Mantiraptor

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A reptilian brushes through the thicket. Small and lithe, standing on three-toed legs, the little herbivore seems like a mix between a lizard and a naked bird. This eubolosaur mindlessly nibbles on some ferns when, suddenly, two large, slimy arms materialize out of the forest, clamp down and pick it up. The prey’s futile attempts to flee this deadly grasp only dig the huge keratinous spikes and hooks lining the arms deeper into its flesh, as two-clawed paws curl around its belly. Well-placed bites to the back and neck soon end the bleating cries.

The ambusher is Mantiraptor olsoni. Though it also appears like a two-legged lizard, it shares little else in common with its prey. Its skin is smooth and clammy and it has no claws on its feet. Instead of an earhole there sits a large tympanum behind the skull, like a frog’s. This man-sized forest monster, like many others on Ryl Madol, is an anamniote, not quite a reptile, but also not quite an amphibian.

The retention of metamorphosis in these once-archaic tetrapods has allowed for some quite impressive plasticity in the development of the limbs, both through phylogeny and ontogeny. Unlike any reptile or mammal, this predator has been able to modify its forelimbs through the fusion and extension of the metacarpals to such a degree that they now resemble the raptorial arms of a mantid insect. A perfect weapon for lying in wait and surprising the small reptiles of the forest floor.

These arms and the ambush-tactics they facilitate are also all that the mantiraptor has in its favour. Despite its superficially dinosaur-like appearance, it is neither fast nor agile. Its metabolism is slow and it can spend hours, maybe even days, standing or sitting completely still, hidden behind bushes and ferns thanks to camouflage, its breaths so shallow that they do not even create visible movements in the chest. Only when some sensory stimulus occurs do the arms, fueled by a short burst of anaerobic energy, lunge forward and primal instincts kick into gear, mindlessly holding onto whatever has been caught and methodically disassembling it with the crocodilian maw. Once its belly is filled, it seeks shelter and goes to sleep for long periods until hunger strikes again. Its name may evoke associations with the smarter dromaeosaurs of the Cretaceous, but the mantiraptor’s brain is about as complex as a frog’s, if not dumber. While such a robotic being may seem easier to handle, it is exactly this mechanical mind which makes it so dangerous. Whereas smarter predators may hesitate to attack a new animal they do not know, for they may rightfully sense that something unknown may be dangerous, the mantiraptor will disassemble a human just as much as anything else that walks in front of its cold eyes.

Compared to other mantiraptor species, Olson’s mantiraptor is unique for still having a more “classic” style of reproduction. The larvae develop from waterborne eggs and go through an aquatic phase where the raptorial arms develop first in order to feed on small fish and the tadpoles of other anamniotes. Mantiraptors do not go through multiple distinct sub-adult morphs like other anamniotes on Ryl Madol. After the hindlegs evolve and the gills are reabsorbed, the young quickly leave the water and come to resemble miniature adults. More derived species, like Thylacosaurus, have shortened this life cycle even further by carrying around their eggs and larvae in a brood pouch on the back until they can walk by themselves, a method which resembles the marsupial frogs of Australia.

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.

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.