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Dragons are reptiles of the superfamily Draconoidea. Modern scientists consider them to late-surviving descendants of the aphanosaurs, primitive avemetatarsalian reptiles closely related to the dinosaurs and pterosaurs. Dragons are wonderfully diverse and occupied a variety of niches throughout the Mesozoic and Cenozoic eras, but only seven species in three genera (Drakon, Long and Pterodrakon) survive today; several others have gone extinct in recent times. Throughout history, dragons have served as the inspirations of several mythologies.

Evolution
The three dragon genera alive today are dramatically different in appearance, so for decades scientists have wondered if they were even related at all. However, recent fossil discoveries, as well as reexamination of a few other recently-extinct species, have revealed not only were they all related, but that they are part of a wonderfully diverse clade of basal avemetatarsalians. The following cladogram is from the most recent study, by Bisnett (2021)

Note that Bisnett considers the placement of the cockatrice, fae and kirin tentative.

Description
The three extant species of dragons are remarkably dissimilar in appearance, but closer inspection reveals some similarities between their basic forms. Apart from some osteological details, the obvious similarity is their head shape; it is rounded at the end, much like those of the aphanosaurs they descended from, as well as their long, light, scaly bodies. Everything else, though, is remarkably diverse throughout the clade. However, several vague similarities have allowed scientists to classify them into two subfamilies, Draconinae and Longinae.

Basal dragons
Basal members of the Draconoidea, such as Nipponosuchus, Elliotvenator and Pelecanodraco, greatly resemble their aphanosaurian ancestors, but with longer legs. They resembled other types of basal archosauriforms, especially long-legged pseudosuchians; Nipponosuchus (whose name means "Japanese crocodile") was even named so because it was originally thought to be an unusual basal archosauriform, similar to Polymorphodon.

Later draconoids, such as Cathetosuchus and Ambodraco, are proportioned more similarly to a big cat than earlier forms. Their necks are more erect, and their snouts are relatively short. Their bodies are also more muscular; Cathetosuchus is built like a lion while Ambodraco is more like a cheetah. Their legs were powerfully built and were held erect, like dinosaurs and mammals. However, Ambodraco ' s legs were slightly bowed out, creating a unique stance amongst tetrapods.

Longines
Longines tended to be more snake-like than draconines and basal draconoids, but early forms such as Plateosauroides were much stockier; similar in construction to basal sauropodomorph ("prosauropod") dinosaurs that dominated the Triassic. (The aforementioned Plateosauroides was so named because its fossils were originally though to be a dinosaur's before reinterpreted as those of a dragon ) Later longines were also unique amongst tetrapods for having more than four limbs - seemingly contravening the etymological definition of the word. Paleontologists have reconstructed an evolutionary series of dragons beginning with a four-limbed ancestor, which gained one extra limb per geological stage, culminating in the salamanders of the Oligocene to very recent times, which have eight limbs. However, the longs, including the Asian dragons, have four, sprawling limbs, while the lindwurms of the tribe Kukulkanini included a tripodal form, two-limbed forms that scooted around on their arms, and entirely limbless forms, such as the sea serpent and the quetzalcoatl. Interestingly, this implies that a one-limbed form existed sometime between the Pliocene and Pleistocene, but no evidence of such a taxon has been discovered.

Longines had unique feather-like structures that seemed to be derived from scales (and thus homologous to dinosaur feathers) but were more spine-like in morphology. In addition to normal scales, they appear on all longines; on salamanders and lindwurms around the face and on longs as "horns", "mustaches", back frills, and in the case of the Korean dragon, a "beard". In the sea serpent, they are modified into fins. In the quetzalcoatl, they seem to cover the body instead of scales.

Draconines
Draconines were closer in form to the ancestral draconoids, being vaguely lion- or cheetah-like in body and limb build. Early forms, such as Acinonychosuchus (and convergently the Drakon+Paradrakon clade), were quadrupedal and greatly resembled the rauisuchids of the Triassic or the sebecid crocodylomorphs of the Mesozoic-Cenozoic. Over time, some members of the group have adopted to a bipedal stance, with the Paleocene transitional form Montypythonosuchus showing adaptations for both bipedal and quadrupedal locomotion. Members of the wyvern clade are so adapted to bipedal locomotion that they even lost their front limbs entirely, seemingly convergent with abelisaurid dinosaurs.

Like the longines, most draconines had spine- or feather-like structures, which have sometimes been called "dracofibers". These were usually present on the vertebral line, sometimes connected by skin, creating the appearance of a "sail" as in Spinosaurus or Dimetrodon. In some forms, such as the wyvern and amphithere, they are paired and expanded into wing-like shapes, resembling the pectoral fins of a flying fish, but there is no evidence that they were suitable enough to fly.

Indeterminate draconoids
Three species of dragon, the cockatrice, fae and kirin, have such an unusual anatomy that scientists aren't sure how they fit into the family tree, or whether they are even dragons at all. Most recent analyses, such as Bisnett (2021), consider them to be draconines closely related to wyverns, European dragons, and drakes, respectively, but due to their unique anatomy they are discussed here in their own section.

The kirin is not as aberrant as the cockatrice and fae, being a reptilian version of a large mammal, but instead of being a predator-mimic, it seemed to be the dragon equivalent to a horse or giraffe. Unlike all known dragons, it had a toothless beak, hooves instead of claws, and even a mane made of dracofibers.

The cockatrice and fae are easily the most unusual dragons, having a morphology that is so far unknown amongst draconoids. The cockatrice, as its name suggests, is very similar to a rooster, down to having dracofibers that are indistinguishable from the pennaceous feathers of birds. It was long known only from myths, legends, and partial subfossils, but only a relatively complete, well-preserved specimen revealed its draconic nature; among other things, its skull and claws were found to be more similar to those of other draconoids than those of birds. The similar fae, only known from gardens in Yorkshire, England, is much smaller, but it has large, dracofiber wings similar in shape to a butterfly's. Indeed, it was originally mistaken for a butterfly before a live specimen was scientifically analyzed in 1986, confirming that it was a dragon. Due to the similarities between their general build and integument, it was long thought that were relatives, but further analysis indicates that the cockatrice was closely related to wyverns while the fae was closer to European dragons. More specimens and/or further analysis would be required to determine their precise evolutionary relationships.

Ecology
The three extant genera of dragons are all carnivores, but they hunted different prey in different ways. European dragons were the forest equivalent of big cats; they hunted bigger prey than wolves and smaller prey than bears. Wyverns had no equivalent in other continents, at least not today; paleontologists believe they filled in the niche of the extinct terror birds. Longs, on the other hand, are quite unique; they require low temperatures and high humidity, so they mostly stay in montane cloud forests, where they crawl after small insects. They only go to lower elevations during rainstorms, which may explain why longs are associated with rainstorms.

In contrast, extinct dragons are far more diverse. While most dragon species, like the recently-extinct drakes (originally thought to simply be female European dragons) and amphitheres, were terrestrial hunters similar to European dragons and wyverns, a greater number did not fit into this niche. Among the forms that went extinct during the Holocene, there was the quetzalcoatl, an arboreal snake-like carnivore, and a number of fish hunters, such as salamanders, lindwurms and hydras, and the fully aquatic sea serpent. Fish-eating dragons were not limited to the Holocene; the Middle Jurassic Pelecanosuchus seemingly had a seabird-like niche, and Tripododraco was similar to a pinniped.

There was also a number of dragons that were not mostly carnivorous; Nipponosuchus and the cockatrice were likely omnivores. The kirin and likely Plateosauroides were herbivores. The fae, although it hasn't been conclusively proven, is generally agreed to be a pollinator similar to butterflies and hummingbirds.

History of study
Although dragons have coexisted with humans for most of our existence, even permeating into the mythologies of many cultures, scientists dismissed them as mythical creations perhaps inspired by encounters with already-known animals. The first scientist to seriously consider the existence of dragons was English naturalist Robert Charleston, who, on a 1763 expedition to Central Europe, noted that citizens of a now-unknown village feared the local ruins of a castle due to beliefs that it was used as a den by a dragon. It captivated him and he wrote a short article speculating about the dragon.

The first physical proof of a dragon came when William Kidd discovered an unusual reptile on his farm in 1824. Over the next three years, he attempted to catch it, and, according to his estimates, thousands of his chickens, pigs, and cows were lost to the beast. Finally, in 1827, he managed to catch it, and presented it alive to members of the Royal Society. The top naturalists of the day were just as astounded as Kidd; in build and habits it seemed to mimic a cheetah, but its skin was like a reptile's and it had a "sail" that resembled the wings of a mythical dragon. In light of this, it was described as the Dragon, Drakon acinonychoides; the binomial name means "cheetah-formed dragon." It was believed to be a relative of crocodiles. In the following years, more live specimens were collected across England, Wales, and continental Europe, including some without sails, which were thought to be females. However, they were shortly reassigned to their own species, the drake, Drakon tubiformis.

The discovery of the European dragon was a well-publicized event, and it quickly became a fixture of the public imagination. Interest in the large, mysterious reptiles were further boosted by tales of adventurers to China, who reported large, crawling, crocodile-like reptiles peaks of the Tian Shan and Himalayas of China, which only descend to lower altitudes when it rains. Due to the similarity of their horns, mustaches and back frills to the sails of European dragons, these creatures were also considered to be dragons. While they easily entered the public imagination as a new type of a dragon, they still remained elusive, akin to a cryptid. Their existence was only proven in the late 1860's when a live specimen turned up in British Hong Kong. It was shipped to England where it was extensively studied; in 1881, the first scientific paper about it was published; it was christened the Chinese dragon, binomial name Long serpentoides, meaning "serpent-formed long".

From then on, dragon science, or "dragonology", marched on very slowly; it only picked up speed in the 1890's when American paleontologist Othello Karst and Norwegian naturalist Jorgen Mannelig described two sets of bones, from the Eastern Seaboard and western Iceland, respectively, as Hydroserpentis atlanticus and Jormungandr giganteus. Karst humiliated Mannelig by pointing out that the bones of Jormungandr were fundamentally identical; being named first, Karst's Hydroserpentis would be the official name of the transatlantic genus. In a paper published in 1894, Karst thanked Mannelig for proving his theory that Hydroserpentis migrated across the Atlantic. At first, Mannelig didn't care too much; he even alerted to Karst to new deposits in Newfoundland and southern Greenland. Karst happily named them Manneligisaurus cornutus and Manneligisaurus groenlandicus, in honor of the man who alerted him to their remains. However, it was a ruse; very shortly after their describing article was published, Mannelig wrote to Karst, revealing that the Manneligisaurus remains were in fact planted by him, and that they actually belonged to the wyrm, Apododraco cerorhinus, that he described a few years earlier. Angered, Karst's relationship with Mannelig soured; they soon wished to fiercely outcompete each other as the best dragonologists. Their conflict was soon nicknamed the "Dragonbone Wars", in the same vein as the Bone Wars that had recently finished.

Unlike the "combatants" in the Bone Wars, the two participators in the Dragonbone Wars were much more sophisticated, spending lots of money on traveling around the world, and collecting living as well as fossil specimens. Although they saw it only as competition, the Dragonbone Wars did much to accelerate the research on dragons. Multiple species were described, including the wyvern, quetzalcoatl, salamander, hydra, lindwurm, and amphithere, which may have gone extinct as a direct result of Mannelig's attempts to study it. His and Karst's actions have also increased public awareness of dragons, and may or may not have led to the description of the kirin and the first hypotheses that the fae was a dragon.

From the 1930's to the 1970's, dragonology once again began to slow down. Although new species were still being described, they were often mistaken for other creatures. Cathetosuchus, Acinonychosuchus, Montypythonosuchus and Bipedosuchus, as their names suggest, were thought to be aberrant crocodile relatives, while the cockatrice and Rhachophorus were mistaken for birds (the latter is a deliberate anagram of Phorusrhacos, which its remains were once thought to belong to ). A further obstacle to dragonology was that scientists no longer believed that dragons were a monophyletic clade; the several extinct and extant dragons named so far were thought to be distantly-related reptiles that so happened to be the basis of dragon myths. Those who did believe in a monophyletic Draconidae (as the family was called) were left with the question of what the dragons were related to; they were certainly archosaurs, but scientists disagreed whether they were relatives of crocodiles, dinosaurs or something else entirely.

Dragonology only got more popular in early 1980 when the remains of Tripododraco were discovered in what was then the Kazakh SSR. Resembling a pinniped in posture and likely lifestyle, its claim to fame was that it was the only animal that was naturally tripodal; it had no trace of a fourth limb, as its only hind limb was modified into an organ that resembled a seal's rotated tail. Its discovery, along with those of Brachiodraco and Plesiolintwurm, sent shockwaves around the world, and inspired many experienced and budding naturalists to study dragons. These fresh dragonologists made more groundbreaking discoveries, such as the basal longines with five to eight limbs. The renewed attention made dragons the second-most popular group of fantastic reptiles in public, behind only the dinosaurs. Focus also shifted to ecology, with a study on the intelligence of the fae receiving widespread attention.

In the 1990's and beyond, the increased use of cladistics and comparative anatomy helped scientists realize that several prehistoric creatures described in previous decades were actually members of the dragon lineage, partially solving the mystery of where the dragons came from. Even in the 21st century, more answers are being found. Chamaeleodraco from Madagascar provided the origin for dracofibers, and Pterospondylodraco revealing the possible purpose of draconines' sails: swimming organs. Discoveries by Chinese scientists revealed how the longs gained their distinctive features and that the longs and draconines were related after all. Dragonology has also gone microscopic, with molecular data revealing that the European dragon and Chinese dragon were splittable into a number of species, and that the wyvern, amphithere and drake are their own genera. Molecular and fossil data were also incoporated into a novel expansive phylogenetic analysis of archosauriformes, proving that dragons were in fact late-surving aphanosaurs, solving the decades-old mystery of what dragons are.