The realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the mid-1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers. Since then, more than twenty genera of dinosaurs, mostly theropods, have been discovered to have been feathered. Most fossils are from the Yixian formation in China. The fossil feathers of one specimen, Shuvuuia deserti, have tested positive for beta-keratin, the main protein in bird feathers, in immunological tests.
Early hypotheses 
Shortly after the 1859 publication of Charles Darwin's On the Origin of Species, British biologist Thomas Henry Huxley proposed that birds were descendants of dinosaurs. He compared the skeletal structure of Compsognathus, a small theropod dinosaur, and the 'first bird' Archaeopteryx lithographica (both of which were found in the Upper Jurassic Bavarian limestone of Solnhofen). He showed that, apart from its hands and feathers, Archaeopteryx was quite similar to Compsognathus. In 1868 he published On the Animals which are most nearly intermediate between Birds and Reptiles, making the case. The leading dinosaur expert of the time, Richard Owen, disagreed, claiming Archaeopteryx as the first bird outside dinosaur lineage. For the next century, claims that birds were dinosaur descendants faded, with more popular bird-ancestry hypotheses including 'crocodylomorph' and 'thecodont' ancestors, rather than dinosaurs or other archosaurs.
In 1964, John Ostrom described Deinonychus antirrhopus, a theropod whose skeletal resemblance to birds seemed unmistakable. Ostrom became a leading proponent of the theory that birds are direct descendants of dinosaurs. Further comparisons of bird and dinosaur skeletons, as well as cladistic analysis strengthened the case for the link, particularly for a branch of theropods called maniraptors. Skeletal similarities include the neck, the pubis, the wrists (semi-lunate carpal), the 'arms' and pectoral girdle, the shoulder blade, the clavicle and the breast bone. In all, over a hundred distinct anatomical features are shared by birds and theropod dinosaurs.
Other researchers drew on these shared features and other aspects of dinosaur biology and began to suggest that at least some theropod dinosaurs were feathered. The first restoration of a feathered dinosaur was Sarah Landry's depiction of a feathered "Syntarsus" (now renamed Megapnosaurus or considered a synonym of Coelophysis), in Robert T. Bakker's 1975 publication Dinosaur Renaissance. Gregory S. Paul was probably the first paleoartist to depict maniraptoran dinosaurs with feathers and protofeathers, starting in the late 1970s.
By the 1990s, most paleontologists considered birds to be surviving dinosaurs and referred to 'non-avialan dinosaurs' (all extinct), to distinguish them from birds (Avialae). Before the discovery of feathered dinosaurs, the evidence was limited to Huxley and Ostrom's comparative anatomy. Some mainstream ornithologists, including Smithsonian Institution curator Storrs L. Olson, disputed the links, specifically citing the lack of fossil evidence for feathered dinosaurs.
Fossil evidence 
After a century of hypotheses without conclusive evidence, well-preserved fossils of feathered dinosaurs were discovered during the 1990s, and more continue to be found. The fossils were preserved in a Lagerstätte — a sedimentary deposit exhibiting remarkable richness and completeness in its fossils — in Liaoning, China. The area had repeatedly been smothered in volcanic ash produced by eruptions in Inner Mongolia 124 million years ago, during the Early Cretaceous Period. The fine-grained ash preserved the living organisms that it buried in fine detail. The area was teeming with life, with millions of leaves, angiosperms (the oldest known), insects, fish, frogs, salamanders, mammals, turtles, and lizards discovered to date.
The most important discoveries at Liaoning have been a host of feathered dinosaur fossils, with a steady stream of new finds filling in the picture of the dinosaur–bird connection and adding more to theories of the evolutionary development of feathers and flight. Norell et al. (2007) reported quill knobs from an ulna of Velociraptor mongoliensis, and these are strongly correlated with large and well-developed secondary feathers.
Behavioural evidence, in the form of an oviraptorosaur on its nest, showed another link with birds. Its forearms were folded, like those of a bird. Although no feathers were preserved, it is likely that these would have been present to insulate eggs and juveniles.
Genuine feathers? 
There have been claims that the supposed feathers of the Chinese fossils were a preservation artifact. Despite doubts, the fossil feathers have roughly the same appearance as those of birds fossilized in the same locality, so there is no serious reason to think they are of different nature; moreover, no non-theropod fossil from the same site shows such an artifact, but sometimes show unambiguous hair (some mammals) or scales (some reptiles).
The "Archaeoraptor" fake 
In 1999, a supposed 'missing link' fossil of an apparently feathered dinosaur named "Archaeoraptor liaoningensis", found in Liaoning Province, northeastern China, turned out to be a forgery. Comparing the photograph of the specimen with another find, Chinese paleontologist Xu Xing came to the conclusion that it was composed of two portions of different fossil animals. His claim made National Geographic review their research and they too came to the same conclusion. The bottom portion of the "Archaeoraptor" composite came from a legitimate feathered dromaeosaurid now known as Microraptor, and the upper portion from a previously-known primitive bird called Yanornis.
Evidence in amber 
In 2011, samples of amber were discovered to contain preserved feathers from 75 to 80 million years ago during the Cretaceous era, with evidence that they were from both dinosaurs and birds. Initial analysis suggests that some of the feathers were used for insulation, and not flight. More complex feathers were revealed to have variations in coloration similar to modern birds, while simpler protofeathers were predominantly dark. Only 11 specimens are currently known. The specimens are too rare to be broken open to study their melanosomes, but there are plans for using non-destructive high-resolution X-ray imaging.
Current knowledge 
List of dinosaur species preserved with evidence of feathers 
A number of non-avialan dinosaurs are now known to have been feathered. Direct evidence of feathers exists for the following species, listed in the order currently accepted evidence was first published. In all examples, the evidence described consists of feather impressions, except those genera inferred to have had feathers based on skeletal or chemical evidence, such as the presence of quill knobs (the anchor points for wing feathers on the forelimb) or a pygostyle (the fused vertebrae at the tail tip which often supports large feathers).
- Avimimus portentosus (inferred 1987: quill knobs)
- Pelecanimimus polydon? (1994)
- Sinosauropteryx prima (1996)
- Protarchaeopteryx robusta (1997)
- GMV 2124 (1997)
- Caudipteryx zoui (1998)
- Rahonavis ostromi (inferred 1998: quill knobs; possibly avialan)
- Shuvuuia deserti (1999)
- Sinornithosaurus millenii (1999)
- Beipiaosaurus inexpectus (1999)
- Caudipteryx dongi (2000)
- Caudipteryx sp. (2000)
- Microraptor zhaoianus (2000)
- Nomingia gobiensis (inferred 2000: pygostyle)
- Psittacosaurus sp.? (2002)
- Yixianosaurus longimanus (2003)
- Dilong paradoxus (2004)
- Sinornithosaurus haoiana (2004)
- Pedopenna daohugouensis (2005; possibly avialan)
- Jinfengopteryx elegans (2005)
- Juravenator starki (2006)
- Sinocalliopteryx gigas (2007)
- Velociraptor mongoliensis (inferred 2007: quill knobs)
- Similicaudipteryx yixianensis (inferred 2008: pygostyle; confirmed 2010)
- Anchiornis huxleyi (2009)
- Tianyulong confuciusi? (2009)
- Concavenator corcovatus? (inferred 2010: quill knobs?)
- Xiaotingia zhengi (2011)
- Yutyrannus huali (2012)
- Microraptor hanqingi (2012)
- Sciurumimus albersdoerferi (2012)
- Ornithomimus edmontonicus (2012)
- Ningyuansaurus wangi (2012)
- Eosinopteryx brevipenna (2013)
- Citipati osmolskae (inferred 2013: pygostyle)
- Conchoraptor gracilis (inferred 2013: pygostyle)
- Note, filamentous structures in some ornithischian dinosaurs (Psittacosaurus, Tianyulong) and pterosaurs may or may not be homologous with the feathers and protofeathers of theropods.
Primitive feather types 
Integumentary structures that gave rise to the feathers of birds are seen in the dorsal spines of reptiles and fish. A similar stage in their evolution to the complex coats of birds and mammals can be observed in living reptiles such as Iguanas and Gonocephalus Agamids. Feather structures are thought to have proceeded from simple hollow filaments through several stages of increasing complexity, ending with the large, deeply rooted, feathers with strong pens (rachis), barbs and barbules that birds display today.
Some evidence suggests that the original function of simple feathers was insulation. In particular, preserved patches of skin in large, derived, tyrannosauroids show scutes, while those in smaller, more primitive, forms show feathers. This may indicate that the larger forms had complex skins, with both scutes and filaments, or that tyrannosauroids may be like rhinos and elephants, having filaments at birth and then losing them as they developed to maturity. An adult Tyrannosaurus rex weighed about as much as an African Elephant. If large tyrannosauroids were endothermic, they would have needed to radiate heat efficiently, and feathers would have interfered with this.
There is now an increasing body of evidence that supports the display hypothesis, which states that early feathers were colored and increased reproductive success. Coloration could have provided the original adaptation of feathers, implying that all later functions of feathers, such as thermoregulation and flight, were co-opted. This hypothesis has been supported by the discovery of pigmented feathers in multiple species. Additionally, some specimens have iridescent feathers. Pigmented and iridescent feathers may have provided greater attractiveness to mates, providing enhanced reproductive success when compared to non-colored feathers.
The fact that precursors of feathers appeared and then were co-opted for insulation is already present in Gould and Vrba, 1982. The reason why such precursors appeared could be explained by a theory which is based on metabolic issue. Feathers are made of protein and contain substantial amounts of certain amino acids, especially cysteine. The protein complex at the base of the composition of the feather is keratin, which has disulfide bonds between amino acids that confer unique properties of stability and elasticity. The metabolism of amino acids containing sulfur proved to be toxic to the organism. If the sulfur amino acids are not catabolized at the final products of urea or uric acid but used for the synthesis of keratin instead, the release of hydrogen sulfide is extremely reduced or avoided. For an organism whose metabolism works at high internal temperatures of 40 °C or greater can be extremely important to prevent the excess production of hydrogen sulfide. This hypothesis could be consistent with the need for high metabolic rate of theropod dinosaurs.
It is not known with certainty at what point in archosaur phylogeny the earliest simple “protofeathers” arose, or if they arose once or, independently, multiple times. Filamentous structures are clearly present in pterosaurs, and long, hollow quills have been reported in specimens of the ornithischian dinosaurs Psittacosaurus and Tianyulong. In 2009 Xu et al. noted that the hollow, unbranched, stiff integumentary structures found on a specimen of Beipiaosaurus were strikingly similar to the integumentary structures of Psittacosaurus and pterosaurs. They suggested that all of these structures may have been inherited from a common ancestor much earlier in the evolution of archosaurs, possibly in an ornithodire from the Middle Triassic or earlier.
Display feathers are also known from dinosaurs that are very primitive members of the bird lineage, or Avialae. The most primitive example is Epidexipteryx, which had a short tail with extremely long, ribbon - like, feathers. Oddly enough, the fossil does not preserve wing feathers, suggesting that Epidexipteryx was either secondarily flightless, or that display feathers evolved before flight feathers in the bird lineage. Plumaceous feathers are found in nearly all lineages of Theropoda common in the northern hemisphere, and pennaceous feathers are attested as far down the tree as the Ornithomimosauria. The fact that only adult Ornithomimus had wing-like structures suggests that pennaceous feathers evolved for mating displays.
Phylogeny and the inference of feathers in other dinosaurs 
Fossil feather impressions are extremely rare and they require exceptional preservation conditions to form. Therefore only a few feathered dinosaur genera have been identified. However, these cover nearly all of the major groups of the theropods, though as of 2013 no feathered ceratosaurian has been discovered. (Ceratosaurians are mostly of the Southern Hemisphere, where no appropriately preserved fossil beds are known.) All fossil feather specimens have been found to show certain similarities. Due to these similarities and through developmental research, almost all scientists agree that feathers could only have evolved once in dinosaurs. Feathers would then have been passed down to all later, more derived species, unless some lineages lost feathers secondarily. If a dinosaur falls at a point on an evolutionary tree within the known feather-bearing lineages, then its ancestors had feathers, and it is quite possible that it did as well. This technique, called phylogenetic bracketing, can also be used to infer the type of feathers a species may have had, since the developmental history of feathers is now reasonably well-known. All feathered species had filamentaceous or plumaceous (downy) feathers. Pennaceous (vaned) feathers are now attested in adults down to the Ornithomimosauria. The following cladogram is adapted from Weishampel et al., 2004.
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