Mesotylos

I remember still one of the very first expeditions I undertook on Ryl Madol. I was still a young researcher, given rather mundane tasks. There was an area close to the coast that was designated as relatively “safe”, where I routinely documented the local population and diversity of carnivorous cycads (yes, the plant, not the insect). Technically it was a task I could have done by myself, I knew my way with a gun in case any dangerous animal might have appeared, but, of course, the research base did not fully trust me, so they signed up one of the hired mercenaries to escort me each time I went. It was pretty simple. We donned our gasmasks, took a small motorboat out from the base islet and drove it upriver to the research area. Most days were quite peaceful. Most of the big, scary things live further inland and most of the aquatic life gets scared by the sound of the boat. The guy, Johnson, was sadly not that talkative. He seemed rather annoyed to be my “babysitter”. He often just sat there on the boat, gun in hand, and quietly watched me as I walked around on shore and studied the plants. The plants, you ask? Oh, they’re only dangerous if you are a bug. Ryl Madol may be full of biological wonders but even it doesn’t have man-eating plants straight out of Hollywood. At least as far as we have observed.

So, there I was one day, counting cycads (or were they benettitaleans?), when, absentmindedly, I moved a little outside the research area and, out of the view of Johnson, deeper into the forest. I just roamed around a little when, suddenly, three men walked out of the underbrush and circled me. Their faces, like mine, were obscured by gasmasks to protect against the toxic spores. They looked rugged and they carried high-calibre weapons. One of them, visibly unwell, had an insanity fish still stuck to his neck. I’m not even sure he noticed the death sentence, perhaps he was this numb from all the pain he had already endured. They clearly were not my guys. They were stalkers, come here to raid the island for artefacts. Sometimes they kidnap or take hostage unsuspecting scientists, thinking that we have more secret knowledge on where to find the good stuff. Today it seems it was my turn. I tried to cry out for Johnson, but one of them put a gun to my head before I could even raise my voice. I knew I’d be dead if I screamed. They asked me if I knew where the “Dream Pool” was. I did not know what that meant and to this day I still do not. But of course they thought I was lying and began to beat me up. My mask nearly broke. In that moment, out of the corner of my eye, I saw something moving between the trees. It was some sort of translucent shadow, either camouflaged perfectly or nearly invisible. But I could sense some kind of movement between the tall cycad trees. Out of raw nature itself, it seemed, two hooked arms materialized and pierced through one of the stalkers, lifting him into the air like a praying mantis would a fly. My first thought was some kind of mantiraptor, but we have never seen those being able to change their colour. Though maybe that is exactly the point. The poor guy, claws pierced straight through his shoulder and belly, screamed like hell. He tried to lift his gun and shoot wildly around. I don’t know what his plan was, he was probably not thinking at all in the moment. By a mere miracle he missed me and instead it was now one of his stalker buddies that ended up screaming and bleeding out on the forest floor. The third guy lifted his gun and tried to shoot at the invisible arms puppeteering his friend, but before he could even pull the trigger, something else grabbed him and dragged him silently into the jungle.

The next moments are blurry to me. For unknown reasons I was spared by the ambush and was able to run back to the river. Johnson was sitting there, waiting for me. It was as if nothing had happened. He sat there, upright, with his wannabe military-uniform, oar of the boat’s motor in one hand, the other hand on the pistol holster, ready to pull on any unsuspecting foe. But there was just one problem. His head was gone. Ripped clean off, no trace of it in sight. I don’t think I even saw any blood splatter on his clothes, at least not in the heat of the moment. I did not really have the time to think about how morbidly bizarre that was, I just pushed the boat back in the water, pushed his body out of the way and started the motor. Downriver I think I took a few wrong turns and suddenly ended up in some quite bad, rocky rapids. I crashed the boat and flew overboard. All I can remember next was floating out at sea, not too far from the coast. The current must have swept my unconscious body out. I took my mask off to get enough air.

As I was contemplating my options, perhaps trying to swim back on shore, that wass when I saw it approach me. The huge shadow of a Mesotylos, a nearly 10 metres long carnivorous marine reptile, slowly drawing near me from beneath the waves. I could see it undulate up and down through the water, swinging its large, wing-like fins up and down, like a humpback whale. The long, crocodile-like maw turned towards me and I just knew I was done for. There was no way for me to survive this encounter. I just leaned back in the water and pretended to be a corpse, in the faint hope that it might lose interest. 

The strangest thing then happened. I could feel it poke me with its snout, me doing my best to remain stiff and not panic. I could feel it nudging my body. But it did not attack. It placed its head underneath me and then gently lifted me above the waves. I could feel it breathing through its retracted nose and I sure as hell know that it felt me breathing too. But that is all it did. Lifting my body out of the water and just gently float there with me, for what felt like an eternity.

Eventually I heard a boat approach and, thankfully, it was one of our own research vessels. As the sound drew closer, the reptile sank into the depths again and left me alone. The crew had seen me and were able to pick me up right after. To this day I still think about what happened. Why am I not dead, a pile of half-digested bones at the bottom of the ocean? Was the animal just not hungry? Did it not recognize me as food? Was it just curious? I cannot help but be reminded of those old stories of sailors being saved from drowning by dolphins.

As the name implies, Mesotylos is a mesosaur, as far as could be gathered from stranded specimens. Yes, a mesosaur. Not a mosasaur. Mosasaurs were true marine lizards of the Cretaceous, whereas mesosaurs were much more archaic beings of the Permian period, the very first marine reptiles (though in some phylogenies they even end up on the synapsid side of the amniote family tree). In a world as stuck in the Palaeozoic as Ryl Madol, it is perhaps no wonder that such animals survived here and continued to evolve. Very obviously, Mesotylos and kin have managed to make the full shift into fully marine life, as have many amniotes that came after them. Unable to move anymore on land, they have completely reduced their hindlegs and their forelimbs have become elongate flippers perfect for “underwater-flying” as is also seen in penguins and the extinct plesiosaurs. Uniquely for any amniote, this style of underwater-flying is combined with a vertically undulating, horizontally flattened tailfluke, very much like that of a manatee. Also very strange is that each flipper still retains two prominent claws, something not seen in any other fully marine reptile or mammal. Perhaps these do serve a function in mating, or maybe they have some hydrodynamic effect. Speaking of mating, their reproduction remains unknown, though it is generally assumed that they give live birth, unlike their egg-laying ancestors. Based on the shape of the very crocodile-like snouts, it is also generally assumed that they feed on fish and various other aquatic vertebrates, which makes my survival all the more miraculous.

Apart from this, nothing more can be said about these mysterious animals, as they are rarely encountered in life. Cryptozoologists like Bernard Heuvelmans have suggested that Mesotylos may be the origin behind various sea serpent sightings all around the Indian Ocean and Southeast Asia. While that idea may at first seem compelling, there is so far no solid evidence that the unique fauna around Ryl Madol’s coast has ever ventured to other areas of the globe, unless by human means. It is as if some strange force binds the animals to the island.

Regarding the little “reef friends” I depicted the fellow with: On the very right we can see one of the many strange brachiopods that inhabit the island’s waters. Its long stalk makes it resemble early Cambrian stem-forms, like Yuganotheca, however this appears to be a case of convergent evolution, as it is found to be a true rhynchonellid.

Emerging below it from a small tunnel is a Dermichthys, nicknamed “Dunkeelosteus”, a small antiarch placoderm with an elongated body that lives much like a moray eel, hiding in reef burrows and ambushing prey. Swimming below it is one of the many marine centipedes that swarm the island’s waters, as well as an ophiuroid brittle star.

More interesting is the Arboreaites and her offspring on the lower left. All research indicates that this seems to be a bona fide arboreomorph, a type of Petalonamae from the Ediacaran or Late Precambrian. This makes this unassuming frond actually one of the most interesting organisms in this picture, as it is the most ancient organism on show here. Tests done on these organisms have validated many hypotheses that had already been suspected about the fossil relatives: 1) They are indeed true, diploblastic animals, however outside crown-Eumetazoa. In other words, more derived than a sponge, but still more archaic than a jellyfish (though some genetic tests intriguingly recover it as somewhere near the base of Ctenophora). 2) It is in fact a colonial organism composed of multiple zooids, which together constitute a holdfast, a backing sheet and the many lobed filter-feeding “pods”. The central stem itself that is keeping everything together it in fact a bundle or fascicle of stolons protruding from all the pods. Very intriguingly, the individual zooids of Arboreaites, stuck on the inside of each pod, resemble miniature versions of the solitary rangeomorph Charnia. As can be seen Arboreaites can reproduce asexually using stolons along the ground, very similarly to bryozoans anc cnidarians. However, sexual spawnings have also been observed, where the organisms release eggs and sperms into the water. Even though the organisms lack true gonads, they appear to be able to produce these gametes from undifferentiated body cells, much in the manner of sea sponges.

It is extremely intriguing that Arboreaites survives and even thrives around the reef regions of Ryl Madol, despite plenty of competition with supposedly “better” Palaeozoic life. This brings into question many leading hypotheses about the extinction of these organisms elsewhere in the world during the Cambrian. One wonders what Earth’s oceans might look like if more of these had survived. Now that would be a fun scenario to think about for the enthusiasts of this new-fangled gene I heard of. I think they call it “speculative evolution.”

Neothelodonts

First appearing during the Late Ordovician, thelodonts were once one of the most successful groups of jawless fish, their distinctive scales being one of the most common index fossils for the Silurian. But the fate of any group that is successful is that, at some point, they simple are not anymore. Thelodonts vanish from the fossil record by the Late Devonian, along with most other jawless fish.

Such is not the case on the lost world of Ryl Madol though, where small thelodonts still swarm the waters of the coasts and rivers. Their success story seems to have continued here, where, despite still being jawless, they have managed to expand into all kinds of ecological niches.

Original monochrome

While Thelodonti as a whole may have been a paraphyletic group, perhaps a transitional grade between Heterostraci or Anaspida on one side and Gnathostomata on the other, most or all of the Rylian thelodonts seem to descend from the monophyletic group Furcacaudata. As the name implies, they are all unified by their characteristic tailfin. It is composed of a forked bone, from which sprout multiple, rigid rods, each suspending a membrane, a bauplan not seen in any other fish. Apart from this trait, Neothelodonti, as this island group has been named, varies a lot in morphology and appearance, making their internal classification unclear. A few have paired fins, though it is unclear if these are homologous with the pectoral fins of other fish. It is also unknown if those that lack these fins completely lost them secondarily or never had them in the first place. The same goes for dorsal fins. A few, in lieu of true jaws, have evolved a radula-like tongue, though it is again unclear if this is a shared or convergent trait.

The perhaps most unchanged of the neothelodonts is Aureocauda rylensis, which looks virtually identical to relatives like Furcacauda from the Devonian, except for the fact that its mysterious paired fins are more developed. It can be found suspension-feeding at various water-levels off the coastal reefs.

One of the largest neothelodonts is Biceratopsis albus, a hand-sized, triangular fish with two rhinoceros-like horns growing from its head. It is unclear what these are used for, as they do not seem to be a sexually dimorphic trait. This rhino-fish sports one of the least unusual mouths of the neothelodonts and as such it is most often encountered suspension-feeding in the open water of the reefs.

This is in contrast to the smaller Proboscichthys hippotigris, who is another reef-dweller, but one that mostly swims close to the ground. Its mouth forms a downturned nozzle or trunk, which it uses to sift the soft mud for any detritus that has fallen down from the upper water layers.

Highest in the water-column feeds Gleptodus ridens, in which the furcoid tailfin makes up over half the bodylength. This seems to make the body rather stiff and spiny and in turn also harder to swallow for predators. Also unique for a thelodont is that it has evolved a sort of expanded lower lip, giving it the appearance of a fake jaw. This seems to be an adaptation towards filter-feeding on planktonic algae floating close to the water’s surface.

Swimming at about the same height is Neothelodus homocercus. It is a fast swimmer which mainly feeds on zooplankton, such as copepods and brachiopod larvae. Due to its elongated shape, strongly contrasting with the other neothelodonts, it has been proposed that it actually falls outside of that clade, not being a furcacaudatan but instead being more closely related with the other major Palaeozoic clade Thelodontiformes.

Levicercus rufus is an adorable little fellow that shares some behavioural traits in common with clownfish, mainly the fact that it lives in symbiosis with local sessile cnidarians, including not just anemones but also rugose horn corals (another Palaeozoic survivor of the island). The fish is immune to the their nettle-stings and uses their tentacle-crown as a sheltered home. In turn, it feeds on small radiodont and trilobite parasites that might attack the anemone. On occasion it has even been observed guarding its home against marine dicynodonts, who, just like sea turtles, can feed on the cnidarians with their hardened beaks. Being jawless, Levicercus’ main method of attack against these giants mainly lies in being as annoying as possible, harassing the eyes and face of the synapsid. Unlike clownfish, Levicercus are not social and do not form hierarchical family groups. Each horn coral instead seems to be inhabited by only one or two of the fish, indicating some territoriality.

Hypersubcercus aureus shares a similar association with corals, though in this case living between them rather than in them. It is also more commonly found among tabulate corals, as well as strophomenid brachiopods. With its radula, it mainly feeds on the algae that grow on the shells of these invertebrates. The fish are surprisingly territorial over their grazing grounds, using their hyperelongated lower tail-lobe as striking weapons against unwelcome intruders.

Echineidion ramjeti is comparable to a remora, using its torpedo-like shape to follow and trail behind larger marine animals, be they sharks, acanthodians or mesosaurs, in order to feed on their parasites and loose dead skin with its downturned mouth. It is one of many finless thelodonts, though it is uncertain if these form a unified group. Its shark-like, pointy nose is unique among thelodonts.

Another commensal feeder is Capronia arcuata, which is also finless. Instead of chasing other animals for their waste, it waits for them to come to it. A mated pair of Capronia usually “set up shop” at their own territory in the reef and wait for larger animals to stop by and rest, letting themselves get cleaned by the little fish.

Adontus homocercus is a freshwater-dweller, inhabiting most of the vast inland waterways that innerve the island. Being small, living in murky waters patrolled by far more dangerous creatures, it is not any wonder that very little is known about this fish. The first descriptor of the animal originally wanted to name it after its distinctive three-humped dorsal fins, bafflingly calling it Triceratops, until he had to be informed that there was already a genus of dinosaur with that name, leading to a minor éclat with the ICZN and a subsequent broken nose when unable to accept that fact.

Monocauda is another freshwater thelodont genus, but much more widespread than Adontus and encompassing at least seven different species. It is most closely associated with swamps and mangroves and feeds on the abundant algae and detritus there, itself being prey to the various tadpoles and efts of the anamniotes. It is unique for having lost the distinctive fork-shape of the tail, having only the upper lobe left, with the little finlets extending all the way down to below the head and moving the animal forward in an undulating motion. It is perhaps this adaptation which allows it to be so successful, making it far more flexible, mobile and better able to hide and shelter in the various nooks and crannies of its environment.

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 utilize 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.