Since it's a new year I decided to try out a different style, as well as a revised design of my wyverns. I figured the older version was just too dragon-like, seeing as how they're supposed to be only very distantly related. These are based mainly on early mediaeval depictions of wyverns, and while I'm overall happy with how it turned out, I'm a bit unsatisfied with the golden wyvern, as it ended up looking like a generic theropod with wings... Might end up redoing that one at some point in the near future.
The Volanosauria, or wyverns as they're more commonly known, are a clade of non-pyrosaurian draconiformes. Wyverns are flying reptiles, but despite their superficially similarity to dragons, they are more closely related to certain wyrms. They represent the youngest clade of Draconiformes, having appeared roughly 28 million years ago, and with 130 species across 5 families, are behind only the wyrms (Ophiosuchia) in terms of species diversity.
While at first glance they seem to have more in common with dragons than wyrms, this is simply a result of convergent evolution, and a closer look at their anatomy reveals significant differences between them and their more distant pyrosaurian cousins. In contrast to dragons, wyverns have tetradactyl forelimbs and tridactyl hindlimbs. Digits III & IV of their forelimbs are elongated, with IV being the longest, and support the patagium. Unlike in dragons though, in the majority of wyvern species, the patagium doesn't connect to the hindlimbs, but instead to the flanks. It also lacks the extensive musculature or air sacs seen in dragon patagia, and instead is supported by keratinous "rods" that run longitudinally. This makes wyvern wings much stiffer than those of dragons (comparable to those of birds) and therefore cannot be folded tightly against the body, in turn making walking on all fours somewhat cumbersome. But as the legs are independent of the wings, most wyverns are facultatively bipedal, and can thus stand or run on their legs should they find themselves grounded. Additionally, in contrast to the pyrosauria, wyverns (as with wyrms), lack forked tongues, and rely largely on their nostrils for olfaction.
Wyverns also have an insulating coat of elongated, hollow scales that are analogous to the modified coelofibers of true dragons, but these show a considerable degree of modification for display purposes. Many wyverns have elaborate, ornate scaly crests that are used for both display as well as communication, particularly by males. In every known species, females are larger than the males, an adaptation thought to be helpful when incubating their eggs and later defending their offspring from predators and/or rivals. As with dragons in being endothermic, volant animals, wyverns have a rather high metabolic rate for reptiles, comparable to that of mammals. Their breathing mechanism however, is quite unique among the draconiformes. A muscular, posterior extension of the pleural tissue that surrounds the lungs forms a "proto-diaphragm" of sorts, which separates the lungs from the viscera. This proto-diaphragm is connected to the pectoral muscles, which are the primary muscles used for flight - in contrast to dragons wherein both pectoral & deltoid (shoulder) muscles contribute equal power, such that each upstroke of the wings expands the lungs and each downstroke contracts them. As a result, wyverns breathe as they flap their wings, but even when at rest, the proto-diaphragm has sufficient musculature to do this on its own. Additionally, wyverns also have a muscular gular sac similar to those of monitor lizards, which can force in extra air during strenuous activity. All this results in a bidirectional airflow system analogous to that of mammals, unlike the more efficient unidirectional system seen in dragons and the more distant archosaurs, and while wyverns do have air spaces in some of their larger bones and thinner bones overall, they lack the degree of skeletal pneumaticity seen in these clades, which, in addition to competition with dragons and birds, ultimately restricts them to much smaller sizes, with most growing no larger than a hawk.
Perhaps the most notable trait of wyverns however is their venom. All known species are able to produce toxic salivary secretions, and the majority of them (125 out of 130 species), have modified teeth to facilitate effective envenomation. While toxicity can vary considerably between species, in general their venoms are more potent than those of other venomous reptiles such as snakes of similar size. This is likely due to the fact that, as venoms are biologically expensive to produce, and wyverns are flying animals that require a lot of energy, they can only spare enough energy to produce rather small quantities of it.
Wyverns have a particularly convoluted evolutionary history; although all extant species can fly, at some point in their evolution one group lost this ability but subsequently regained it. The descendants of this group form the clade Boreovolanosauria, while those that retained flight throughout their evolution are classified in the Notovolanosauria.
The Notovolanosauria includes a single family, the Fossoriopterygidae, also known as burrowing wyverns or frogmouth wyverns. With just 5 species endemic to Madagascar and a few other nearby island groups, this is the smallest & least speciose clade of wyverns, and get their name from their occasional tendency to nest underground, usually in burrows dug by mammals and reptiles. They’re also characterised by their large, wide mouths, which bear a passing resemblance to those of frogs. These wyverns are nocturnal insectivores, and perhaps as a result of their diet, appear to have lost their venom-injecting fangs, although their salivary glands do produce toxic secretions. Although nowhere near lethal enough to kill or incapacitate a large animal, all species can still deliver a rather painful bite that can also cause localised swelling and numbness, and therefore may play a role in defence - distracting a potentially predator with sharp pain and/or numbness while the wyvern escapes. These wyverns further differ from others in that their patagia are connected to their legs, but are still quite stiff due to their keratinous support rods, which makes them especially cumbersome on the ground and as such spend most of their time in trees.
The largest known fossoriopterygid is the greater frogmouth wyvern (Batrachocephalus longicauda) of Madagascar. With a wingspan of 70-80 cm, it isn't much bigger than a crow, but is still quite a bit larger than most other members of its clade, which are around half the size. This unusual wyvern hunts in the eastern rainforests of Madagascar, feeding on insects as well as small birds and reptiles, and unlike most other fossoriopterygids, nests on trees rather than on the ground. This is mainly because, with its relatively large wings but otherwise weak legs, it cannot jump high enough off the ground to get airborne, and most therefore take off from a vertical surface or one high above the ground, similar to birds such as swifts and most bats. Being nocturnal, it doesn't have many predators, but does occasionally fall prey to owls and fossas.
In contrast to most other wyverns, both parents help incubate the eggs and look after the offspring for the first month, after which the nest is abandoned. Perhaps due to this, sexual dimorphism isn't nearly as pronounced in this species as it is in some other wyverns. Unlike other fossoriopterygids but like the Boreovolanosaurian wyverns, the young take over a month to properly learn how to fly, but nonetheless instinctively know how to hunt, and thus learn to catch insects on trees even before they can actually fly, indicating that they rely on their parents solely for protection.
The Boreovolanosauria, sometimes referred to as 'true wyverns', represent the descendants of the ancestral wyvern group which went through a flightless phase, which is reflected in their morphology that's better adapted for movement in the ground. Although none possess the acrobatic prowess of the fossoriopterygidae, they are all quite capable fliers, and have a well-developed venom delivery apparatus in the form of 3 pairs of grooved and/or hollow fangs in their upper jaw that are connected to a venom sac, which in turn stores venom synthesized in the parotid salivary glands. The venoms of all of these wyverns have a high concentration of cytotoxins and proteases, which destroy tissues and therefore make the prey easier to digest. This particular trait allowed wyverns to make due with a shorter digestive system and thus save weight, a critical adaptation for flying animals, as well as allowing them to take down prey without risking significant injury
While there are exceptions, most Boreovolanosaurians are nocturnal or crepuscular, a behavioural adaptation that likely arose to minimise competition with large birds. This clade comprises 4 families of wyverns:
The Volanaspidae are a clade of wyverns comprising 25 species distributed though Asia, Africa & southern Europe. These are generally stocky, robustly built wyverns that generally hunt ground-dwelling prey such as rodents, small squamates & birds' eggs. They are characterised by having one of the most advanced venom delivery systems of any tetrapod - 3 pairs of long, hollow, retractable (hinged) fangs in their upper jaw similar to those of vipers that allow them to inject lethal doses of venom deep into their prey's bodies.
The largest member of this family, the African golden wyvern (Avivipera aureus) is an eagle-sized animal with a 2 m wingspan and fangs that can be upto 6 cm long.
Native to the shrublands & savannahs of northern and central Africa, this species is chiefly nocturnal, avoiding the scorching daytime sun and doing much of its hunting in the late evening and night. Equipped with powerful jaws, this wyvern has been noted to be able to kill small prey simply with a powerful bite, and therefore seemingly reserving its venom for larger or more agile prey, which it can chase bipedally on the ground (albeit only for a very short distance). Its venom is a cocktail of cytotoxic and myotoxic peptides, which cause rapid tissue and muscle necrosis, leading to death in a matter of minutes, as well as make digestion much easier.
Golden wyverns are polyandrous, with a single female mating with multiple males. Indeed, females are 20% larger than males on average, and are solitary, while males (usually siblings) tend to form small coalitions of 2-4 individuals, probably to compensate for their smaller size as they still tackle similarly sized prey as the females do. While they have been known to take on large birds and small ungulates, this is a rare occurrence, as their teeth are ill-suited to tear meat, and thus generally tackle smaller prey that can be swallowed whole or in 2-3 large chunks.
The superfamily Volanosauroidea comprises the 3 remaining wyvern families, the Coryphoraptoridae, Volanosauridae, & Aviagamidae, which are quite closely related, having split from a common ancestor just over 8 mya in the late Miocene.
The Coryphoraptoridae, also known as 'helmeted wyverns', include 18 species that are unique to southeast Asia, Australia and the island of New Guinea. These are the largest known wyverns, and are fearsome predators capable of hunting prey generally as large as themselves. They get their name from the bony casques on their heads, which are believed to help protect their heads as they fly through the dense jungles of their range, as those species living in less dense habitats have smaller casques. Uniquely among wyverns, their teeth are laterally compressed & serrated, which allows them to tear off chunks of flesh and thus effectively hunt large game. Their fangs are also unique in having multiple grooves rather than being hollow, which appears to be the ancestral condition in Boreovolanosaurians. While this does make venom delivery somewhat less efficient, it makes the fangs themselves more durable as they're solid, instead of hollow and fragile, a useful adaptation considering that these wyverns have very strong jaws that can dispatch small prey through brute force alone. Indeed, these wyverns are mostly restricted to isolated regions & islands where they have few competitors, allowing them to claim the role of apex predators, which is certainly the case with the New Guinean giant wyvern (Haplorhynchus pennocristatus), the largest known wyvern species.
With an impressive 2.7-3 m wingspan, this rather strikingly coloured wyvern is the top predator of its island habitat, where there isn't enough large game available to sustain a population of large dragons. It avoids competition with the sympatric Papuan eagle by hunting primarily in open highlands, grasslands and sparse forest, as its large wings are ill-suited to the dense forest preferred by the latter, as well as being cathemeral in its hunting habits, in contrast to the strictly diurnal eagle. In areas where their ranges overlap however, eagles may kill young wyverns, and vice versa. Most of its diet consists of the many large rodents and marsupials native to the island, as well as monitor lizards and the occasional fish. Although they prefer small to medium-sized prey, on rare occasions, wyverns have been known to tackle prey many times their size, such as cassowaries and the diprotodontids Hulitherium & Maokopia. When attacking such large and dangerous game, the wyvern bites the animal's neck and immediately retreat to avoid injury, and simply wait until its myo- & haemotoxic venom kills its victim by paralysing the muscles necessary for breathing, as well as exsanguination, brought on by its lacerating teeth. In addition to its toxicity, the massive quantity of venom it can inject with a single bite is what makes it lethal enough to hunt such large game.
New Guinean giant wyverns mate for life, a trait that is unique to this particular wyvern genus. Only the female incubates the eggs, but both parents help raise the hatchlings for the first 2 months.
The Volanosauridae, consisting of 45 species, is the most speciose clade of wyverns. Mostly native to Eurasia, Australia & North America, these generally prefer sparse forests and open areas, and the term 'wyvern' more often than not refers to them. Many species possess ornate crests, frills and other display structures made of modified scales, which in most cases are either unique to, or more prominent in males. They have hollow fangs, which appear to have been developed independently from the volanaspidae, and are fixed rather than being retractable, similar to the fangs of elapids than those of vipers.
The ocellated wyvern (Volanosaurus ocellatus) is a typical member of this family, with a wingspan of 1-1.2 m, it is similar in size to a large hawk, but with a rather gracile build, and inhabits much of Europe, Western Asia and even northern Africa, being the only volanosaurid present there. It gets its name from the prominent eye-like markings on its wings, present in both sexes, though their exact function is unknown. Both sexes also possess a fan-like crest of scales on their heads, which makes it difficult to distinguish them, as there are no obvious differences apart from the female's greater size (~10% larger).
Although predominantly nocturnal, occellated wyverns will often hunt small reptiles at dawn, a time at which they can be found basking in exposes spaces, while those wyverns living in more forested areas are more crepuscular, hunting birds at dawn and dusk. This wyvern's venom contains neurotoxins, cytotoxins & haemotoxins, reflecting its broad diet. The neurotoxins appear to be the primary effector molecules, causing rapid paralysis of the major skeletal muscles, particularly in birds, but seem to have a greatly reduced effect on mammals and reptiles, which in turn seem to be more adversely affected by the haemo- & cytotoxins. Consequently, as with snakes, populations show considerable variation in venom composition depending on their distribution as well as prey preference. It's likely that the neurotoxins evolved specifically for bird predation, and is understandable considering that small birds are quite agile even compared to the wyvern, and as such need to be subdued quickly.
A polygamous species as with the majority of wyverns, parental care in occellated wyverns is restricted to the female, which may mate with any number of males and consequently lays eggs that may not have shared parentage. Oddly, this species nests in close proximity to the European spotted gryphon (Gryphon aquiloides), a diurnal species wherein both parents return to guard their nest (and thus the wyvern's) at night, thus allowing the wyvern to spend a considerable amount of time foraging at night while the gryphons deter nocturnal predators. It's unclear whether the wyvern is merely a commensal or if the gryphons are mutually benefitted by this association.
The Avilacertidae are the sister clade of the volanosauridae, but while the latter are more diverse in the eastern hemisphere than the western, these show the opposite trend. The 37 species of wyvern in this family are mostly neotropical in distribution, with 25 species in central and south America, 10 in North America and 2 in east Asia. Their general habitat preferences and ecological niches are very similar to those of the volanosauridae, to the point that members of the 2 clades almost never have overlapping ranges, and it's believed that their present distributions are actually a result of competitive exclusion. Avilacertids show marked sexual dimorphism, with males and females of certain species looking so different that they've been previously thought of as separate species. Males in most cases have extensive ornamentation in the form of frills, crests, vibrant colours and markings, whereas females rarely if ever possess such display features. As with the volanosaurids, these wyverns have fixed, hollow fangs as part of their venom delivery apparatus. The group gets its name from their distinctly bird-like heads which end in a narrow, beak-like snout.
The peacock wyvern (Avilacerta magnificens) undoubtedly shows the most extravagant dimorphism of any wyvern. Males are bright blue with black markings, and possess an elaborate fan-like crest of scales on their tails as well as a smaller crest on their heads which all add up to a dazzling display of colour, which gives them their name. Females by comparison, are a dull green with similar black markings and lack these crests, though are similar in size to the males with a 1.5 m wingspan, making them among the largest avilacertids. Native to the Amazon rainforest, female peacock wyverns live in small groups of 3-4, whereas males are generally solitary, and while the former show cathemeral behaviour, the latter are strictly nocturnal, which is believed to be due to the males' obviously poor camouflage during the daytime, which not only impedes foraging, but also makes them a beacon for sharp-eyed predators. Another notable trait of this species is its venom. Peacock wyverns possess what's probably the most potent venom of any known wyvern species in the world, with as little as 9 mg being sufficient to kill an adult human, and the wyvern may inject over 10 times this amount in a single bite. This toxicity is comparable to that of the Australian coastal taipan, one of the world's most toxic snakes. The venom is a mixture of several very fast-acting neurotoxins which spread rapidly throughout the body, and specifically target cardiac and skeletal muscles, causing death in anywhere from half a minute to several minutes, depending on the size of their prey. It was previously thought that such toxic venom evolved not just as a predatory but also a defensive weapon, seeing as how during the mating season, male wyverns are especially vulnerable to predators as they have to display to the females during the daytime, when their lack of camouflage can become a serious detriment, however it is now considered more likely that the lethality of the venom evolved primarily for predation. These wyverns prey upon birds, small reptiles, rodents and even small monkeys, all of which are extremely agile and/or dangerous, and as such need to be incapacitated as quickly as possible. At the same time, despite their lethality, both sexes of these wyverns have plenty of predators of their own; jaguars, large eagles & constrictor snakes are fast & powerful enough to restrain or even cripple them, while the armoured skins of dragons and crocodylians are virtually impervious to their rather small fangs. Their primary predator is the arboreal sparassodont Tshenkutshen, which appears to possess a good degree of resistance to the venom.
Peacock wyverns are immune to their own venoms, and this may explain why males possess such extravagant display structures, i.e., similar to their namesake, it evolved as a consequence of sexual selection. Females of a group generally build a single, large nest in a tree hollow, and take turns foraging and guarding the eggs. The eggs hatch in as little as 3 weeks, much faster than those of most other wyverns, likely due to the hot climate of the rainforest, and while male offspring typically go their own separate ways after a few months, females generally stay together.