Feathered dinosaurs

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 early 1990s that clearly nonavian dinosaur fossils were discovered with preserved feathers. Today there are more than a dozen genera of dinosaurs with fossil feathers, all of which are theropods. Most are from the Yixian formation in China. The fossil feathers of one specimen, Shuvuuia deserti, have even 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 The Origin of Species, British biologist and evolution-defender Thomas Henry Huxley proposed that birds were descendants of dinosaurs. He cited skeletal similarities, particularly among some saurischian dinosaurs, fossils of the 'first bird' Archaeopteryx and modern birds. 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 has since become 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 1980s.

By the 1990s, most paleontologists considered birds to be surviving dinosaurs and referred to 'non-avian dinosaurs' (all extinct), to distinguish them from birds (aves or avian dinosaurs). Direct evidence to support the theory was missing, however. Some mainstream ornithologists, including Smithsonian Institution curator Storrs L. Olson, disputed the links, citing the lack of fossil evidence for feathered dinosaurs.

Fossil evidence

After a century of hypotheses without hard evidence, particularly well-preserved (and legitimate) fossils of feathered dinosaurs were discovered during the 1990s and 2000s. 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, lizards and crocodilians 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, 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.

Current knowledge

List of dinosaur genera preserved with evidence of feathers

A number of non-avian dinosaurs are now known to have been feathered. Direct evidence of feathers exists for the following genera, listed in order of publication. In all examples, the evidence described consists of feather impressions, except those marked with an asterisk (*), which denotes genera known to have had feathers based on skeletal or chemical evidence, such as the presence of quill knobs or beta-keratin.

1. Avimimus* (1987)
2. Sinosauropteryx (1996)
3. Protarchaeopteryx (1997)
4. Caudipteryx (1998)
5. Rahonavis* (1998)
6. Shuvuuia* (1999)
7. Sinornithosaurus (1999)
8. Beipiaosaurus (1999)
9. Microraptor (2000)
10. Nomingia* (2000)
11. Epidendrosaurus (2002)
12. Cryptovolans (2002)
13. Scansoriopteryx (2002)
14. Yixianosaurus (2003)
15. Dilong (2004)
16. Pedopenna (2005)
17. Jinfengopteryx (2005)
18. Sinocalliopteryx (2007)
19. Velociraptor* (2007)
20. "Epidexipteryx" (2008)

Primitive feather types

The evolution of feather structures is 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.

It is logical that the simplest structures were probably most useful as insulation, and that this implies homeothermy. Only the more complex feather structures would be likely candidates for aerodynamic uses.

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 a specimen of Psittacosaurus from Liaoning. It is thus possible that the genes for building simple integumentary structures from beta keratin arose before the origin of dinosaurs, possibly in the last common ancestor with pterosaurs – the basal Ornithodire.

Most of the theropods known to have feathers are maniraptorans. Only a few non-maniraptoran theropods are known to have them as well. At present, the most primitive (known) theropod dinosaur with integumentary filaments is Sinosauropteryx, a compsognathid (Jurassic/Cretaceous, 150-120 mya), whose body was covered with feather-like structures that look like hollow tubes, or hairs. They may or may not have had barbs, like downy (plumaceous) feathers. Another very primitive theropod, Dilong paradoxus (Early Cretaceous), an ancestor of Tyrannosaurus rex, had similar simple feather structures. The alvarezsaurid Shuvuuia is sometimes found to be outside the maniraptora, but consensus right now places it as a maniraptoran.

The first dinosaur fossils from the Yixian formation found to have true flight-structured feathers (pennaceous feathers) were Protarchaeopteryx and Caudipteryx (135-121 mya). Due to the size and proportions of these animals it is more likely that their feathers were used for display rather than for flight. Subsequent dinosaurs found with pennaceous feathers include Pedopenna and Jinfengopteryx. Several specimens of Microraptor, described by Xu et al. in 2003, show not only pennaceous feathers but also true asymmetrical flight feathers, present on the fore and hind limbs and tail. Asymmetrical feathers are considered important for flight in birds. Before the discovery of Microraptor gui, Archaeopteryx was the most primitive known animal with asymmetrical flight feathers.

Phylogeny and the inference of feathers in other dinosaurs

Feathered dinosaur fossil finds to date, together with cladistic analysis, suggest that many types of theropod may have had feathers, not just those that are especially similar to birds. In particular, the smaller theropod species may all have had feathers and possibly even the larger theropods (for instance T. rex) may have had feathers, in their early stages of development after hatching. Whereas these smaller animals may have benefitted from the insulation of feathers, large adult theropods are unlikely to have had feathers, since inertial heat retention would likely be sufficient to manage heat. Excess internal heat may even have become a problem, had these very large creatures been feathered.

Fossil feather impressions are extremely rare; therefore only a few feathered dinosaurs have been identified so far. However, through a process called phylogenetic bracketing, scientists can infer the presence of feathers on poorly-preserved specimens. 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 (although it is possible that some lineages lost feathers secondarily). If a dinosaur falls at a point on an evolutionary tree within the known feather-bearing lineages, scientists assume it too had feathers, unless conflicting evidence is found. This technique 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.

The scientists who described the (apparently unfeathered) Juravenator performed a genealogical study of coelurosaurs, including distribution of various feather types. Based on the placement of feathered species in relation to those that have not been found with any type of skin impressions, they were able to infer the presence of feathers in certain dinosaur groups. The following simplified cladogram follows these results, and shows the likely distribution of plumaceous (downy) and pennaceous (vaned) feathers among theropods. Note that the authors inferred pennaceous feathers for Velociraptor based on phylogenetic bracketing, a prediction later confirmed by fossil evidence.

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References
See also
Origin of birds
External links
 Downy Dinos on EvoWiki
 DinoBuzz, dinosaur-bird controversy explained, by UC Berkeley.
 Journal of Dinosaur Paleontology, with many articles on dinosaur-bird links.
 Life-like images of feathered dinosaurs
 Feathered dinosaurs at the American Museum of Natural History