Feathered dinosaurs

Feathered dinosaurs is a term used to describe dinosaurs, particularly maniraptoriform dromaeosaurs, that were covered in plumage, either filament-like intergumentary structures with few branches, to fully developed pennaceous feathers complete with shafts and vanes. Feathered dinosaurs first came to realization after it was discovered that dinosaurs are closely related to birds. Since then, the term "feathered dinosaurs" has widened to encompass the entire concept of the dinosaur–bird relationship, including the various avian characteristics some dinosaurs possess, including a pygostyle, a posteriorly oriented pelvis, elongated arms and forelimbs and clawed hand, and clavicles fused to form a furcula. A substantial amount of evidence demonstrates that birds are the descendants of theropod dinosaurs, and that birds evolved during the Jurassic from small, feathered maniraptoran theropods closely related to dromaeosaurids and troodontids (known collectively as deinonychosaurs). Less than two dozen species of dinosaurs have been discovered with direct fossil evidence of plumage since the 1990s, with all coming from Cretaceous deposits in China, most notably Liaoning Province. Together, these fossils represent an important transition between dinosaurs and birds, which allows paleontologists to piece together the origin and evolution of birds.

Despite integumentary structures being limited to non-avian dinosaurs, particularly well-documented in maniraptoriformes, fossils do suggest that a large number of theropods were feathered, and it has even been suggested that based on phylogenetic analyses, Tyrannosaurus at one stage of its life was covered in down-like feathers, although there is no direct fossil evidence of this. Based on what is known of the dinosaur fossil record, paleontologists generally think that most of dinosaur evolution happened at relatively large body size (a mass greater than a few kilograms), and in animals that were entirely terrestrial. Small size (&lt;1 kg) and arboreal habits seem to have arisen fairly late during dinosaurian evolution, and only within maniraptora.

Birds were originally linked with other dinosaurs back in the late 1800s, most famously by Thomas Huxley. This view remained fairly popular until the 1920s when Gerhard Heilmann's book The Origin of Birds was published in English. Heilmann argued that birds could not have descended from dinosaurs (predominantly because dinosaurs lacked clavicles, or so he thought), and he therefore favored the idea that birds originated from the so-called 'pseudosuchians': primitive archosaurs that were also thought ancestral to dinosaurs and crocodilians. This became the mainstream view until the 1970s, when a new look at the anatomical evidence (combined with new data from maniraptoran theropods) led John Ostrom to successfully resurrect the dinosaur hypothesis. The best and most complete review of bird origin theories is that provided by Witmer (1991).

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 twenty genera of dinosaurs with fossil feathers, nearly 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, the ground-breaking book which described his theory of evolution by natural selection, British biologist and evolution-defender Thomas Henry Huxley proposed that birds were descendants of dinosaurs. He compared 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.

Aboreal dinosaurs
Thanks to the discovery of such theropods as Microraptor and Epidendrosaurus, we do now have small forms exhibiting some features suggestive of a tree-climbing (or scansorial) way of life. However, the idea that dinosaurs might have climbed trees goes back a long way, and well pre-dates the dinosaur renaissance of the 1960s and 70s.

The idea of scansoriality in non-avian dinosaurs has been considered a 'fringe' idea, and it's partly for this reason that, prior to 2000, nobody had attempted any sort of review on the thoughts that had been published about the subject. The oldest reference to scansoriality in a dinosaur comes from William Fox, the Isle of Wight curator and amateur fossil collector, who in 1866 proposed that Calamospondylus oweni from the Lower Cretaceous Wessex Formation of the Isle of Wight might have been in the habit of 'leaping from tree to tree'. The Calamospondylus oweni specimen that Fox referred to was lost, and the actual nature of the fossil remains speculative, but there are various reasons for thinking that it was a theropod. However, it's not entirely accurate to regard Fox's ideas about Calamospondylus as directly relevant to modern speculations about tree-climbing dinosaurs given that, if Fox imagined Calamospondylus oweni as resembling anything familiar, it was probably as a lizard-like reptile, and not as a dinosaur as they are currently understood.

During the early decades of the 20th century the idea of tree-climbing dinosaurs became reasonably popular as Othenio Abel, Gerhard Heilmann and others used comparisons with birds, tree kangaroos and monkeys to argue that the small ornithopod Hypsilophodon (also from the Wessex Formation of the Isle of Wight) was scansorial. Heilmann had come to disagree with this idea and now regarded Hypsilophodon as terrestrial. William Swinton favored the idea of a scansorial Hypsilophodon, concluding that 'it would be able to run up the stouter branches and with hands and tail keep itself balanced until the need for arboreal excursions had passed', and in a 1936 review of Isle of Wight dinosaurs mentioned the idea that small theropods might also have used their clawed hands to hold branches when climbing.

During the 1970s, Peter Galton was able to show that all of the claims made about the forelimb and hindlimb anatomy of Hypsilophodon supposedly favoring a scansorial lifestyle were erroneous, and that this animal was in fact well suited for an entirely terrestrial, cursorial lifestyle. Nevertheless, for several decades Hypsilophodon was consistently depicted as a tree-climber.

In recent decades, Gregory Paul has been influential in arguing that small theropods were capable climbers, and he not only argued for and illustrated scansorial abilities in coelurosaurs, he also proposed that as-yet-undiscovered maniraptorans were highly proficient climbers and included the ancestors of birds. The hypothesised existence of small arboreal theropods that are as yet unknown from the fossil record later proved integral to George Olshevsky's 'birds come first' (BCF) hypothesis. Olshevsky argued that all dinosaurs, and in fact all archosaurs, descend from small, scansorial ancestors, and that it is these little climbing reptiles which are the direct ancestors of birds.

Ostrom, Deinonychus and the Dinosaur Renaissance
In 1964, the first specimen of Deinonychus antirrhopus was discovered in Montana, and in 1969, John Ostrom of Yale University described Deinonychus as a theropod whose skeletal resemblance to birds seemed unmistakable. Since that time, Ostrom had 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). 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.

Modern research and feathered dinosaurs in China
The early 1990s saw the discovery of spectacularly preserved bird fossils in several Early Cretaceous geological formations in the northeastern Chinese province of Liaoning. In 1996, Chinese paleontologists described Sinosauropteryx as a new genus of bird from the Yixian Formation, but this animal was quickly recognized as a theropod dinosaur closely related to Compsognathus. Surprisingly, its body was covered by long filamentous structures. These were dubbed 'protofeathers' and considered to be homologous with the more advanced feathers of birds, although some scientists disagree with this assessment. Chinese and North American scientists described Caudipteryx and Protarchaeopteryx soon after. Based on skeletal features, these animals were non-avian dinosaurs, but their remains bore fully-formed feathers closely resembling those of birds. "Archaeoraptor," described without peer review in a 1999 issue of National Geographic, turned out to be a smuggled forgery, but legitimate remains continue to pour out of the Yixian, both legally and illegally. Feathers or "protofeathers" have been found on a wide variety of theropods in the Yixian, and the discoveries of extremely bird-like dinosaurs, as well as dinosaur-like primitive birds, have almost entirely closed the morphological gap between theropods and birds.

A small minority, including ornithologists Alan Feduccia and Larry Martin, continues to assert that birds are instead the descendants of earlier archosaurs, such as Longisquama or Euparkeria. Embryological studies of bird developmental biology have raised questions about digit homology in bird and dinosaur forelimbs. However, due to the cogent evidence provided by comparative anatomy and phylogenetics, as well as the dramatic feathered dinosaur fossils from China, the idea that birds are derived dinosaurs, first championed by Huxley and later by Nopcsa and Ostrom, enjoys near-unanimous support among today's paleontologists.

Feather structures and anatomy
Feathers vary in length according to their position on the body, with the filaments of the compsognathid Sinosauropteryx being 13 mm and 21 mm long on the neck and shoulders respectively. In contrast, the structures on the skull are about 5 mm long, those on the arm about 2 mm long, and those on the distal part of the tail about 4 mm long. Because the structures tend to be clumped together it is difficult to be sure of an individual filament's morphology. The structures might have been simple and unbranched, but Currie &amp; Chen (2001) thought that the structures on Sinosauropteryx might be branched and rather like the feathers of birds that have short quills but long barbs. The similar structures of Dilong also appear to exhibit a simple branching structure.

Evolution
Models of feather evolution are often proposed that the earliest prototype feathers were hair-like integumentary filaments similar to the structures of Sinosauropteryx and Dilong. In Prum's model of feather evolution, hollow quill-like integumentary structures of this sort were termed Stage 1 feathers.

The idea that feathers started out as hollow quills also supports Alan Brush's idea that feathers are evolutionary novelties, and not derived from scales. However, in order to determine the homology of Stage 1 feathers, it is necessary to determine their proteinaceous content: unlike the epidermal appendages of all other vertebrates, feathers are almost entirely composed of beta-keratins (as opposed to alpha-keratins) and, more specifically, they are formed from a group of beta-keratins called phi-keratins. No studies have yet been performed on the Stage 1 structures of Sinosauropteryx or Dilong in order to test their proteinaceous composition, however, tiny filamentous structures discovered adjacent to the bones of the alvarezsaurid Shuvuuia have been tested for beta-keratin, and the structures were discovered to be composed of beta-keratin. Alvarezsaurids have been of controversial phylogenetic position, but are generally agreed to be basal members of the Maniraptora clade. Due to this discovery, paleontologists are now convinced that beta-keratin-based protofeathers had evolved at the base of this clade at least.

While basal coelurosaurs possessed these apparently hollow quill-like 'Stage 1' filaments, they lacked the more complex structures seen in maniraptorans. maniraptorans possessed vaned feathers with barbs, barbules and hooklets just like those of modern birds. However, their bodies were not covered in vaned feathers as are those of the majority of living birds: instead, it seems that they were at least partly covered in the more simple structures that they had inherited from basal coelurosaurs like Sinosauropteryx. This condition may have been retained all the way up into basal birds: despite all those life restorations clothing archaeopterygids in vaned breast, belly, throat and neck feathers, it seems that their bodies also were at least partly covered in the more simple filamentous structures. The Berlin Archaeopteryx specimen appears to preserve such structures on the back of the neck though pennaceous vaned feathers were present on its back, at least.

Though it has been suggested at times that vaned feathers simply must have evolved for flight, the phylogenetic distribution of these structures currently indicates that they first evolved in flightless maniraptorans and were only later exapted by long-armed maniraptorans for use in locomotion. Of course a well-known minority opinion, best known from the writings of Gregory Paul, is that feathered maniraptorans are secondarily flightless and descend from volant bird-like ancestors. While this remains possible, it lacks support from the fossil record, though that may or may not mean much.

Features linking birds and dinosaurs
Over a hundred distinct anatomical features are shared by birds and theropod dinosaurs. Some of the more interesting similarities are discussed here:

Feathers
Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The initial specimen was found in the solnhofen limestone in southern Germany, which is a lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils. Archaeopteryx is a transitional fossil, with features clearly intermediate between those of modern reptiles and birds. Discovered just two years after Darwin's seminal Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for Compsognathus.

Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in Liaoning province, northeastern China, which was part of an island continent during the Cretaceous period. Though feathers have been found only in the lagerstätte of the Yixian Formation and a few other places, it is possible that non-avian dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered non-avian dinosaurs may be due to the fact that delicate features like skin and feathers are not often preserved by fossilization and thus are absent from the fossil record. A recent development in the debate centers around the discovery of impressions of "protofeathers" surrounding many dinosaur fossils. These protofeathers suggest that the tyrannosauroids may have been feathered. However, others claim that these protofeathers are simply the result of the decomposition of collagenous fiber that underlaid the dinosaurs' integument. The Dromaeosauridae family, in particular, seems to have been heavily feathered and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.

Skeleton
Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing link between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important link for paleontologists. Furthermore, it is increasingly clear that the relationship between birds and dinosaurs, and the evolution of flight, are more complex topics than previously realized. For example, while it was once believed that birds evolved from dinosaurs in one linear progression, some scientists, most notably Gregory S. Paul, conclude that dinosaurs such as the dromaeosaurs may have evolved from birds, losing the power of flight while keeping their feathers in a manner similar to the modern ostrich and other ratites.

Comparisons of bird and dinosaur skeletons, as well as cladistic analysis, strengthens the case for the link, particularly for a branch of theropods called maniraptors. Skeletal similarities include the neck, pubis, wrist (semi-lunate carpal), arm and pectoral girdle, shoulder blade, clavicle, and breast bone.

Wishbones
The wishbone (or furcula) was long believed to be a structure unique to birds, formed by the fusion of the two collarbones (clavicles) into a single V-shaped structure that helps brace the skeleton against the stresses incurred while flapping. The reptiles ancestral to birds, therefore, should, at the very least, show well-developed clavicles. But by 1926, when Danish artist and naturalist Gerhard Heilmann published his monumentally influential The Origin of Birds, no clavicles had been reported in any theropod dinosaur. Noting this fact, Heilmann suggested that birds evolved from a more generalized archosaurian ancestor, such as the aptly-named Ornithosuchus (literally, “bird-crocodile”), which is now believed to be closer to the crocodile end of the archosaur lineage. At the time, however, Ornithosuchus seemed to be a likely ancestor of more birdlike creatures.

In the 1980s, paleontologists began to realize that theropods not only possessed clavicles, but also possessed clavicles that were fused into a wishbone. In 1983, Rinchen Barsbold reported the first dinosaurian furcula from a specimen of the Cretaceous theropod Oviraptor. A furcula-bearing Oviraptor specimen had previously been known since the 1920s, but because at the time the theropod origin of birds was largely dismissed, it was misidentified for sixty years. p. 9

Following this discovery, paleontologists began to find furculae in other theropod dinosaurs. Wishbones are now known from the dromaeosaur Velociraptor, the allosauroid Allosaurus, and the tyrannosaurid Tyrannosaurus rex, to name a few.

In 2000, Alex Downs reported an isolated furcula found within a block of Coelophysis bauri skeletons from the Late Triassic Rock Point Formation at Ghost Ranch, New Mexico. While it seemed likely that it originally belonged to Coelophysis, the block contained fossils from other Triassic animals as well, and Alex declined to make a positive identification. Currently, a total of five C. bauri furculae have been found in the New Mexico Museum of Natural History's (NMMNH) Ghost Ranch, New Mexico Whitaker Quarry block C-8-82. Three of the furculae are articulated in juvenile skeletons; two of these are missing fragments but are nearly complete, and one is apparently complete.

Avian air sacs
Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation which was led by Patrick O'Connor of Ohio University. The lungs of theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) likely pumped air into hollow sacs in their skeletons, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said. In a paper published in the online journal Public Library of Science ONE (September 29, 2008), scientists described Aerosteon riocoloradensis, the skeleton of which supplies the strongest evidence to date of a dinosaur with a bird-like breathing system. CT-scanning revealed the evidence of air sacs within the body cavity of the Aerosteon skeleton.

Gizard stones
Another piece of evidence that birds and dinosaurs are closely related is the use of gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.

Heart and sleeping posture
Modern computed tomography (CT) scans of a dinosaur chest cavity (conducted in 2000) found the apparent remnants of complex four-chambered hearts, much like those found in today's mammals and birds. The idea is controversial within the scientific community, coming under fire for bad anatomical science or simply wishful thinking. The type fossil of the troodont, Mei, is complete and exceptionally well preserved in three-dimensional detail, with the snout nestled beneath one of the forelimbs, similar to the roosting position of modern birds. This demonstrates that the dinosaurs slept like certain modern birds, with their heads tucked under their arms. This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.

Reproductive biology
A discovery of features in a Tyrannosaurus rex skeleton recently provided more evidence that dinosaurs and birds evolved from a common ancestor and, for the first time, allowed paleontologists to establish the sex of a dinosaur. When laying eggs, female birds grow a special type of bone in their limbs between the hard outer bone and the marrow. This medullary bone, which is rich in calcium, is used to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of the Tyrannosaurus rex specimen's hind limb suggested that T. rex used similar reproductive strategies, and revealed the specimen to be female. Further research has found medullary bone in the theropod Allosaurus and ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue. Medullary bone has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity rather quickly for such large animals.

Brooding and care of young
Several Citipati specimens have been found resting over the eggs in its nest in a position most reminiscent of brooding.

Numerous dinosaur species, for example Maiasaura, have been found in herds mixing both very young and adult individuals, suggesting rich interactions between them.

A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth (see altricial). This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth.

Gizzard stones
Another piece of evidence that birds and dinosaurs are closely related is the use of gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths. Gizzard stones are also found in some fish (mullets, mud shad, and the gilaroo, a type of trout) and in crocodiles.

Molecular evidence and soft tissue
One of the best examples of soft tissue impressions in a fossil dinosaur was discovered in Petraroia, Italy. The discovery was reported in 1998, and described the specimen of a small, very young coelurosaur, Scipionyx samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.

In the March 2005 issue of Science, Dr. Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone from the Hell Creek Formation in Montana. After recovery, the tissue was rehydrated by the science team. The seven collagen types obtained from the bone fragments, compared to collagen data from living birds (specifically, a chicken), reveal that older theropods and birds are closely related.

When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone marrow cavity (a process called demineralization), Schweitzer found evidence of intact structures such as blood vessels, bone matrix, and connective tissue (bone fibers). Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. The exact nature and composition of this material, and the implications of Dr. Schweitzer's discovery, are not yet clear; study and interpretation of the specimens is ongoing.

The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review, neither of these reports could be confirmed. However, a functional visual peptide of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds. In addition, several proteins have putatively been detected in dinosaur fossils, including hemoglobin.

BADD, BAND, and the Birds Came First hypothesis


The non-standard, non-mainstream Birds Came First (or BCF) hypothesis proposed by George Olshevsky suggests that while there is a close relationship between dinosaurs and birds, but argues that, merely given this relationship, it is just as likely that dinosaurs descended from birds as the other way around. The hypothesis does not propose that birds in the proper sense evolved earlier than did other dinosaurs or other archosaurs: rather, it posits that small, bird-like, arboreal archosaurs were the direct ancestors of all the archosaurs that came later on (proper birds included). George was aware of this fact, and apparently considered the rather tongue-in-cheek alternative acronym GOODD, meaning George Olshevsky On Dinosaur Descendants. This was, of course, meant as opposite to the also tongue-in-cheek BADD (Birds Are Dinosaur Descendants): the term George uses for the 'conventional' or 'mainstream' view of avian origins outlined in the first two paragraphs above. 'BADD' is bad, according to BCF, as it imagines that small size, feathers and arboreal habits all evolved very late in archosaur evolution, and exclusively within maniraptoran theropod dinosaurs.

By taxon
Most groups of maniraptoriformes exhibited some form of plumage. Ornithischian dinosaurs including Psittacosaurus and the recently described Tianyulong had integumentary structures present, but these were not fully-developed feathers.

Therizinosaurs
The existence of hypothetical scansorial dinosaurs was also proposed by Nesov (1995) who thought that therizinosauroids - the mostly large to gigantic coelurosaurs often likened to ground sloths - might have gone through an arboreal phase in their evolution. Supposedly, these small, tree-climbing sloth dinosaurs were necessary (in the theoretical sense) given the absence from the fossil record of early, primitive members of the therizinosauroid lineage. Implied by Nesov's hypothesis is the idea that the large, ground sloth-like therizinosauroids must have descended from tree sloth-like ancestors, but there isn't really any reason to agree with this. Furthermore, we do now have fossils of basal therizinosauroids (like Beipiaosaurus Xu et al., 1999, Nothronychus Kirkland &amp; Wolfe, 2001, Falcarius Kirkland et al., 2005, and Suzhousaurus Li et al., 2007) that don't match Nesov's predictions.

Maniraptora
Maniraptorans are so far the only group of dinosaurs where true, pennaceous feathers were present. More simple integumentary structures were present elsewhere in coelurosaurian theropods, including in compsognathids and tyrannosauroids. Among maniraptoran theropods, Chatterjee (1997) proposed that the stiffened dromaeosaurid tail functioned as a prop, functionally analogous to the stiffened rectrices of woodpeckers and other trunk-climbing birds, and he also noted that the opisthopubic pelvis and strongly hooked manual claws allowed these dinosaurs to be proficient climbers. Rather than restricting tree-climbing to those theropods closest to birds, however, Chatterjee (1997, 1999) went on to propose that scansoriality was primitive for all coelurosaurs, and he has even promoted the downright unlikely idea that such animals as compsognathids and ornithomimosaurs were tree-climbers.

In a similar but far less well known hypothesis, Svend Palm (1997) proposed that small, climbing coelurosaurs used their hooked manual and sharp pedal claws to climb the vertical trunks of cycad-like plants, regarding the crowns of such plants as providing ideal hiding and nesting places for such dinosaurs. Palm compared his little scansorial proto-birds with the non-dinosaurian proto-birds imagined by Gerhard Heilmann, and - like Palm - Heilmann had reconstructed his hypothetical animals as well able to ascend vertical trunks.

NGMC 91-A, the Sinornithosaurus-like theropod informally dubbed "Dave", possessed unbranched fibers in additional to more complex branched and tufted structures. Protarchaeopteryx is well known for its fan-like array of 12 rectricial feathers, but it also seems to have sported simple filament-like structures elsewhere on the tail. Soft and downy feathers are preserved in the chest region and tail base, and are also preserved adjacent to the femora.

In 2007, quill knobs were found on the ulna (one of 2 "forearm" bones) of a Velociraptor, and the researchers concluded that this theropod had modern-looking feathers, composed of a rachis (stem) and vanes formed by barbs. They thought this might indicate that Velociraptor had flying ancestors - but pointed out that the feathers might have had functions other than flight, e.g. for display or for shielding a nest.

Oviraptorosaurs
The bodies and limbs of oviraptorosaurs are arranged in a bird-like manner, suggesting the presence of feathers on the arms which may have been used for insulating eggs or brooding young.

Ornithomimosaurs
In 1997, filament-like integumentary structures were reported to be present in the Spanish ornithomimosaur Pelecanimimus polyodon. Furthermore, one published life restoration depicts Pelecanimimus as having been covered in the same sort of quill-like structures as are present on Sinosauropteryx and Dilong. However, a brief 1997 report that described soft-tissue mineralization in the Pelecanimimus holotype has been taken by most workers as the definitive last word 'demonstrating' that integumentary fibers were absent from this taxon.

However, the report describing soft-tissue mineralization described soft-tissue preservation seen in one small patch of tissue, and the absence of integument here does not provide much information about the distribution of integument on the live animal. This might explain why a few theropod workers (notably Paul Sereno and Kevin Padian) have continued to indicate the presence of filamentous integumentary structures in Pelecanimimus. Feduccia et al. (2005) argued that Pelecanimimus possessed scaly arms and figured some unusual rhomboidal structures in an effort to demonstrate this. The objects that they illustrate do not resemble scales and it remains to be seen whether they are anything to do with the integument of this dinosaur. A full description/monograph on this dinosaur has yet to be published, which might have more information on this subject.

Fossil evidence
After a century of hypotheses without conclusive evidence, especially well-preserved (and legitimate) 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, 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 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.

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 the order currently accepted evidence was first published. 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.


 * 1) Avimimus* (1987)  p. 536
 * 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) Cryptovolans (2002)
 * 12) Scansoriopteryx (2002)
 * 13) Epidendrosaurus (2002)
 * 14) Psittacosaurus? (2002)
 * 15) Yixianosaurus (2003)
 * 16) Dilong (2004)
 * 17) Pedopenna (2005)
 * 18) Jinfengopteryx (2005)
 * 19) Sinocalliopteryx (2007)
 * 20) Velociraptor* (2007)
 * 21) Epidexipteryx (2008)
 * 22) Anchiornis (2009)
 * 23) Tianyulong? (2009)

* 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
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.

Nearly all paleontologists regard birds as coelurosaurian theropod dinosaurs. Within Coelurosauria, multiple cladistic analyses have found support for a clade named Maniraptora, consisting of therizinosauroids, oviraptorosaurs, troodontids, dromaeosaurids, and birds. Of these, dromaeosaurids and troodontids are usually united in the clade Deinonychosauria, which is a sister group to birds (together forming the node-clade Eumaniraptora) within the stem-clade Paraves.

Other studies have proposed alternative phylogenies in which certain groups of dinosaurs that are usually considered non-avian are suggested to have evolved from avian ancestors. For example, a 2002 analysis found oviraptorosaurs to be basal avians. Alvarezsaurids, known from Asia and the Americas, have been variously classified as basal maniraptorans,  paravians, the sister taxon of ornithomimosaurs, as well as specialized early birds. The genus Rahonavis, originally described as an early bird, has been identified as a non-avian dromaeosaurid in several studies. Dromaeosaurids and troodontids themselves have also been suggested to lie within Aves rather than just outside it. p. 472

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.