Sealacanth

As the sun sets and the waters calm down, a strange creature hauls itself onto a beach. Its limbs end in fins, yet they already have joints which foreshadow elbows and knees. Its two dorsal fins now hang to the sides, useless outside the sea. It breathes heavily, almost strained, but it is doing better than the other water-dwellers that have been swept here, dead ammonites, clams and fish, which have been crushed by the gravity and heat. It succeeds where others fail. Beyond the beach lies a lush rainforest, filled with big invertebrates. To the victor will go the spoils.

Only the presence of scaly, bipedal creatures on the beach and the distant cries of seabirds betray that this is not an ancient scene from the middle of the Devonian period. This is the very modern life of a particular fish on the island of Ryl Madol. The sealacanth (Erpetolepis littoralis) looks like an ancient holdover but is in many ways also something quite new, perhaps among the most advanced of the walking fish that live on Earth.

Despite the name and appearance, Erpetolepis is not a close relative of the coelacanths which also swim around in the Indian Ocean. The presence of internal nostrils and labyrinthodont teeth identify it not as an actinistian but as a tetrapodomorph, though closer to basal forms such as Eusthenopteron than to more famous fossils like Tiktaalik or the true tetrapods. Its many adaptations towards littoral life seem to be parallel developments. Unlike archaic lobe-finned fish, the sealacanth has clearly defined limb-joints, though not yet true fingers, instead just a flexible fan of lepidotrichia like other fish. It is a very subtle difference, but the shoulder-girdle is also not fully connected anymore to the skull, allowing it to move its head independently of the body, an important development which in true tetrapods led to the evolution of a true neck. It also sports fully developed lungs in addition to gills.

Actual seals are largely absent from the coastlines of Ryl Madol, on account of the presence of the toxic spores. While there is no shortage of aquatic and semiaquatic amphibians and reptiles on these beaches, very few of them have seemingly managed to utilized the resulting opportunity quite as successfully as these clumsy-looking fish, earning them their name. Like pinnipeds, sealacanths hunt in the shallow and open waters for food but when they want to sleep and rest, they come onto land and sometimes even sleep in groups. Though gregarious, they are not exactly social, as the lack of ears limits their ability to communicate. Towards human explorers they are largely apathetic, though some degree of curiosity is reported on occasion.

The prey of sealacanths consists of slow-swimming or crawling invertebrates and small vertebrates, largely fish, lizard-sized marine reptiles and washed-up carcasses. Sometimes sealacanths can be observed digging up the rocky nests of Spheniscodapedon, the penguin-like reptile seen here in the background, in order to feed on their eggs. This does not always go well, as these “rhynchoguins” tend to guard their nests and have pretty mean hooked beaks, more than capable of ripping chunks of flesh out of even a large fish such as this. Though it is larger and stronger, if a flock of these reptiles gangs up on a sealacanth, they can very easily rip it to shreds. The further the fish crawls into the island, the more dangerous the threats become and so too the farther out into the sea it swims. Perhaps being stuck, ecologically and evolutionarily, on the water’s edge is good enough, at least for now.

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.