Castorosaurus

Ryl Madol is able to support a large amount of amphibious and aquatic life thanks to its many pools, ponds and streams which pay tribute to large lakes and rivers. For many of the tetrapods and arthropods, these are vital for their reproduction, as their eggs and larvae lack shells and need to be laid in water. However, many of these breeding pools are not necessarily provided by the island topography. They are made by the wildlife itself, though, ironically, not by amphibians.

Encountered in almost every waterway are waterlogged wooden structures, which early explorers at first mistook as as bridges or fish-traps created by the island’s intelligent natives. But no. These are dams built by an aquatic reptile called Castorosaurus, whose name literally translated to “beaver-lizard”. Castorosaurs are derived members of the Captorhinidae, a group of Palaeozoic reptiles that, on account of missing postorbital windows, were traditionally grouped with the “Anapsida” alongside the turtles, though recent work has recovered them as being eureptiles close to the Diapsida. Even more recent work has declared them stem-amniotes, though this remains controversial.

Just like beavers, castorosaurs are ecosystem-engineers who prolifically build dams along narrow waterways, creating many pools throughout the island in which other organisms can thrive and reproduce. Their dams usually have a hollow chamber that can only be accessed from underwater. Here the castorosaurs lay their eggs and raise their young. The dam structure prevents terrestrial or aerial predators from easily gaining access to the nest, though the parents still tend to be on the look-out for any of the creeping amphibians that may dive and slither below.

Unlike the terrestrial captorhinids of the Palaeozoic, castorosaurs feature many aquatic adaptations such as webbed feet and a dorso-ventrally flattened tail like a platypus. This tail-shape is of course quite curious, as reptiles usually swim through sideways undulation and so one would expect a laterally flattened tail, but the tail seems to be mainly used as a sort of hammer in dam-building. What castorosaurs have retained from their ancestors are the long, kinked snouts with specialised teeth at the front. Said teeth have become quite large and spiky. Many people easily make the mistake of assuming that castorosaurs used their large front-teeth just like beavers to gnaw at wood, but that is in fact incorrect. They are entirely unsuited for such a task and are instead used to capture fish.

If they cannot gnaw at trees and branches, where do castorosaurs get the wood for their dams then? The answer is that they simply pick it up from the forest or lake floor and so do not need to saw up fresh wood. But why it is that simple for them is actually interesting. Vast areas of Ryl Madol are covered in thick carpets of wood-litter that degrade into smaller pieces over time as animals trample over them, but never truly rot and decay. Some pools and moors have as a consequence devolved into large, malignant peat bogs in which unwary animals may stumble and get trapped. It is highly unusual and not something seen in any other forests in the world. Only recently has the reason behind this been revealed by botanists and microbiologists: Ryl Madol entirely lacks any organisms, be they bacteria or fungi, that can digest lignin, the main polymer that makes up wood. This used to be true for all of Earth in the Carboniferous period, from which huge coal deposits still survive. While it explains the primordial nature of Ryl Madol’s forests, it just opens up many more questions. Why has lignin-digestion never evolved or arrived on Ryl Madol? Or did it exist here once but the respective microorganisms have since somehow gone extinct? The lack of notable coal deposits in the island’s geological formations may be evidence of the latter, pointing towards this being a fairly recent development. It seems almost like time on the island is moving backwards.

Walky Tully

Ryl Madol is already home to a variety of animals which by all rights should not exist, at least not anymore, but even among them the walky tully (Micrormus holidayi) stands out, as it has evaded proper classification to this day.

What really can be said about an animal whose jaw sits at the end of an extendable proboscis, has eyes on stalks like a snail, a body like a tadpole and three clumsy legs? It has a backbone, so at the very least it can be considered some kind of vertebrate, but that is where consensus ends. Unlike any other vertebrate, there is no direct bone-connection between the jaws and the cranium, the “neck” is just a floppy tube made of cartilage and muscle, attached to what resembles the hyomandibular bone of sharks. Despite living in water, it has lungs but no gills. Its “fingers” have no resemblance to those of tetrapods, instead having evolved from fin-rays, the internal anatomy of the forelimbs somewhat resembling the alternating bone-structure of lungfish-fins. The single hindlimb is located behind the cloaca, meaning that it is possibly homologous with the anal fin found in most fish. However, in the vertebrae, the pleurocentrum dominates, which is a trait more typical of stegocephalian tetrapods than of fish.

With its proboscis and stalked eyes, many researchers have obviously tried linking this animal to the notorious fossil Tullimonstrum gregarium of Illinois, which is where its common name comes from. However, the classification of Tullimonstrum is itself controversial, as it is not even clear if this organism was a vertebrate or an invertebrate of some kind. If Tullimonstrum was a vertebrate, it would have been one of the most basal cyclostome kinds, a relative of lampreys and hagfish. It notably has no fins whatsoever, at least none that were ever able to be identified from the fossils. If Micrormus is indeed a descendant or close relative of the tully monster, then it must have evolved limbs, lungs, a loss of gills and various other characteristics that are not present in its Carboniferous ancestor independently of other vertebrates.

This suggests that the resemblance is merely due to convergent evolution and that Micrormus is some kind of highly aberrant bony fish, possibly sarcopterygian in origin. Due to its amphibious characteristics, the most radical proposal has been that it may descend from some type of tetrapodomorph that, like the coelacanth, still possessed a muscular anal fin but for whatever reason had lost its pelvic fins, which in true tetrapods evolved into our hindlegs. This hypothesis is not at all popular, but other suggestions have not been less crazy, such as the idea that Micrormus is a vertebrate-mimicking cephalopod or a relative of the dancing worms of Turkana. Genetic studies that could shed more light on the matter are unfortunately lacking.

In contrast, the actual life habits of the walky tully are surprisingly unspectacular. It is a small animal, able to fit comfortably inside a human hand. Like most fish on the island it has developed an amphibious lifestyle and spends a lot of time crawling around or even sleeping on lake shores. When “walking”, the single hindlimb is not used merely as support for the forelimbs but also helps the animal push forward, earning it the alternative name “mud-tripod”. In the water it hunts smaller fish and tadpoles, such as the mantiraptor larva seen here. Some researchers have proposed that it actually is a specialized tadpole-predator, but there is no conclusive data that it prefers this prey over any other small aquatic critters. Among its own enemies are various stegocephalians, predatory fish and the stork-like gruisaurs.

Tullys reproduce through external fertilization and lay their spawn inside protected alcoves along riverbanks. The young hatch as miniature adults without first going through a larval stage, which is why gleaning its evolutionary history from embryological data has also proven difficult.

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.