Draconology: The Therosuchia

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I must say that this was the most fun piece in this series so far, literally just getting to play around with somewhat familiar concepts and creatures to make them look familiar yet also unique in a way that reflects their evolutionary histories. I’m happy with how all of these turned out, but I’m really happy with the forest drake, which, looking back, was probably the very first spec evo creature that I had come up with, so I really wanted to do it justice, so I took a good week off before getting to work on it…maybe I should take art breaks more often!

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The members of the Therosuchia, known colloquially as drakes and less often “wingless dragons”, are large reptiles that are in fact closely related to dragons of the Draconia clade, with both clades falling within the Pyrosauria. While certain species can breathe fire, this ability isn’t as developed as it is in the true dragons (suborder Eudraconia), and several species have lost it altogether. They get their name from their armored, osteoderm-covered skin, which resembles that of crocodilians.

With just 20 species in 4 families, they're the least speciose clade of Draconiformes, however, these species show remarkable morphological & functional diversity, in contrast to the high degree of morphological conservation seen in dragons. So high is this diversity that at one point it was thought that the Therosuchia was a paraphyletic clade, especially since some species show remarkable similarity to those belonging to extinct clades. Subsequent studies however showed that this similarity is merely superficial, the result of convergent evolution rather than descendance, and that Therosuchia is in fact monophyletic.

Although they seem to have little in common with dragons except for their fire-breathing trait, evidence of their phylogenetic affinities becomes apparent in their anatomy. Interestingly, drakes have a pillar-erect gait, wherein the hip socket faces downward to form a bony “shelf” with which the femur articulates. The only other clade of vertebrates known to have had such a configuration were the Rauisuchia, which were completely unrelated. It also appears to have been present in early dragons, though no extant taxa have retained it. Drakes and dragons also have a very similar breathing mechanism: a complex system of air sacs aided by a muscular diaphragm that allows them to ventilate their lungs with a unidirectional air-flow system, like birds, crocodiles and monitor lizards, but with a very different anatomy. These traits allow them to maintain a much higher metabolic rate than most reptiles, but nowhere near as high as that of most mammals and dragons, thus making them mesotherms at best.

The skins of drakes is also covered in hard osteoderms that are almost identical in structure to those of dragons, though none possess coelofibers. These characteristics, in addition to fossils & molecular lines of evidence, indicate that the drakes are stem Pyrosaurians, sharing a common ancestor with dragons but retaining most of the basal traits (plesiomorphies) with limited modification.

Fossil evidence indicates that the last common ancestor of the Pyrosauria gave rise to several distinct lineages during the early Triassic, with one lineage evolving powered flight while the rest remained earth-bound. The flight capable clade ultimately gave rise to the dragons, while the Therosuchia were the only flightless clade to survive to the present day.

Over the course of their evolution, several species among the 4 surviving families independently took to the water, becoming semi-aquatic, perhaps due to increased competition from mammals and dragons on land.

Perhaps the oddest and most specialised of these is the Barysuchidae, a lineage which split off from the rest of the Therosuchia around 70 mya in the late Cretaceous but today is survived by a single species, the Taraihonu (Barysuchus cheloniformis), sometimes also called the turtle drake. Endemic to the rivers and wetlands of Lemuria, this is the only exclusively herbivorous drake known. At 5m long and weighing over a ton, it is a large drake, and gets its name from both its semi-aquatic lifestyle as well as its osteoderm-covered back, which gives it a vaguely turtle/tortoise-like shape, although it's horned head also gives it a passing resemblance to ankylosaurs. As with its namesake, the turtle drake possesses large, broad ribs which, although aren't fused to each other, are fused with the dorsal osteoderms. This structure is less a turtle's shell and more like the armored back of an ankylosaur, but with a flattened, almost crocodile-like tail, making it a very strange-looking animal indeed. This armor as well its gregarious tendancies serves as its primary defense, as it is incapable of breathing fire.

Although capable of holding their breath for nearly an hour, and spending much of their time underwater, taraihonu are rather poor swimmers, and instead walk along the river bed as they feed on aquatic vegetation, which they mow through with their battery of short, leaf-like teeth. In particular, they feed on a type of Azolla aquatic fern called chokerweed native to the region. Chokerweed is very poor in nutrients and while not toxic, it is, as with other Azolla, extremely fast growing, and can prevent the growth of other plants essential for other aquatic herbivores. This is where the taraihonu fills the role of a keystone species - by keeping the density of chokerweed in check, it allows other aquatic plants to grow, and thus sustain an impressive diversity of aquatic organisms.

Because of the fern's low nutritional value, a drake needs to eat roughly 20 kg of it per day. Taraihonu also have the lowest metabolic rate of any pyrosaurian known, presumably an adaptation to help them subsist on such a low-quality diet. However, because few other sympatric large herbivores feed on chokerweed, this drake has virtually no competitors.

Roughly half the known species of drake are semi-aquatic, with at least one semi-aquatic taxon from each family. However, the Thalattoraptoridae are unique in that all 6 species are specialized semi-aquatic reptiles. This group is believed to have originated during the late Eocene to early Oligocene (35-30 mya).

Often called sea drakes, these are characterised by their elongated, flattened tails which in many cases are hypocercal to aid in swimming, thick skin with a layer of fatty tissue underneath for better insulation, ovoviviparity, a modified auditory system to detect sounds underwater as well as a modified sense of touch to detect underwater vibrations, and an ability to obtain at least some oxygen from the water via a highly vascularised buccal cavity. Although all species do possess well-muscled legs for moving about on land, it appears that they will only come ashore to escape large oceanic predators or storms. Although they cannot maintain a constant body temperature, thalattoraptorids are able to keep themselves warmer than the surrounding water thanks to countercurrent heat exchangers. Inspite of this, all except the largest species are restricted to warmer waters around the world.

Not much is known about the life histories of most sea drakes, but the Makara (Thalattoraptor capricornis) is an exception, thanks to its circumtropical distribution, tendancy to hunt close to the surface (rarely venturing below 100 m) and general boldness.

At 4-4.5 m long and weighing 220-270 kg, the Makara isn't the largest member of its clade, but is undoubtedly the fastest, able to swim at speeds of upto 65 kmph. A trait unique to this genus is the large crest on its head and neck, which hydrodynamically, serves a similar function as a dorsal fin. Its webbed limbs also allow it to turn on a dime, making it a highly effective pursuit predator. When in shallow, coastal waters, Makara generally hunt alone for small fish and invertebrates, but when venturing into open waters, will congregate in pods numbering anywhere from 7 to 30 and target rather large and dangerous prey, from small sharks to tuna, large squids and even billfish. However, they aren't apex predators, and frequently fall prey to everything from sharks to macropredatory dolphins (orcas in particular) and other, larger drakes. They can hold their breath for roughly an hour, but during intense activity, will surface every few minutes.

During the breeding season following mating, female Makara will congregate in small female-only pods around mangrove swamps and estuaries, where they'll give birth to 12-20 pups. After 2 months or so of living in the relative safety of the mangroves, they'll rejoin the rest of their pods at sea, alongwith their offspring.

The Therosuchidae aren't nearly as derived as the previous 2 clades, seemingly having taken the ancestral Pyrosaurian bauplan and simply built upon it. Among the unique, derived traits (autapomorphies) that separate them from other drakes are their digitigrade/semi-digitigrade hindlimbs, ornamentation in the form of elaborate crests, spines and tubercles (though no single species possesses all 3) and a unique skull anatomy that allows for a high degree of cranial kinesis, much more than other drakes. Notably, the members of this clade have retained the ability to breathe fire, though quite underdeveloped compared to that of dragons and in most cases simply acts as a deterrent or intimidation tactic to startle a potential predator and thus buying enough time to escape, although in the largest species it can serve as an offensive weapon simply by virtue of the size of the flames produced.

With 7 known species in 2 subfamilies distributed across Asia, Africa, Europe and South America, the therosuchids are the most speciose clade of drakes. All species are active predators of small to medium-sized game, and it is thought that their kinetic skulls evolved primarily to assist in rapidly engulfing prey items.

The 2 subfamilies are morphologically quite distinct, with one consisting of mostly semi-aquatic species with a more varanoid appearance while the other consists of terrestrial and semi-arboreal species with longer limbs and a passing resemblance to early dinosauromorphs.

The members of this latter group are sometimes referred to as questing beasts, or more commonly forest drakes, due to their preference for forested habitats where dragons are much rarer. Of these, the best known and most widespread is the Oriental forest drake (Therosuchus orientalis). At

3 - 3.7 m long and weighing 60 to 75 kg, they are larger than their closest relative, the European forest drake aka the Questing beast (Therosuchus longicollis), though this is quite small by drake standards. They inhabit much of South and Southeast Asia, and generally hunt in small packs numbering around 4 to 7 individuals, but aren't apex predators, as they do fall prey to big cats and dragons, and their kills are sometimes stolen by packs of wild dogs.

As with most therosuchids, forest drakes are facultative bipeds, and can actually sprint at speeds of upto 35 kmph to either run down prey or escape from a threat. The species also shows marked sexual dimorphism, with males being 30% larger than the females on average and possessing a sail-like crest at the base of their tails, and turn bright golden-yellow and red during the breeding season at which time they often engage in vicious fights over mating rights. However, this makes them especially vulnerable to bring spotted by predators. While normally an adult male drake faces few threats except large dragons and tigers, males that are severely injured following a fight with each other are easy prey to everything from leopards to wolves & wild dogs. Following mating, the entire pack will help care for the eggs laid by all the females in the group throughout the months-long incubation period, although the hatchlings are only cared for the first 2-3 months.

A unique feature of this genus is the presence of robust, semi-opposable first digits on both the fore- and hindlimbs, a configuration eerily reminiscent of primates. These are tipped with talon-like claws, and while it is believed that they evolved to assist in climbing, they can be used to deadly effect during prey capture, helping to restrain rather large game while they tear off chunks of flesh with their teeth. When hunting alone, they generally feed on small prey such as rodents and small reptiles as well as invertebrates, and though not as good swimmers as certain other therosuchids, they can catch fish underwater, making them supreme generalists, which is presumably the reason they've managed to thrive in a landscape filled with other, larger predators.

The Dirosuchidae are close relatives of the therosuchidae, with the 2 groups having split off from a basal common ancestor sometime during the Oligocene roughly 28 mya. Of all the extant drake clades, these are by far the most basal, with anatomical features that have undergone little modification from that of the earliest drakes. While the therosuchidae appear to have become more gracile overtime and emphasized agility over brute force, the dirosuchidae have seemingly done the opposite, becoming robust, armored predators with powerful jaws capable of hunting megafauna. Most of the 6 known species are found mainly in the tropical regions of South America, Africa & Australia; with a much lower metabolic rate compared to the therosuchids, but still higher than that of other large reptiles, most still need to bask in sunlight in order to warm up, though not for very long periods. All known species are facultatively bipedal, and as with therosuchids, are capable of breathing fire. Although this ability is roughly on the same level as that of the latter, the generally larger size of these drakes means that it is much more effective, both as a weapon and a deterrent.

The dirosuchids in general are large animals, with even the smallest species growing as large as a Komodo dragon. Even so, the largest species, the titan drake (Dirosuchus maximus), is a true giant among giants. Males can reach lengths of upto 11-13 m and weigh 7 tons, with exceptional individuals approaching 9 tons, making them similar in size to a Tyrannosaurus rex or Purussaurus while females are somewhat smaller, at 9-11 m. This makes it the largest member of the Draconiformes as well as one of the largest extant reptiles. Although a marine reptile, it's not nearly as specialised for a life in the sea as the thalattoraptorids. With its thick skin (which can be over a foot thick in places) and underlying layers of fatty tissue, this species isn't restricted to shallow tropical waters, and is able to venture into colder, deeper waters to prey upon marine mammals as well as deep-sea creatures such as giant squids, sharks, sea drakes and even the mysterious sea serpents.

Armed with serrated, conical teeth, titan drakes can take on even the largest and most fearsome of prey. With a bite force estimated to be twice that of a saltwater crocodile, they can crack open everything from turtle shells to whale bones. Although not particularly fast, their powerful tails can propel them at speeds of upto 30 kmph in short bursts, sufficient to ambush unsuspecting prey and then deliver a bone-crushing bite to the victim's main locomotory organ, or in case of slower-moving prey the back or belly, tear off a large chunk of flesh and simply wait for the unfortunate animal to die from blood loss or simply go into shock. This technique is surprisingly similar to that used by many other dirosuchids, with some modifications to suit its aquatic habitat and the size of prey.

More often than not however, titan drakes feed on schools of small fish and squid rather than aquatic megafauna. An expandable gular sac allows them to suction feed by taking in a large volume of water along with a large number of fish, and then simply expelling the water out between their teeth, while the serrations keep the fish within the mouth. Despite their size and power, these drakes do have predators. Juveniles fall prey to everything from dolphins, large fish and sharks while adults are known to be harassed off carcasses by orcas. Large pods of orcas have also been documented attacking titan drakes and on at least one occasion an old individual was even killed by such a pod.

The large spines on their backs are modified osteoderms, a trait possessed by all dirosuchids but are largest in this species. By flushing blood into these, the drake can shed excess heat during periods of intense activity in warm waters, the only time when its gigantothermy becomes an issue. However, it appears that they are more important as a blood buffering system. Capable of holding its breath for nearly 3 hours, the accumulation of CO2 in the drake's blood can easily lead to acidosis during frequent, long dives, and the calcium stored within the osteoderms helps neutralize this.

Little is known about the titan drake's mating habits. Unlike the thalattoraptorids, this species needs to come ashore to lay eggs as with other dirosuchids. Females generally travel alone to isolated islands, and will not tolerate the presence of others. Following a 2-month incubation period during which the female won't eat, the eggs hatch. The mother will protect her young for a month, before abandoning them. The juveniles spend much of their time on land feeding on just about anything they can catch to fuel their rapid growth. In as little as 5-7 years, they're too large to hunt on land and will become fully aquatic hunters, and may reach adult size in as little as 12-15 years, though very, very few actually survive till then.

Male drakes by comparison will rarely come ashore, only doing so to rid themselves of the millions of parasites that they've picked up during their time in the water. A landed titan drake therefore bears a bountiful feast for seabirds and other small predators, so much so that it is possible to locate one simply by following a larger than normal flock of them. As with other dirosuchids, titan drakes can stand bipedally, using their tails as a support, but while juveniles can run this way, adults are simply far too bulky to manage anything more than a very slow amble.

 
The Draconimorpha