Aistoconstrictor

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There are no snakes on Ryl Madol. In their stead slither other, more ancient creatures through the underbrush. Most numerous among these legless vertebrates are aistopods, an ancient lineage of stegocephalians. These were the very first tetrapods to completely lose all of their limbs and in these primeval jungles they have further converged on the serpents that replaced them elsewhere in the world.

Largest among them is Aistoconstrictor latagnathus, which, it has been reported, can grow up to nine or ten metres long, larger than any anaconda. Most individuals are smaller though, usually maxing out at around six or seven metres. Despite technically being an amphibian (in the classic paraphyletic sense), Aistoconstrictor shares many characteristics with actual constricting snakes, such as boas. It kills and captures small prey, such as lystrodos, by biting their head and then ensnaring them with its body, crushing the poor victim under its weight.

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Looking at the skull we can see a combination of both old and new. Overall, the cranium still bears great resemblance to ancient aistopods such as Phlegethontia, with large orbits, large fenestrae and a light construction at the back of the skull that gives the jaw-joints a larger range of movement. Differing from ancient aistopods, the “aistoboids” evolved an extra jaw-joint in their lower jaw, allowing the dentary to articulate with the surangular bone. Like in constricting snakes this allows the predator to “shove” prey down its gullet with its teeth by moving the lower jaw back and forth. Unlike in snakes, the mandibles are still connected at the tips, so they cannot open as widely.

Click to enlarge. Life stages are not drawn to scale.

Being a stegocephalian, likely of the reptiliomorph variety, Aistoconstrictor develops from an aquatic larval stage. It does not raise its young on land in burrows like a giant caecilian, as older textbooks have often wrongly stated. After internal fertilization, the eggs are instead laid into a breeding pond, which the mother often guards until hatching, as some toads are known to do. Upon hatching, the larvae emerge as little eel-like creatures with surprisingly long external gills. At this stage they bear a great resemblance to the larvae of caecilians, though this is surely a coincidence. These aistoboid larvae feed on aquatic insects and algal scum before they grow in size and enter the next life stage, the “spade eel”, named after the shape of its snout. Its gills have shrunk and become covered by a soft skin-flap, while its swim-bladder has expanded into a simple lung, allowing it to breathe both in and out of water. The spade eel lives much like a predatory fish, feeding on many smaller vertebrates by use of ambush attacks. They are also surprisingly gregarious, often swimming in small swarms for protection. Unlike the sub-adult stages of some other Rylian reptiliomorphs, spade eels cannot become reproductively active and are always destined to grow into fully adult “aistoboids” once they lose their gills and live on land.

As a human can in some ways resemble other bipedal animals on Ryl Madol like the lystrodos or eubolosaurs, it may not come as a surprise that stalkers and other explorers are frequently attacked by Aistoconstrictor and relatives, whose coloration conceals it behind the underbrush or lianas. While the creatures are capable of killing people through constriction, human shoulders are usually too wide to fit through the jaws, so the beasts tend to give up after the head and just leave behind a mangled corpse for the scavengers.

Marine Centipedes

Myriapods have a long history, stretching all the way back into the Palaeozoic, though they would not become truly successful until the Age of Dinosaurs. That success seems to have been repeated on Ryl Madol, attested to by the island’s diverse fauna of millipedes and centipedes found nowhere else on Earth.

Among these are the marine centipedes, Platychilopodidae, which inhabit the waters surrounding the island. Living their whole lives at sea, their bodies have become streamlined, their legs became flattened flippers and their caudal legs form something that could be called a tail fluke. They even evolved so-called blood lungs inside their tracheae, a trait otherwise only known from aquatic insect larvae.

Chilocaris venefecus is one member of this family. Its preferred prey are other arthropods and small agnathan fish, such as neothelodonts, which it captures with its large jaws and injects with venom. It itself is preyed on by large fish and marine reptiles and amphibians. As many would-be explorers have painfully found out, if the “marinopede” cannot defend itself with its venom, it will use its “tail-fluke” as a pincer.

Among the Platychilopodidae, C. venefecus is one of the smaller species, growing about 20 centimetres long. It is far outdone by the horrifically large Con Rit, Cetioscolopendra aeliani.

Cancrichthys

Ryl Madol teems with all types of amphibious life in its many waterways. This includes actual amphibians, armoured proto-reptiles, various arthropods and even some derived sarcopterygian fish. But out of the swamps can sometimes also crawl a quite peculiar creature, which seems like a wild mix of all of the above. Cancrichthys zemani, whose name literally means “crabfish” is a placoderm, a member of an ancient group of armour-encased fish which went extinct elsewhere on Earth during the end of the Devonian period. More precisely, it seems to be an antiarch, unmistakenly related in some form to ancient fossil genera like Pterichthyodes or Bothriolepis.

What earns it its name are its unique limbs. Although, when viewed from the side, it appears as if it has six crab-like legs jutting out from the body, these are actually all attached to a main “fin”, a stylopodium. It therefore technically only has two legs, but this does not diminish the fact that no other type of vertebrate has such limbs, as it would correspond to a single upper arm/humerus out of which multiple lower arms sprout.

When on land, Cancrichthys breathes using a simple lung, which gives credit to the controversial idea that fish like Bothriolepis already evolved this trait independently of the ancestors of tetrapods. It seems therefore possible that these ancient placoderms were already somewhat amphibious creatures sifting through mud in lakes which regularly turned oxygen-poor. Evading extinction on what would become Ryl Madol, time could have put more selection pressure on the limbs to become more capable of terrestrial locomotion, in order to travel between ponds and lakes more easily. As antiarchs like this had no autopodium (what becomes the hand in tetrapods), evolution had to make-do with what was available. Even if unique, the mutation that leads to Cancrichthys’ odd limbs may be similar to the hox-genes that split the tetrapod autopodium into fingers, but instead applied one level higher to the zeugopodium. As Placodermi are a paraphyletic grade from which all later bony fish, including us, likely descended, it does not seem implausible that the genetic tool-set for this could have already been present or re-evolved in these fish.

Cancrichthys leads an unassuming life, most of it spent plying mud for invertebrate prey, such as worms, snails or freshwater trilobites. On land, its carapace gives it perfect protection against most predators. If that is not enough, the fish has sometimes been seen striking at threats with its tail. Like many of the fossil placoderms, Cancrichthys reproduces through internal fertilization and gives birth to live young. This is in a somewhat humorous contrast to many of the more terrestrial, reptile-like creatures of the island that have to lay their eggs in water.

Arachnosaurus

While giant spiders are unknown here, arachnophobes are still not safe on Ryl Madol. High up in the canopy lurks a creature appropriately christened Arachnosaurus nychuros. Its prehensile tail makes up over half the body length and is used to lower the animal head-down into the air like a lantern.  The arms and legs are strong and built for climbing, like a chameleon's. The hindlegs are relatively short and tucked close to the body, but the toes are disturbingly long, splayed and tipped with curved claws. Cryptically camouflaged among the vines, Arachnosaurus hangs in wait for hours with its feet stretched out waiting for prey. This may include insects even smaller flying reptiles that happen to fly through the jungle, though the predator also seems to be content with arthropods and “lizards” that happen to waddle a few tree-branches below. Once in reach, Arachnosaurus envelops its prey in a deadly hug it cannot escape from and uses its pointy mouth to feed on it while it is still alive. After the first few bites the prey-animals seem like they are paralyzed, suggesting the use of some type of venom, but autopsies have so far been unable to find venom glands in Arachnosaurus, making this rather mysterious.

Arachnosaurus are surprisingly gregarious, often hanging from the trees in medium-sized groups like bats do, though they do not cooperate when it comes to hunting or fending off predators. They make no audible sounds and instead seem to communicate by changing the color of their headcrest.  Mating usually entails the male presenting the female with a gift in the form of food. After mating, the female builds a nest in a hollowed-out tree-hole and lays its eggs there. The mother guards the nest and even leaves some food behind shortly before the eggs hatch, but once the young emerge they are left to themselves. 

Unlike the anamniotes of the forest floor, Arachnosaurus is a true reptile that lays hard-shelled eggs, though its exact affinity remains uncertain. It appears to descend from some group of basal diapsids, perhaps Araeoscelidia similar to ancient Araeoscelis and Petrolacosaurus.

Basilosuchus

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Huge, ferocious and carnivorous, Basilosuchus imperiosus, though rare in its adult form, is a creature feared across the whole island. Here we see it, among the ruins, feeding on a dead Megarhinosaurus, a poor dinocephalian that was probably slain by the giant predator.

Basilosuchus is a bipedal creature, often walking with a horizontally held spine, with prominent armour plates along the back. Its tail ends in a flattened paddle, painted with a prominent spot probably used for social signalling. The three horns on the skull may also serve a similar purpose, though they might also be used for interspecies territorial fights.

Click to enlarge. Life stages are not to scale.

The most fascinating aspect about this creature is its life cycle. Despite appearances, Basilosuchus is not a dinosaur nor even a true reptile. It is an “anamniote”, a reptiliomorph stegocephalian, more closely related to us than to true amphibians, but still outside the Amniota. As such, while it has many reptilian characteristics, it still lays its eggs in water, where they hatch into tiny, gill-bearing tadpoles. While it seems archaic, this has allowed Basilosuchus and many other Rylian anamniotes to evolve a complex life cycle which prevents the juveniles from competing for the same limited resources of the island as the adults.

After the tadpole loses its gills and grows its hindlegs, it turns into a large amphibian with simple armour-plates along the back. This stage strongly resembles creatures from the fossil record known as Chroniosuchia, which may show where the origins of Basilosuchus lie. This stage lives as a crocodile-like ambush-predator in swamps and rivers.

After some time, the hindlegs of the chroniosuchoid grow longer and the limbs are tucked underneath the body. The jaw becomes more crooked and horns start growing on the snout. This new “suchoid” stage leaves the water and starts living inland, especially the cluttered rainforest, where it preys on smaller animals. This stage is morphologically and ecologically perhaps most comparable to one of the extinct land-crocodilians.

With time, the “suchoid” increases in size, the hindlegs become longer, more robust, and it starts walking on them more and more until it becomes fully bipedal and ventures into the open forest galleries and fern prairies. This is generally where adult life begins. But, fascinatingly, depending on environmental conditions, both the chroniosuchoid and suchoid stages can already become reproductively active, at which point they actually remain in this stage for the rest of their life. “Juvenile” Basilosuchus are thus encountered far more often than the ferocious adults and take on their own specific roles in the ecosystems they inhabit.

Gorgoraptor

 Although true dinosaurs are sadly absent from Ryl Madol, many similar-looking creatures have convergently evolved in their stead. Among them are the lycaenoraptors, of which Gorgoraptor pilosus is a member. Though bipedal and digitgrade like theropods, the saberteeth and faint hairs along the back show that these are in fact some kind of therapsid, descending from the likes of gorgonopsians, therocephalians or perhaps even cynodonts. 

Using their great sense of smell and decent eyesight, these small-to-medium-sized predators opportunistically stalk the meadows and forests in search of prey. While its smaller relatives live in packs, Gorgoraptor often hunts alone. When it captures prey like lystrodos, it pins them down with its powerful legs and arms and deals the killing blow by severing their windpipe with its characteristic saberteeth. It itself can fall prey to the much larger Carptophoneus, the anteosaurid(?) "tiger" of the island, and of course the titanic Basilosuchus and Decarnodon, anamniote reptiliomorphs, the latter of which could give even large theropods like T. rex a run for their money.

As, unlike true mammals, these synapsids still have full colour-vision like other amniotes, they can often be found with striking patterns and colour schemes. Apart from charismatic stripes, the mane along the neck and back can also shine in bright colours. Perhaps, long ago in the distant past, this is how hair first got its start, not as a means of insulation but as a display feature.

Didontornis

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In the horsetail meadows of Ryl Madol, a strange fellow can be encountered, A biped, both feather- and furless. But, contra Plato, it is not man, nor even really mammal. Although now heavily altered and resembling more a naked, flightless bird than anything else, the two small tusks growing behind the beak still give away the true ancestry of this Paleozoic survivor. It is a dicynodont therapsid which has learned to walk on two almost humanoid legs. With these, as well as its horizontal pupils, it is well-equipped to evade the predators which may lurk among the tall seed-fern-flora. Yet, these little herbivores still tend to fall prey to the likes of lycaenoraptors and aistoboids.

Didontornis is a small browser that feeds on horsetails and other shrubby plants. These are cut off with the sharp beak and grinded up in the stomach using gastroliths. Only males have the distinctive tusks, which made classification of this species initially difficult when the first encountered specimens were females. The tusks probably serve as either a weapon or display feature during fights for mating-rights. The arms and legs are short and small and rarely used during foraging. When two didontornes get into a scuffle, they sometimes try to slap each other, which can look adorable.

Didontornis is just one member of a wider group of bipedal dicynodonts, the Diornithodonta, which on Ryl Madol have taken over a similar ecological role as ground birds and fowl would do elsewhere in the world. Their common name is “lystrodos”, a portmanteau of Lystrosaurus and dodo.