsare used in traditional medications.[59]In India, feathers of theIndian peacockhave been used in traditional medicine for snakebite, infertility, and coughs.[60][61]

Members of Scotland'sClan Campbellare known to wear feathers on their bonnets to signify authority within the clan.Clan chiefswear three, chieftains wear two and anarmigerwears one. Any member of the clan who does not meet the criteria is not authorized to wear feathers as part of traditional garb and doing so is considered presumptuous.[62]

During the 18th, 19th, and early 20th centuries, there was a booming international trade in plumes for extravagant women's hats and other headgear (including inVictorian fashion).Frank Chapmannoted in 1886 that feathers of as many as 40 species of birds were used in about three-fourths of the 700 ladies' hats that he observed in New York City.[63]For instance, South Americanhummingbirdfeathers were used in the past to dress some of the miniature birds featured insinging bird boxes.This trade caused severe losses to bird populations (for example,egretsandwhooping cranes). Conservationists led a major campaign against the use of feathers in hats. This contributed to passage of theLacey Actin 1900, and to changes in fashion. The ornamental feather market then largely collapsed.[64][65]

More recently, rooster plumage has become a popular trend as ahairstyleaccessory, with feathers formerly used as fishing lures now being used to provide color and style to hair.[66]

Feather products manufacturing in Europe has declined in the last 60 years, mainly due to competition from Asia. Feathers have adorned hats at many prestigious events such as weddings and Ladies Day at racecourses (Royal Ascot).

Evolution

Main article:Origin of avian flight
Late Jurassicfossil feather of an unidentified dinosaur, once thought to beArchaeopteryx.
Rachis-dominated feathers inside mid-CretaceousBurmese amber

Functional considerations

The functional view on the evolution of feathers has traditionally focused on insulation, flight and display. Discoveries of non-flying Late Cretaceous feathered dinosaurs in China,[67]however, suggest that flight could not have been the original primary function as the feathers simply would not have been capable of providing any form of lift.[68][69]There have been suggestions that feathers may have had their original function in thermoregulation, waterproofing, or even as sinks for metabolic wastes such as sulphur.[70]Recent discoveriesare argued to support a thermoregulatory function, at least in smaller dinosaurs.[71][72]Some researchers even argue that thermoregulation arose from bristles on the face that were used as tactile sensors.[73]While feathers have been suggested as having evolved from reptilianscales,there are numerous objections to that idea, and more recent explanations have arisen from the paradigm ofevolutionary developmental biology.[2]Theories of the scale-based origins of feathers suggest that the planar scale structure was modified for development into feathers by splitting to form the webbing; however, that developmental process involves a tubular structure arising from a follicle and the tube splitting longitudinally to form the webbing.[1][2]The number of feathers per unit area of skin is higher in smaller birds than in larger birds, and this trend points to their important role in thermal insulation, since smaller birds lose more heat due to the relatively larger surface area in proportion to their body weight.[5]The miniaturization of birds also played a role in the evolution of powered flight.[74]The coloration of feathers is believed to have evolved primarily in response tosexual selection.In fossil specimens of theparavianAnchiornis huxleyiand thepterosaurTupandactylus imperator,the features are so well preserved that themelanosome(pigment cells) structure can be observed. By comparing the shape of the fossil melanosomes to melanosomes from extant birds, the color and pattern of the feathers onAnchiornisandTupandactyluscould be determined.[75][76]Anchiorniswas found to have black-and-white-patterned feathers on the forelimbs and hindlimbs, with a reddish-brown crest. This pattern is similar to the coloration of many extant bird species, which use plumage coloration for display and communication, including sexual selection and camouflage. It is likely that non-avian dinosaur species utilized plumage patterns for similar functions as modern birds before the origin of flight. In many cases, the physiological condition of the birds (especially males) is indicated by the quality of their feathers, and this is used (by the females) inmate choice.[77][78]Additionally, when comparing differentOrnithomimus edmontonicusspecimens, older individuals were found to have a pennibrachium (a wing-like structure consisting of elongate feathers), while younger ones did not. This suggests that the pennibrachium was a secondary sex characteristic and likely had a sexual function.[79]

Molecular evolution

Severalgeneshave been found to determine feather development. They will be key to understand the evolution of feathers. For instance, some genes convert scales into feathers or feather-like structures when expressed or induced in bird feet, such as the scale-feather convertersSox2,Zic1,Grem1,Spry2,andSox18.[80]

Feathers and scales are made up of two distinct forms ofkeratin,and it was long thought that each type of keratin was exclusive to each skin structure (feathers and scales). However, feather keratin is also present in the early stages of development ofAmerican alligatorscales. This type of keratin, previously thought to be specific to feathers, is suppressed during embryological development of the alligator and so is not present in the scales of mature alligators. The presence of thishomologouskeratin in both birds andcrocodiliansindicates that it was inherited from a common ancestor.[81]

This may suggest that crocodilian scales, bird and dinosaur feathers, and pterosaurpycnofibresare all developmental expressions of the same primitive archosaur skin structures; suggesting that feathers and pycnofibers could be homologous.[82]Molecular dating methods in 2011 show that the subfamily of feather β-keratins found in extant birds started to diverge 143 million years ago, suggesting the pennaceous feathers ofAnchiorniswere not made of the feather β-keratins present in extant birds.[83]However, a study of fossil feathers from the dinosaur Sinosauropteryx and other fossils revealed traces of beta-sheet proteins, using infrared spectroscopy and sulfur-X-ray spectroscopy. The presence of abundant Alpha -proteins in some fossil feathers was shown to be an artefact of the fossilization process, as beta-protein structures are readily altered to Alpha -helices during thermal degradation.[84]In 2019, scientists found that genes for the production of feathers evolved at the base of archosauria, supporting that feathers were present at early ornithodirans and is consistent with the fossil record.[85]

Feathered dinosaurs

Main article:Feathered dinosaurs
Archaeopteryx lithographica(Berlin specimen)

Several non-aviandinosaurshad feathers on their limbs that would not have functioned for flight.[67][2]One theory suggests that feathers originally evolved on dinosaurs due to theirinsulationproperties; then, small dinosaur species which grew longer feathers may have found them helpful in gliding, leading to the evolution of proto-birds likeArchaeopteryxandMicroraptorzhaoianus.Another theory posits that the original adaptive advantage of early feathers was their pigmentation or iridescence, contributing to sexual preference in mate selection.[86]Dinosaurs that had feathers or protofeathers includePedopennadaohugouensis[87]andDilong paradoxus,atyrannosauroidwhich is 60 to 70 million years older thanTyrannosaurus rex.[88]

The majority of dinosaurs known to have had feathers or protofeathers aretheropods,however featherlike "filamentous integumentary structures" are also known from theornithischiandinosaursTianyulongandPsittacosaurus.[89]The exact nature of these structures is still under study. However, it is believed that the stage-1 feathers (seeEvolutionary stagessection below) such as those seen in these two ornithischians likely functioned in display.[90]In 2014, the ornithischianKulindadromeuswas reported as having structures resembling stage-3 feathers.[91]The likelihood of scales evolving on early dinosaur ancestors are high. However, this was by assuming that primitive pterosaurs were scaly.[92][93]A 2016 study analyzes the pulp morphology of the tail bristles ofPsittacosaurusand finds they are similar to feathers but notes that they are also similar to the bristles on the head of theCongo peafowl,the beard of theturkey,and the spine on the head of thehorned screamer.[94]A reestimation of maximum likelihoods by paleontologistThomas Holtzfinds that filaments were more likely to be the ancestral state of dinosaurs.[95]

In 2010, acarcharodontosauridnamedConcavenator corcovatuswas found to haveremigeson the ulna suggesting it might have had quill-like structures on the ams.[96]However, Foth et al. 2014 disagress with the publication where they point out that the bumps on the ulna ofConcavenatorare on theanterolateralwhich is unlike remiges which are in aposterolateralon the ulna of some birds, they consider it more likely that these are attachments for interosseous ligaments.[97]This was refuted by Cuesta Fidalgo and her colleagues, they pointed out that these bumps on the ulna are posterolateral which is unlike that of interosseous ligaments.[98]

Since the 1990s, dozens of feathered dinosaurs have been discovered in the cladeManiraptora,which includes the clade Avialae and the recent common ancestors of birds,OviraptorosauriaandDeinonychosauria.In 1998, the discovery of a feathered oviraptorosaurian,Caudipteryx zoui,challenged the notion of feathers as a structure exclusive to Avialae.[99]Buried in the Yixian Formation in Liaoning, China,C. zouilived during the Early Cretaceous Period. Present on the forelimbs and tails, their integumentary structure has been accepted[by whom?]as pennaceous vaned feathers based on the rachis and herringbone pattern of the barbs. In the clade Deinonychosauria, the continued divergence of feathers is also apparent in the familiesTroodontidaeandDromaeosauridae.Branched feathers with rachis, barbs, and barbules were discovered in many members includingSinornithosaurus millenii,a dromaeosaurid found in the Yixian formation (124.6 MYA).[100]

Previously, a temporal paradox existed in the evolution of feathers—theropods with highly derived bird-like characteristics occurred at a later time thanArchaeopteryx—suggesting that the descendants of birds arose before the ancestor. However, the discovery ofAnchiornis huxleyiin the Late Jurassic Tiaojishan Formation (160 MYA) in western Liaoning in 2009[101][102] resolved this paradox. By predatingArchaeopteryx,Anchiornisproves the existence of a modernly feathered theropod ancestor, providing insight into the dinosaur-bird transition. The specimen shows distribution of large pennaceous feathers on the forelimbs and tail, implying that pennaceous feathers spread to the rest of the body at an earlier stage in theropod evolution.[103]The development of pennaceous feathers did not replace earlier filamentous feathers. Filamentous feathers are preserved alongside modern-looking flight feathers – including some with modifications found in the feathers of extant diving birds – in 80 million year old amber from Alberta.[104]

Two small wings trapped in amber dating to 100 mya showplumageexisted in some bird predecessors. The wings most probably belonged toenantiornithes,a diverse group of avian dinosaurs.[105][106]

A largephylogeneticanalysis of early dinosaurs by Matthew Baron,David B. Normanand Paul Barrett (2017) found thatTheropodais actually more closely related toOrnithischia,to which it formed thesister groupwithin thecladeOrnithoscelida.The study also suggested that if the feather-like structures of theropods and ornithischians are of common evolutionary origin then it would be possible that feathers were restricted to Ornithoscelida. If so, then the origin of feathers would have likely occurred as early as theMiddle Triassic,[107]though this has been disagreed upon.[108][109]The lack of feathers present in large sauropods and ankylosaurs could be that feathers were suppressed by genomic regulators.[110]

Evolutionary stages

Diagram illustrating stages of evolution

Several studies of feather development in the embryos of modern birds, coupled with the distribution of feather types among various prehistoric bird precursors, have allowed scientists to attempt a reconstruction of the sequence in which feathers first evolved and developed into the types found on modern birds.

Feather evolution was broken down into the following stages by Xu and Guo in 2009:[90]

  1. Single filament
  2. Multiple filaments joined at their base
  3. Multiple filaments joined at their base to a central filament
  4. Multiple filaments along the length of a central filament
  5. Multiple filaments arising from the edge of a membranous structure
  6. Pennaceous feather with vane of barbs and barbules and central rachis
  7. Pennaceous feather with an asymmetrical rachis
  8. Undifferentiated vane with central rachis

However, Foth (2011) showed that some of these purported stages (stages 2 and 5 in particular) are likely simply artifacts of preservation caused by the way fossil feathers are crushed and the feather remains or imprints are preserved. Foth re-interpreted stage 2 feathers as crushed or misidentified feathers of at least stage 3, and stage 5 feathers as crushed stage 6 feathers.[111]

The following simplified diagram of dinosaur relationships follows these results, and shows the likely distribution of plumaceous (downy) and pennaceous (vaned) feathers among dinosaurs and prehistoric birds. The diagram follows one presented by Xu and Guo (2009)[90]modified with the findings of Foth (2011)[111]The numbers accompanying each name refer to the presence of specific feather stages. Note that 's' indicates the known presence of scales on the body.

Dinosauria

In pterosaurs

Pterosaurswere long known to have filamentous fur-like structures covering their body known aspycnofibres,which were generally considered distinct from the "true feathers" of birds and their dinosaur kin. However, a 2018 study of two small, well-preserved pterosaur fossils from theJurassicofInner Mongolia,Chinaindicated that pterosaurs were covered in an array of differently-structured pycnofibres (rather than just filamentous ones), with several of these structures displaying diagnostic features of feathers, such as non-veined grouped filaments and bilaterally branched filaments, both of which were originally thought to be exclusive to birds and other maniraptoran dinosaurs. Given these findings, it is possible that feathers have deep evolutionary origins in ancestralarchosaurs,though there is also a possibility that these structures independently evolved to resemble bird feathers viaconvergent evolution.[112]Mike Benton, the study's senior author, lent credence to the former theory, stating "We couldn't find any anatomical evidence that the four pycnofiber types are in any way different from the feathers of birds and dinosaurs. Therefore, because they are the same, they must share an evolutionary origin, and that was about 250 million years ago, long before the origin of birds."[113][114][115][116]But the integumentary structures of theanurognathidspecimens is still based gross morphology as Liliana D'Alba pointed out. The pycnofibres of the two anurognathid specimens might not be homologous with the filamentous appendages on dinosaurs.[117]Paul M. Barrett suspects that during the integumentary evolution of pterosaurs, pterosaurs primitively lost scales and pycnofibers started to appear.[93]

Cascocaudawas almost entirely covered in an extensive coat of pycnofibres, which appear to have come in two types. The first are simple, curved filaments that range in length from 3.5–12.8 mm long. These filaments cover most of the animal, including the head, neck, body, limbs and tail. The second type consists of tufts of filaments joined near the base, similar to the branchingdown feathersof birds and othercoelurosauriandinosaurs, around 2.5–8.0 mm long and only cover the wing membranes. Studies of sampled pycnofibres revealed the presence of microbodies within the filaments, resembling themelanosomepigments identified in other fossil integuments, specifically phaeomelanosomes. Furthermore,infrared spectral analysisof these pycnofibres show similarabsorption spectrato redhuman hair.These pycnofibres likely provided both insulation and may have helped streamline the body and wings during flight.[118] The identity of these branching structures as pycnofibres or feathers was challenged by Unwin & Martill (2020), who interpreted them as bunched-up and degraded aktinofibrils–stiffening fibres found in the wing membrane of pterosaurs–and attributed the melanosomes and keratin to skin rather than filaments.[119]These claims were refuted by Yang and colleagues, who argue that Unwin and Martill's interpretations are inconsistent with the specimen's preservation. Namely, they argue that the consistent structure, regular spacing, and extension of the filaments beyond the wing membrane support their identification as pycnofibres. Further, they argue that the restriction of melanosomes and keratin to the fibres, as occurs in fossil dinosaur feathers, supports the case they are filaments and is not consistent with contamination from preserved skin.[120]Protofeathers likely evolved in early archosaurs, not long after the P-T extinction event during the time metabolic rates of early archosaurs and synapsids were increasing, postures becoming erect, and sustained activity.[85]

See also

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